Steep-slope asphalt shingle roofing field guide

Steep-slope shingle roofing, and why it sheds rather than seals

A steep-slope asphalt shingle roof is a system of overlapping shingles laid in courses on a sloped, nailable deck, where each course laps the one below so water runs off the surface instead of soaking through. The word that matters is shed. Shingles shed water. They are not a waterproof membrane, and treating them like one is the root of most of the failures you chase later.

Because the shingles only shed, the slope is doing real work. Gravity moves the water down and off before it can sit on a seam, find a nail head, or back up under a course. Take the slope away and the same shingles that keep a house dry for thirty years will let water stand, wick backward, and leak at every lap. That is why a shingle roof has a floor under it, a minimum slope below which you do not use shingles at all and switch to a membrane built to hold water rather than shed it.

The system is a stack of layers, and each one has a job. The deck carries the load and holds the nails. The underlayment is the secondary water barrier. The ice and water shield defends the spots where water backs up. The flashing seals where the field stops at a wall or a pipe. The shingles are the wear surface that takes the sun and the rain. Skip a layer or cheat one and the roof still goes on, looks finished, and fails on a schedule you set the day you installed it.

Why do asphalt shingles need slope?

Asphalt shingles need slope because they shed water by overlap, not by sealing the surface, so water has to keep moving downhill or it gets behind the shingle. Each course covers the nails and the top edge of the course below it. As long as the water is running down the face, that overlap is a roof. The moment water slows, pools, or backs up, it has time to creep sideways and up under the laps to the nail holes and the unsealed top edges, and now the overlap is just a stack of paper over a leak.

Two forces fight the slope. Wind-driven rain pushes water uphill and under the tabs, and standing snow and ice dam the water against the eave so it backs up the slope. Both are worse on a low slope, because the flatter the roof, the slower the water leaves and the farther it can travel backward before gravity wins. Steeper roofs shed faster, dry faster, and give wind-driven and dammed water less time to find a way in.

This is the whole reason the trade splits roofs into steep-slope and low-slope. A steep-slope roof is built to shed, with shingles, tile, slate, or metal. A low-slope roof is built to hold water without leaking, with a continuous membrane and sealed seams. They are different crafts with different failure modes, and the slope is the line between them.

What is the minimum slope for asphalt shingles?

The standard minimum slope for asphalt shingles is commonly 4:12, meaning 4 in of rise for every 12 in of run, and most manufacturers write their warranties around that figure. You can go down to a low-slope range of 2:12 up to but not including 4:12, but only with extra protection: a double layer of underlayment, or a self-adhered membrane over the whole field, set up as a low-slope detail the manufacturer and the code spell out. Below 2:12 you do not use shingles. You switch to a membrane roof.

The reason the low-slope range needs double underlayment is the same backup problem. Water leaves a 2:12 roof slowly, so wind-driven rain and ice dams have more time to push past the shingles, and the doubled or self-adhered underlayment becomes the layer that actually keeps the water out while the shingles just slow it down. On that range the underlayment is the roof and the shingles are the cladding, which is the opposite of how a steep roof works.

Confirm the exact slope limits and the low-slope build-up against the shingle manufacturer's printed instructions and the adopted building code, because the residential code carries both the 2:12 floor and the double-underlayment requirement for the low-slope range, and the manufacturer can be stricter. Where the slope drops below 2:12, the work belongs to the low-slope membrane trade, a different system covered by topic in the deck and membrane guides, not a shingle job pushed past where shingles work.

The nailable deck the whole roof rides on

Everything above the deck depends on the deck, because a shingle roof is held down by nails driven into it. A steep-slope deck is almost always wood structural panels, plywood or OSB, or solid sawn plank on older homes. The nail has to find solid, sound wood and hold against uplift, so the deck thickness, condition, and spacing of the rafters drive the whole fastening job before the first shingle goes on.

