Roofing system types: steep-slope vs low-slope, and how to choose

How to think about roofing systems: the slope decides

A roofing system is the whole assembly that keeps water out of a building, from the deck up through the covering you can see, and it falls into two families that the slope of the roof decides between. Steep-slope roofs shed water with overlapping materials. Low-slope roofs hold water back with a continuous membrane. Before you argue shingle versus metal or TPO versus EPDM, you settle which family the roof is in, because the slope already answered the question and the rest follows from it.

Steep-slope is the shedding world. Shingles, metal panels, tile, slate, and wood all work the same way: each piece laps the one below, gravity carries the water down the face and off the edge, and nothing under the covering ever has to be watertight on its own. Low-slope is the membrane world. The water leaves slowly or sits, so the roof cannot rely on overlap. It needs a continuous, sealed waterproof skin, single-ply, built-up, modified bitumen, or sprayed foam, that holds water without leaking.

This guide is the map, not the detail. It lays out the families, what each one is for, and how the parts under the covering make a system instead of a surface. The two systems people pick most have their own guides: the low-slope membrane selection guide for TPO, EPDM, and PVC, and the steep-slope asphalt shingle guide for the dominant residential roof. Use this hub to get to the right family, then go deep where the decision lives.

What slope makes a roof steep-slope or low-slope?

The line the trade draws between low-slope and steep-slope sits around 2:12 to 3:12, where 2:12 means 2 in of vertical rise for every 12 in of horizontal run. Below roughly 2:12 the roof is low-slope and wants a membrane. Above roughly 3:12 to 4:12 it is solidly steep-slope and sheds with overlapping materials. The 2:12-to-3:12 band is the transition, where some shedding materials are allowed only with extra underlayment and some are not allowed at all.

The reason the slope governs is the same physics for every covering. Overlapping materials shed water as long as it keeps moving downhill. Flatten the roof and the water slows, wind-driven rain pushes it back up under the laps, and snow and ice dam it against the edge, so it finds the nail holes and the unsealed top edges. A membrane is built for the opposite job, to stay watertight when the water is slow or standing. Put a shedding material on too flat a roof and it leaks. Put a membrane on a steep roof and you have paid for the wrong system.

The exact minimums vary by material and by authority, and they are the most important numbers to confirm, not assume. NRCA generally recommends 4:12 or steeper for asphalt shingle, tile, metal shingle, slate, and wood, while the building code allows several of those lower with added protection. The IRC lets asphalt shingles down to 2:12 with a double underlayment or a self-adhered membrane. Mechanically seamed standing seam metal can run far flatter than snap-lock or exposed-fastener panels. Confirm the slope minimum against NRCA, the manufacturer's printed instructions, and the adopted code edition before you commit a covering to a slope.

Steep-slope: asphalt shingles, the default residential roof

Asphalt shingles are the dominant steep-slope covering on houses in North America, and for good reason. They are the lowest first cost of the steep-slope materials, they install fast with a small crew, and they come in 3-tab, architectural, and premium lines that span a wide range of wind ratings and looks. An architectural shingle on a typical house is the volume default, and most steep-slope re-roofs are shingle-to-shingle.

The system is more than the shingle. It is a nailable deck, an underlayment as the secondary barrier, a self-adhered ice-and-water membrane at the eaves and valleys, a starter course, flashing at every wall and penetration, balanced attic ventilation, and the field shingles nailed in the line. The covering takes the credit, but the underlayment, the flashing, and the nailing are where the roof is made or lost. The number-one shingle failure in the field is the nail, not the shingle.

Shingles shed, they do not seal, so the slope minimum is real. They run standard at 4:12 and steeper, down to 2:12 with low-slope detailing, and below 2:12 you switch systems entirely. The full install, the slope limits, the underlayment, the nailing, and the flashing, lives in the steep-slope asphalt shingle guide. Use this section to place shingles in the family and go there for the work.

Steep-slope: metal roofing

Metal roofing covers three different systems that get lumped under one name, and the difference matters for both slope and life. Standing seam is the premium system: formed panels with vertical ribs at the edges that lock together over concealed clips, so there are no fasteners through the face to leak. Exposed-fastener panels, the through-fastened corrugated and rib panels, screw down through the face with gasketed screws, which is cheaper and faster but puts hundreds of penetrations in the field. Metal shingles and tiles are stamped panels that mimic shingle, tile, or slate.

The systems split on slope and on where they leak. Mechanically seamed standing seam can run very low, commonly down near 1:12 or even flatter for some profiles, while snap-lock standing seam, exposed-fastener panels, and metal shingles commonly want 3:12 or steeper. Standing seam's weak point is the clip spacing and the panel expansion. Exposed-fastener's weak point is the gaskets under the screw heads, which dry out and back out under UV and thermal cycling and need re-fastening over the years. Confirm the slope minimum and the fastening against the panel manufacturer, because the profile sets it.