Thickness sets the hold. Codes and manufacturers set a minimum panel thickness for the rafter spacing, commonly around 15/32 in for sheathing at typical spacing, with thicker panels where the span or the wind load calls for it. Confirm the minimum against the adopted code and the panel span rating, because thinner-than-rated deck flexes between rafters, works the nails loose, and telegraphs every panel edge through the shingles. Panels are laid with a small gap at the edges, commonly around 1/8 in, so they can expand without buckling and ridging the shingles above.

The killer on any deck is water and rot. A soft spot underfoot is a deck that has lost its fastener hold and maybe its strength, and you do not shingle over it, you replace it. The deck type, its thickness, the nailable-versus-non-nailable question, and the moisture and rot checks are their own subject, covered in the roof deck substrate guide. Read the deck before you load shingles, because the deck decides whether the nails hold at all.

What is the difference between 3-tab and architectural shingles?

A 3-tab shingle is a single flat layer of asphalt mat cut into three tabs, so the whole roof reads as one thin, uniform plane. An architectural shingle, also called dimensional or laminated, is built from two or more layers bonded together, which gives it a thicker, shadowed, random look and a heavier, stiffer body that lies tighter to the deck. The thickness is not just appearance. It is wind resistance and life.

The wind ratings split the two clearly. A 3-tab is commonly warranted in the range of about 60 mph, while architectural shingles run roughly 110 to 130 mph, and premium or designer lines and impact-rated products go higher still. The laminated body resists lifting because it sits closer to the deck and its varied tab pattern denies the wind the single continuous edge it uses to peel a 3-tab back. That is why the trade moved to architectural as the default on most homes, and why 3-tab now shows up mostly on budget work and repairs to match an existing roof.

Read the ratings off the standards, not the brochure. Asphalt shingles are made to ASTM D3462 as a product standard, wind resistance is tested by ASTM D3161 or D7158, and impact resistance against hail is rated by UL 2218 in classes 1 through 4, with Class 4 the most resistant. A stated wind or impact rating only holds if the shingle is installed the way the test assumed, with the right nail count and placement. Pick the shingle for the wind zone and the hail exposure, then install it to the rating, or the number on the box means nothing.

Underlayment: felt versus synthetic

Underlayment is the sheet that goes down over the deck and under the shingles as the secondary water barrier, the layer that keeps water out when wind-driven rain gets past a shingle or while the deck is exposed during the tear-off and dry-in. It is not the primary roof. The shingles are. But it is the backup that decides whether a wind event or a slipped course becomes a leak or a non-event.

Felt is the traditional choice, an asphalt-saturated paper in the common 15 lb and 30 lb weights, made to standards like ASTM D226 and D4869. It works, and it is cheap, but it is water-resistant, not waterproof. Felt tears in wind, wrinkles when it gets wet and then dries, degrades in sun if it is left exposed, and seals poorly around the nails that go through it. Leave felt open to weather for a week and it can be junk before the shingles ever cover it.

Synthetic underlayment is a woven or spun polymer sheet, lighter, far stronger in tear, with longer allowed exposure and better footing on the slope, qualified under ASTM D8257. It lies flatter, holds up to wind during the dry-in, and does not wrinkle the way felt does. It costs more per roll and covers more per roll, and on most jobs the labor and the weather protection pay for it. Whichever you use, lap it shingle-fashion so the upper course laps over the lower, follow the manufacturer's exposure and fastening, and remember it is the secondary barrier, sized to back up the shingles, not to replace them.

What is ice and water shield?

Ice and water shield is a self-adhered, rubberized-asphalt membrane that sticks directly to the deck and seals around the nails driven through it, used at the eaves, the valleys, and the penetrations where water is most likely to back up or pool. It is the one layer in the system that is genuinely waterproof, and it goes exactly where the shingles' shed-only design is weakest. Made to a standard like ASTM D1970, it is the ice-dam and backup defense, not a field underlayment.

At the eaves in cold climates it has a specific job and a specific reach. An ice dam forms when snow melts up on the warm part of the roof, runs down to the cold overhang, and refreezes, building a ridge of ice that ponds meltwater behind it and drives that water up under the shingles. The membrane has to extend from the eave edge up the slope to a point past the inside face of the exterior wall, so that the backup is over the waterproof layer by the time it reaches the heated, leak-prone part of the roof. The common rule is coverage to at least 24 in inside the warm wall line, more on steeper or colder roofs, and the residential code spells out where it is required.

Beyond the eaves it goes in every valley, centered down the full length before the shingles, and around penetrations and at wall transitions where water concentrates. Confirm the coverage, the warm-wall reach, and where it is required against the adopted code and the manufacturer, because in a cold climate the missing eave membrane is the difference between a wet winter and a dry one. Skip it where ice dams form and you own the ceiling stains.

Drip edge at the eaves and the rakes

Drip edge is the metal angle run along the eaves and the rakes that carries water off the edge of the deck and into the gutter instead of letting it wick back under the shingles and rot the fascia and the deck edge. It is a small piece of metal that prevents a slow, hidden failure at the most exposed edge of the roof, and the building code now requires it on most shingle roofs.

The order of the layers at the edge is the part crews get backward, and it matters because of how water moves. At the eaves, the drip edge goes down first, directly on the deck, and the underlayment laps over the top of it, so any water that reaches the underlayment runs out onto the metal and off the edge. At the rakes it flips: the underlayment goes down first and the drip edge goes over it, so wind-driven rain cannot get under the metal and run back onto the deck. Eave metal under the underlayment, rake metal over it. Get that wrong and the drip edge funnels water the wrong way.

Fasten it to the deck, commonly at 8 to 12 in on center, and lap the pieces in the direction water runs, upper over lower on the rakes and shingle-fashion at corners. Where ice and water shield is used at the eave, the common detail runs the membrane over the eave drip edge and the rake drip edge over the membrane. Confirm the lap and the fastening against the manufacturer and the adopted code.

The starter course

The starter course is the first course of shingles at the eave, laid before the field shingles, and it does two jobs the field shingles cannot do for themselves at the edge. It seals the bottom edge of the first visible course with an adhesive strip placed right at the eave, and it backs up the cutouts and the joints of that first course so water cannot run straight through the gaps to the deck.

A field shingle has its sealant strip up the face, positioned to bond the course above it. At the very bottom edge of the roof there is no course below to seal to, so without a starter the bottom edge of the first field course is loose, the wind gets under it, and the cutouts between the first tabs sit directly over the deck. A purpose-made starter strip, or a field shingle cut and reversed per the manufacturer, puts a glue line right at the eave to lock that first course down and shifts the seam so it does not line up with the field cutouts.

The starter also runs up the rakes on many systems, glue line to the outside, to seal the raking edge against the wind that wants to lift the field shingles off the gable. Set the starter so its sealant lands near the eave edge, not buried up the slope where it does no good, and keep it back from the very edge by the small reveal the manufacturer calls for. No starter, or a starter installed upside down with the glue in the wrong place, is a wind failure waiting at the edge of the roof where wind loads are highest.

Field shingles, exposure, and the offset

Exposure is the part of each shingle left visible to the weather after the next course laps over it, and it is what sets how much overlap protects the roof. Standard three-tab shingles expose 5 in, and many architectural shingles expose about 5-5/8 in, but the manufacturer's printed exposure controls, and changing it changes the coverage. Run the exposure too long to stretch a bundle and you have thinned the overlap to where water reaches the nails and the top edges. Run it short and you waste material and can throw off the seal-strip alignment.

The offset, also called the stagger, is how far each course is shifted sideways from the one below so the joints do not stack up. Stacked joints make a straight channel for water down the roof. The manufacturer specifies the offset pattern for the shingle, often a set number of inches per course or a racking pattern, and it is built into how the seams and the seal strips are meant to land. Follow the printed pattern, not a habit from a different shingle.