Metal earns its premium on life and on low maintenance, commonly 40 to 70 years for a quality standing seam roof, well past a shingle roof's run. It is light, it reflects heat well in a coated or bare finish, and it carries snow and sheds it. Metal is its own deep subject, covered by topic in the metal roofing guides, so weigh standing seam against exposed-fastener on the building's slope, budget, and how long the owner intends to hold it.

Steep-slope: tile, slate, and wood

Tile, slate, and wood are the heavy and the natural steep-slope systems, the long-life roofs that ask more of the structure and the budget. Clay and concrete tile shed with interlocking or overlapping pieces and last a long time, commonly 50 years for concrete and well past a century for clay. The catch is weight. Tile commonly runs 6 to 15 lb per square foot depending on profile, several times a shingle roof, so the structure has to be designed or verified to carry the dead load before tile goes on a building.

Slate is the long-life leader. A natural slate roof commonly lasts 75 to 200 years, outliving the building's other systems several times over, which is why you still see slate on century-old buildings. It is also the heaviest common covering, measured in hundreds of pounds per square and sometimes more than a thousand, and the highest first cost, so it lands on premium and historic work where the life and the look justify it and the structure can hold it. Slate commonly wants 4:12 minimum and performs better at 6:12 and steeper.

Wood shake and shingle is the traditional natural roof, cedar most often, with a life commonly in the 25-to-40-year range that depends heavily on climate, maintenance, and fire exposure. It needs a slope to shed and a deck detail that lets the wood breathe, and in many wildfire-exposed areas the fire rating limits or rules it out. Each of these is its own craft, covered by topic in the tile, slate, and wood roofing guides. The shared lesson for selection is that tile and slate buy decades of life at the cost of weight and money, so the structural capacity and the budget gate the choice before the aesthetics do.

The steep-slope assembly: the shed-water stack

A steep-slope roof is a stack of layers over a nailable deck, and every layer has a job even though only the top one shows. The deck, almost always wood structural panels or plank, carries the load and holds the fasteners. The underlayment is the secondary water barrier, the layer that keeps water out when wind-driven rain gets past the covering or while the deck is open during the work. A self-adhered ice-and-water membrane goes at the eaves, the valleys, and the penetrations where water backs up and the covering is weakest.

Over that goes the covering, the shingles, panels, tile, or slate that take the sun and the rain, and around the edges and openings goes the flashing that seals every transition to a wall, a pipe, or a chimney. Flashing makes the seal and sealant only supplements it, because caulk always fails first. The deck and the underlayment are common to every steep-slope covering, which is why they are their own deep subjects, covered by topic in the roof deck substrate and underlayment guides.

The shared principle across all steep systems is the same as the slope rule. Nothing under the covering has to be watertight on its own, because the overlap and the slope keep the water moving off the surface. The underlayment and the ice-and-water membrane are the backup for the spots where overlap is not enough. Treat the covering as the whole roof and skip a layer below it, and the roof still looks finished the day it goes on and fails on a schedule you set that day.

Low-slope: single-ply membranes (TPO, EPDM, PVC)

Single-ply membranes are the dominant low-slope system on commercial buildings, a single sheet of waterproof material rolled out over the roof and seamed into a continuous skin. The three are TPO, a white reflective thermoplastic that has taken the largest share of the new market; EPDM, the black rubber roof with the longest field track record and the best cold flexibility; and PVC, the thermoplastic that resists grease and chemicals the other two cannot. TPO and PVC seam by hot-air weld. EPDM seams with splice tape and primer.

Single-ply leads on commercial low-slope because it installs fast, weighs little, and the white versions reflect heat to meet a cool-roof requirement straight off the roll. On a single-ply roof the field of the sheet almost never leaks. The seams, the flashings, and the penetrations do, which is why the seam QA on a welded roof is its own discipline. There is no single best membrane. Each wins in specific conditions, and the building's chemical exposure, climate, attachment, and warranty pick it.

Which of the three, and where mod-bit and built-up still beat them, is the whole subject of the low-slope membrane selection guide. Use this section to place single-ply as the default low-slope family, then go there to choose TPO, EPDM, or PVC against the building instead of by brand habit.

Low-slope: built-up roofing and modified bitumen

Built-up roofing and modified bitumen are the asphalt-based low-slope systems, and both build their waterproofing by stacking plies rather than relying on one sheet. Built-up roofing, BUR, is the classic tar-and-gravel roof: alternating layers of bitumen, hot asphalt or coal tar, and reinforcing felt or fabric, finished with a gravel or mineral surface that protects the asphalt from the sun. It is a multi-ply system, so a breach in one ply does not necessarily reach the deck, and that redundancy is its argument.