Keep the courses straight and level across the roof with chalk lines, both horizontal lines for the course exposure and the offset control, and check them against the eave and the ridge as you go up. A roof that drifts out of line by a fraction each course arrives at the ridge crooked, with the exposure wandering and the cuts at the hips and valleys fighting you. The lines are cheap. Tearing off a roof that ran out of square is not.

How many nails per shingle?

Most asphalt shingles take a minimum of 4 nails per shingle for standard installation and 6 nails per shingle in high-wind zones or wherever the manufacturer calls for it, commonly above design wind speeds in the range of 110 mph. The residential code carries the 4-versus-6 requirement, and the manufacturer's instructions can require 6 to hold the wind rating on the warranty. When in doubt on a windy site, 6 nails is the safe call, and on a high-wind product 6 is not optional.

Where the nail goes matters as much as how many. Every shingle has a nail line or nail zone, a narrow band the manufacturer marks across the shingle, and the nails go in that band, low enough to be covered by the next course and high enough to catch the top of the course below so each nail holds two shingles. A nail driven high, above the zone, holds only its own shingle and leaves it free to lift, and a nail driven into the exposure below the zone is exposed to the weather and leaks. Nail in the line, every time.

The number-one shingle failure in the field is the nail, not the shingle. An overdriven nail, set too deep so the head cuts through or crushes the mat, no longer holds anything and gives the wind a free edge. An underdriven or high nail leaves the head proud to telegraph and lift the course above. A nail set crooked, angled so part of the head bites and part stands off, does both. Gun pressure is the usual culprit, set too high so half the nails overdrive, and the only fix is to set the pressure, watch the heads, and hand-nail the ones the gun gets wrong. The fastener also has to be long enough to penetrate the deck, commonly at least 3/4 in into the wood or fully through thinner decking, and confirm the length against the deck and the manufacturer.

Inspectors and manufacturer reps check the nailing first when a wind warranty is on the line. They lift a few shingles, count the nails, look at the line, and check whether the heads are driven flush, because a roof that fails the nailing fails the warranty no matter how good the shingles are. The nailing is where the roof is made or lost.

Valleys: open metal, closed-cut, and woven

A valley is where two roof planes meet in an inside angle, and it carries the combined runoff of both slopes, so it is one of the hardest-working and most leak-prone parts of a shingle roof. There are three common ways to build it, and they differ in how the shingles and the metal handle that concentrated water. Whichever you use, line the valley first with ice and water shield down its full length before anything else.

An open valley leaves a channel of exposed metal down the center, with the shingles from both planes trimmed back to a line on each side of it and the water running on the metal. It is the most durable for high-flow valleys because the water runs on metal, not on cut shingle edges, and it sheds debris and ice well. The metal can be galvanized steel, aluminum, copper, or coated steel, and the shingles are clipped at the top corner and set in sealant along the trim line so water cannot drive under them into the channel.

A closed-cut valley runs the shingles from one plane all the way through the valley, then laps the shingles from the second plane over the first and cuts them back to a line just off the valley center. It is clean-looking and common on residential work. A woven valley laces the shingles from both planes alternately through the valley without cutting either, for a continuous woven look. Woven works better with a flexible three-tab than with a stiff architectural shingle, which resists lying down in the valley and can leave a void. On stiffer architectural shingles the open or closed-cut valley is usually the better build. Follow the manufacturer's valley details, because the warranted method is theirs to specify.

Penetrations and flashing: pipe boots, step flashing, and chimneys

Flashing is how a shingle roof seals where the field of shingles stops at a pipe, a wall, or a chimney, and it is where the roof leaks if it leaks. The field of shingles sheds fine on its own. The trouble is always the transition, the spot where a person had to make water turn a corner and stay out. The rule across all of it: flashing makes the seal, sealant only supplements it. A detail that relies on a bead of caulk instead of metal lapped the right way leaks when the caulk fails, and caulk always fails first.