Modified bitumen, mod-bit, is the modern evolution of the same idea in rolled sheets. Polymers are added to the asphalt to make it more flexible and durable, the sheet is reinforced with fiberglass or polyester, and it is applied by torch, hot mop, cold adhesive, or as a self-adhered peel-and-stick. A granulated cap sheet gives a tough, walkable surface that takes foot traffic and abuse well, which is why mod-bit holds up on high-traffic roofs and as a base under amenity decks and pavers.

Both lost the volume market to single-ply because single-ply installs faster, weighs less, and reflects heat the dark asphalt systems do not. But the multi-ply redundancy and the tough trafficked surface are real, and on the right roof they still win. A specifier who only knows single-ply is missing part of the toolkit. These systems are covered by topic in the built-up and modified bitumen guides, so weigh the redundancy and the traffic resistance against the weight and the heat.

Low-slope: spray polyurethane foam (SPF)

Spray polyurethane foam is the low-slope system that goes on as a liquid and cures into a continuous, monolithic roof with no seams and no fasteners through the field. Two chemicals, an isocyanate and a polyol, are sprayed together, react, and expand into a closed-cell foam that bonds tightly to the substrate. A protective coating, usually a silicone or acrylic elastomeric, goes over the foam to take the UV and the weather, because the foam itself degrades in sunlight if it is left bare.

SPF does two jobs at once that other systems split. It waterproofs and it insulates, adding real R-value in the same layer, and because it is sprayed it can build slope to drain into low spots and around penetrations without separate tapered insulation. It also bonds to a wide range of existing substrates, BUR, metal, single-ply, concrete, which makes it a recover option where tearing off is costly. The lack of seams removes the most common leak path on a low-slope roof.

The catch is the coating and the maintenance. SPF lives on its coating, and the coating wears, so the system needs periodic inspection and re-coating to reach its long life, commonly 20 years and beyond when it is maintained. A neglected coating exposes the foam and the roof goes downhill fast. SPF is covered by topic in the spray foam roofing guide. Treat it as a real low-slope option where the insulation value, the slope-building, and the recover-without-tear-off matter, and budget for the re-coats.

How a low-slope membrane is held down: adhered, mechanical, or ballasted

A low-slope membrane is held to the roof one of three ways, and the wind, the deck, and the building drive which one. A fully adhered system glues the whole sheet down with adhesive and has no field fasteners. A mechanically attached system fastens the membrane to the deck with screws and plates, usually in rows that fall in the seam laps. A ballasted system lays the membrane loose and holds it with stone or pavers. All three work with single-ply, with practical limits by system.

Attachment is mostly a wind decision. Fully adhered systems generally reach the highest wind-uplift ratings, because the bond is spread across the whole sheet instead of concentrated at fastener rows, so they show up on tall buildings, coastal exposures, and the corners and perimeters where uplift peaks. Mechanically attached systems depend entirely on the fastener pattern and density, which is engineered to the wind load and tightened at the edges and corners. Ballasted systems use weight, which limits them to roofs with the slope and the structural capacity to carry the stone.

On a mechanically attached roof the seam carries both the watertightness and the wind load, because the fasteners live in the lap, which changes what the seam has to do. The membrane choice and the attachment choice are not independent, and the attachment moves the cost more than the membrane does. The attachment decision is covered alongside the membrane choice in the low-slope membrane selection guide. Confirm the attachment and the fastening pattern against the engineered wind-uplift requirement, because the pattern is designed, not chosen on the roof.

The low-slope assembly: deck, insulation, cover board, membrane

A low-slope roof is a buildup, and the membrane on top is the last layer, not the whole roof. Under it sits the deck, the insulation, often a cover board, and only then the membrane. The deck can be steel, concrete, or wood, and it carries the load and takes the attachment. The insulation, usually rigid boards of polyiso or another foam, sits above the deck and carries most of the roof's R-value. A cover board, a thin dense layer, goes over the insulation to take foot traffic, hail, and the membrane attachment without crushing the soft insulation below.

This above-deck insulation is what makes a low-slope roof a compact roof, with the insulation outside the structure and the membrane right over it. That layout has consequences the steep-slope world does not face. There is no attic and no vent space between the insulation and the membrane, so the assembly cannot breathe the way a vented attic does, and the dew point and the vapor control move into the buildup itself. A wet insulation layer in a compact roof has nowhere to dry, which is why the vapor retarder and the detailing matter more than people expect.