A pipe penetration gets a boot, a flashing collar that fits over the pipe with a base flange that laps into the shingles, woven in so the shingles above lap over the top of the flange and the shingles below lap under the bottom, the same shingle-fashion overlap as everything else. Where a roof plane runs into a sidewall, you use step flashing: a separate bent piece of metal at every single course, each piece lapping the course below and tucked up behind the wall cladding, so the wall sheds onto the metal and the metal sheds onto the shingle. One piece per course, woven in as you go, never a single long strip run up the wall and caulked, because the continuous strip has nothing to lap each course and leaks at every seam.

A chimney needs the full set: step flashing up the sides, a base or apron flashing across the downslope face, a cricket or saddle behind the upslope face to split the water around the chimney, and a counterflashing set into or onto the masonry that overlaps the top of all the base flashing so the metal edge is covered, not exposed. The same step-flashing and counterflashing logic carries onto dormers, headwalls, and skylights. These wall and penetration details are their own deep subject, covered in the roof penetration flashing guide. Build the flashing to lap the water out, and let the sealant be the backup, never the seal.

Why does attic ventilation matter for a shingle roof?

Attic ventilation matters because it moves outside air through the space under the roof to carry off heat and moisture, and a shingle roof without it runs hotter and wetter, which shortens shingle life and feeds ice dams. The shingles cook from below when the attic traps summer heat against the deck, and the trapped winter moisture condenses on the cold underside of the deck and rots it. Ventilation is not a comfort add-on. It is part of how long the roof lasts.

The system has to be balanced, with intake low and exhaust high. Intake comes in at the soffit or the eave, rises as it warms, and leaves at the exhaust near the ridge, so cool air sweeps the whole underside of the deck on its way through. The key is balance: the intake at the soffit should equal or exceed the exhaust at the ridge, and in no case should the exhaust outrun the intake, or the system starts pulling air from inside the house instead of from outside. A ridge full of exhaust with blocked or missing soffit intake is the most common ventilation mistake, and it does worse than nothing.

Size it to the code rule and pick the exhaust to suit the roof. The common requirement is a net free vent area of 1 sq ft for every 150 sq ft of attic floor, which can drop to 1 in 300 when the intake and exhaust are balanced and split high and low, or with a vapor retarder, confirmed against the adopted code. A continuous ridge vent paired with continuous soffit intake gives the most even sweep and the cleanest look, and it integrates under the ridge cap. Box vents and other point exhausts work but vent less evenly, and mixing exhaust types on one attic can short-circuit the airflow. In a cold climate, good balanced ventilation keeps the deck cold and even, which is half the ice-dam defense, the ice and water shield being the other half.

Hip and ridge caps

The hip and ridge caps are the finishing course of individual shingles bent over the hips and the ridge to seal the very top of the roof where the two planes meet. It is the last lap in the system, and it covers the cut top ends of the field shingles from both sides so wind and water cannot get into the gap at the peak. Caps are installed lapping in the direction away from the prevailing wind, so the wind drives over the laps rather than into them.

Use cap shingles made for the job, not field shingles folded over. Purpose-made hip and ridge caps are thicker and more flexible so they bend over the ridge without cracking, and on cold days especially a field shingle folded over a sharp ridge will crack at the bend and fail. The caps have their own sealant and their own nailing, with the nails placed so the next cap covers them, and the exposure matched to the cap product. On a high-wind roof the cap nailing and the cap product both have to suit the wind rating, because the ridge is where uplift is highest and a blown-off cap opens the whole peak.

Where there is a ridge vent, the cap shingles go over the vent and are nailed through it into the deck on each side with fasteners long enough to reach solid wood through the vent material. The vent and the cap are one assembly: the vent does the breathing, the cap sheds the water over it. Confirm the cap nailing length and pattern against the vent manufacturer, because a cap nail too short to bite the deck through the vent is a cap that leaves in the first real wind.

Wind, the seal strip, and the warranty

Every modern shingle has a self-seal adhesive strip, a band of thermal sealant that bonds each course to the one above once the sun warms it, so the field becomes one bonded surface instead of loose flaps. That seal is most of the shingle's wind resistance. Until it sets, the shingles are held only by their nails, and a roof installed right before a cold snap or a storm can blow off before the strips ever bond. The seal does the wind work the nails alone cannot.

The strip activates with heat and time, so in cold weather it may not seal on its own for weeks, or until the next warm spell. On a cold-weather install, or anywhere the wind warranty requires it, you hand-seal each shingle with a spot of roofing cement under the tab in the manufacturer's specified amount and location, so the shingle is bonded now instead of waiting on the weather. Too much cement is as bad as too little, because it can blister the shingle, so use the dab size the manufacturer calls for.

The wind warranty rides on all of it together: the right shingle for the wind speed, the right nail count and placement, the starter sealed at the edges, and the seal strips bonded or hand-sealed. Miss any one and the manufacturer can deny a wind claim, because the rated wind resistance assumed the whole set. The warranty is not a promise about the shingle alone. It is a promise about the shingle installed exactly the way the rating was tested, which is why the install drives the warranty more than the product does.

Cold-weather and hot-weather installation

Temperature changes how shingles behave under your hands and feet, and ignoring it costs you either at install or later. In the cold, asphalt shingles stiffen and turn brittle, the seal strips will not activate, and a shingle bent over a hip or snapped on the cutting line can crack instead of fold. You keep the bundles warm until just before they go up, score and snap rather than force the cold mat, hand-seal because the strips will not, and you do not walk a frozen roof harder than you have to. A cold roof that is not hand-sealed is a roof waiting for the first wind before the strips ever bond.

In the heat the problem flips. Hot shingles go soft, so the granules scuff off under boots and knees, footprints mar the surface, and a careless crew can mark up a new roof just walking it. The sealant can also bond so fast that a misplaced shingle is hard to lift and reset without tearing. You work softer in the heat, stage your movement, use pads where you kneel, and avoid dragging bundles across laid shingles.

There is a low end below which you do not install at all without special handling, and there is no benefit to fighting a roof in the wrong conditions when a day's wait changes everything. Confirm the manufacturer's temperature guidance, because the install temperature and the hand-seal requirement are theirs to set, and a roof laid outside their window can lose its warranty even if it looks fine going down.

Should you tear off or roof over the old shingles?

Tear-off means stripping the old roof down to the deck and starting clean. An overlay, or recover, means laying new shingles over one existing layer without stripping it. Tear-off is the better roof almost every time, because it is the only way to see and fix the deck, the underlayment, the ice and water shield, and the flashing, all the layers an overlay buries and trusts blind.

The code sets a hard limit: most jurisdictions allow a maximum of two layers of roofing, so you can overlay once over a single existing layer, and a roof already carrying two layers must be torn off. Confirm the layer limit against the adopted code, because some climates and some structures are stricter. There are also conditions that rule out an overlay regardless of the count: a curled, cupped, or uneven old roof telegraphs through the new shingles, and a soft or wet deck cannot be left under a new roof and called good.

The honest case against overlay is what it hides. You cannot replace the ice and water shield, you cannot fully reflash the walls and penetrations, and you cannot inspect the deck for the rot that is often the reason the old roof failed. An overlay over a bad deck is a new roof built on a failing foundation, and it leaks at the same details the old one did. The reasons people overlay are real, lower cost and less debris, but they are paying for it in a shorter, blinder roof. Where the deck is sound, the old roof is flat, and the layer count allows it, an overlay can be a reasonable call. Where any of those is in question, tear off.

Steep-slope access and fall protection

A steep roof is a fall hazard first and a roofing job second, and the steeper it is the less margin you have when a boot slips on loose granules or frost. Fall protection on a steep slope is not optional and it is not a formality. A worker who comes off a steep roof has nothing to grab on the way down, and the trade's worst injuries come off roofs.

The system is a personal fall arrest setup on the steeper and higher work: a full-body harness, a lanyard or rope grab, and an anchor fastened into the structure, the rafters or the ridge, not into sheathing alone. On lower-pitch work, roof brackets carrying planks give footing and a place to stand and stage material, and toe boards catch tools and feet. The anchor has to land in framing that will hold a fall, and the line has to be rigged so a fall is arrested short, before the worker reaches the edge.

OSHA sets the framework, with fall protection generally required for roofing work above 6 ft, and the rules tighten with the slope. Confirm the requirement against the current OSHA standard and any state plan, because the specifics on steep-slope work, anchor strength, and acceptable systems are theirs to set and they do get enforced. The fall protection goes on before the work starts, not after someone slips, because the first slip is the one that does not give a second chance.

Steep-slope shingles on commercial and large buildings

Steep-slope shingle work is not just houses. It shows up on light-commercial and larger buildings wherever there is a steep face to cover: mansard roofs that wrap the top floor of an apartment or hotel, the steep entry canopies and porte-cocheres on a commercial front, the sloped screen walls and architectural roofs over an otherwise flat building. The slope is steep, so shingles shed there the same way they do on a house, but the scale and the access change the job.

A mansard is the case that bites crews. It is nearly vertical, so the water runs off fast and the shingles are mostly cladding, but the wind load on that near-vertical face is high and the seal strips matter even more, because there is little slope to help hold a loose tab down. Nailing and hand-sealing get stricter on a mansard, and the starter and the edges carry more of the wind load than they do on a low-pitch house roof.

On these buildings the steep-slope shingle scope usually sits next to a low-slope membrane roof on the flat areas, and the tie-in between the two systems is where the leaks hide. Where a mansard or a steep canopy meets the flat roof or a parapet, the shingle flashing has to lap correctly into the membrane and the wall, and that transition is a flashing problem covered by topic in the penetration flashing guide. Coordinate the two trades at the tie-in, because a shingle face and a membrane roof that were each built fine on their own will leak at the seam between them if nobody owned it.

Material versus workmanship warranty

There are two warranties on a shingle roof and they cover different failures. The manufacturer's material warranty covers the shingles themselves failing, a manufacturing defect, premature granule loss, that kind of thing. The contractor's workmanship warranty covers the install, the leaks and failures that come from how the roof was put on. They are separate promises from separate parties, and most roof problems are workmanship, not material.

The catch every owner and every installer should understand: the install drives the material warranty too. The manufacturer's coverage assumes the roof was installed to their printed instructions, with their starter, their nailing, their ventilation, and their flashing details. Vent the attic wrong and cook the shingles from below, nail them outside the line, or skip the ice and water shield where it was required, and the manufacturer can deny a material claim because the failure traces to the install, not the product. The strongest coverage, a full system warranty, usually requires the whole system from one manufacturer installed by a certified contractor, and it is only as good as the install behind it.

Document the install to defend both warranties. The shingle, the underlayment, the ice and water shield, the nailing, the ventilation, and the flashing are the file a manufacturer rep reads when a claim comes in years later. A roof with no record of how it was built is a roof with no defense when the question is whether the failure was the product or the work.

What to document

Write down what went on the roof and how, because the layers get covered the day the crew finishes and the record is the only proof of what is underneath. A warranty claim and a callback both turn on whether the roof was built to the manufacturer's instructions and the code, and the only evidence is the log made while it was open.

Common mistakes

• Overdriving the nails or driving them high above the nail line, so the shingles hold only by the gun's overpressure and lift in the first wind.
• Skipping ice and water shield at the eaves in a cold climate, or stopping it short of the inside face of the exterior wall.
• Installing the wrong starter, no starter, or the starter upside down with the sealant buried up the slope instead of at the eave.
• Venting the attic with exhaust at the ridge and little or no matching intake at the soffit, which starves the airflow and cooks the deck.
• Relying on a continuous strip of metal and a bead of caulk at a sidewall instead of step flashing woven one piece per course.
• Overlaying new shingles over a soft, wet, or uneven deck, or over a second existing layer, instead of tearing off.
• Running the exposure long to stretch the bundles, which thins the overlap and exposes the nails and the top edges.
• Reversing the drip edge order, rake metal under the underlayment or eave metal over it, so it funnels water back onto the deck.
• Weaving a stiff architectural shingle into a valley where it will not lie down, leaving a void under the weave.
• Folding field shingles over the ridge instead of using cap shingles, so they crack at the bend in cold weather.

Field checklist

Standards and references

The shingle manufacturer's printed installation instructions govern the roof and the warranty, full stop. They set the slope limits, the underlayment, the starter, the exposure, the nail count and placement, the valley method, the flashing details, and the ventilation the warranty assumes, and the manufacturer rep judges a claim against those instructions years later. Confirm every figure in this guide against the actual product literature, because the printed instruction is the authority and it can be stricter than any rule of thumb here.

The product and test standards behind the shingles sit under that. Asphalt shingles are made to ASTM D3462, wind resistance is tested by ASTM D3161 or D7158, impact resistance is rated by UL 2218 in classes 1 through 4, the self-adhered eave and valley membrane follows a standard like ASTM D1970, felt underlayment follows ASTM D226 or D4869, and synthetic underlayment follows ASTM D8257. Cite the standard that controls the point, and remember a rating only holds if the shingle is installed the way the test assumed.

The trade and the code carry the rest. The NRCA and the ARMA publish the steep-slope application practices, the low-slope shingle details, the drip-edge, valley, and flashing methods, and the ventilation guidance, as recommended industry practice. The International Residential Code carries the enforceable requirements: the 2:12 floor and the double-underlayment low-slope provision, the ice-barrier requirement and where it applies, the drip-edge requirement, the 4-versus-6 nailing, and the attic ventilation net-free-area ratios. OSHA carries the fall protection. The code is adopted and amended by jurisdiction, so confirm each requirement against the adopted edition and any local amendments, and let the manufacturer's instructions and the adopted code override the rule of thumb.

Units, terms, and conversions

Steep-slope shingle work carries its own vocabulary, and the same thing reads differently across a manufacturer's instruction sheet, a code section, and a spec, so the terms are worth pinning down.

Slope, also called pitch, is given as rise over run, inches of vertical rise per 12 in of horizontal run, written like 4:12. Shingle coverage is by the square, where one square is 100 sq ft of finished roof. Exposure is the inches of each shingle left to the weather, commonly around 5 in. Wind ratings are in miles per hour, impact in UL 2218 classes 1 to 4. Ventilation is in net free area, the actual open area of a vent, sized against the attic floor area by a ratio like 1 in 150 or 1 in 300. Nail length is in inches, commonly 1-1/4 in for new shingles over standard deck, longer over thicker build-ups.

Slope / pitch

Rise over run in inches per foot, written like 4:12; the standard shingle minimum is commonly 4:12

Square

100 sq ft of finished roof area, the unit shingles are sold and measured by

Exposure

The portion of each shingle left visible to the weather, commonly around 5 in, set by the manufacturer

Underlayment

The secondary water barrier under the shingles, felt or synthetic; it backs up the shingles, it is not the roof

Ice and water shield

Self-adhered waterproof membrane at eaves, valleys, and penetrations that seals around the nails through it

Starter course

The first sealed course at the eaves and rakes that locks the bottom edge and backs the field cutouts

Nail line / nail zone

The marked band where nails go, so each fastener holds two courses; nails above or below it fail

Step flashing

One bent metal piece per course at a sidewall, woven into the shingles, never a single caulked strip

Net free area

The actual open vent area for attic ventilation, balanced intake to exhaust against the attic floor area

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