The deck, the insulation, and the cover board are common to every low-slope membrane, single-ply, BUR, mod-bit, or coated foam, and they are their own deep subjects, covered by topic in the roof deck and roof insulation guides. The lesson for selection is that two roofs with the same membrane can perform very differently because of what is under it. Spec the assembly, not just the sheet.

What is the difference between steep-slope and low-slope roofing?

Steep-slope roofing sheds water with overlapping materials on a slope steep enough to keep the water moving off the surface, while low-slope roofing holds water back with a continuous, sealed membrane on a roof too flat to shed reliably. That single difference, shed versus hold, drives everything else: the materials, the failure modes, the inspection, and the trade that does the work. They are different crafts, and the slope is the line between them.

The failure modes split cleanly. A steep-slope roof leaks at the laps, the nails, the valleys, and the flashing, the spots where the overlap is not enough, and almost never in the open field. A low-slope roof leaks at the seams, the terminations, and the penetrations, the spots where the continuous skin is interrupted, and almost never in the open membrane. An inspector reads each roof differently because of that: on a steep roof, check the flashing and the nailing; on a low-slope roof, probe the seams and the details.

Where each lands is mostly the building. Steep-slope dominates houses and the visible pitched roofs of light commercial, because the slope is there and the look matters. Low-slope dominates commercial, industrial, and large flat-roofed buildings, where the roof is out of sight and the span is wide. Many buildings carry both, a low-slope field with steep-slope mansards, canopies, or screen walls, and the tie-in between the two systems is where those buildings leak.

The roof is a system, not just the covering

The biggest mistake in roofing, on either side of the slope line, is treating the covering as the whole roof. The shingle or the membrane is one part of an assembly that also includes the deck it rides on, the insulation, the underlayment or vapor and air barrier, the flashing at every edge and penetration, and the drainage that gets the water off. A roof fails at the weakest of those parts, and it is rarely the field of the covering.

Each part has a job and a guide. The deck carries the load and takes the fasteners or the attachment, and a soft or wet deck fails the roof above it no matter how good the covering is. The insulation controls heat and, on a low-slope compact roof, the dew point. The underlayment or the air and vapor barrier is the backup and the moisture control. The flashing seals the transitions, where most leaks start, not the open field. The drainage moves the water off before it can sit and find a way in.

These parts are deep subjects in their own right, covered by topic in the roof deck, insulation, flashing, ventilation, and drainage guides. The point for anyone choosing a system is that the covering decision is the start, not the finish. A right covering on a bad deck, a wrong attachment, a missing flashing detail, or a roof that does not drain still fails. Spec and inspect the whole assembly, and put the covering in its place as one layer of it.

Insulation, R-value, and the cool roof

Roof insulation is where most of a building's roof-line R-value lives, and on a low-slope roof it usually sits above the deck as rigid board, commonly polyiso at roughly R-4 to R-6 per inch. Two layers staggered, with the joints offset, beat one thick layer because the offset breaks the thermal path through the joints. Where the roof needs slope to drain and does not have it built into the structure, tapered insulation does double duty: it adds R-value and it builds the slope, sloped boards laid to carry water to the drains.

Tapered insulation has a number people get wrong. When you figure the assembly R-value of a tapered layer, you use the average thickness across the slope, not the thick end, because the thin end at the drain carries far less insulation. Recent energy code editions spell this out. Get it wrong and the roof is short of its specified R-value at exactly the low spots, which is also where moisture concentrates.

The cool roof is a separate lever that lives at the surface. A reflective roof, usually a white low-slope membrane or a reflective coating, bounces solar heat instead of absorbing it, which lowers the surface temperature, cuts the cooling load, and slows the heat aging of the membrane. Many energy codes and rating programs set a minimum solar reflectance or solar reflectance index for low-slope roofs in warm climates, and the Cool Roof Rating Council and ENERGY STAR rate the products. Reflective wins in cooling-dominated climates and where the code demands it. In a cold, heating-dominated climate, a dark roof that absorbs winter heat can be the better call. Confirm the reflectance requirement against the adopted energy code.

How do you ventilate a steep-slope roof versus a low-slope roof?

Steep-slope and low-slope roofs handle ventilation in opposite ways, and confusing the two causes real damage. A steep-slope roof with an attic or a vented cavity is ventilated by moving outside air through that space, intake low at the soffit and exhaust high at the ridge, so cool air sweeps the underside of the deck and carries off heat and moisture. The system has to be balanced, with intake meeting or exceeding exhaust, or the exhaust starts pulling conditioned air out of the house. A ridge full of exhaust with blocked or missing soffit intake is the most common ventilation mistake, and it does worse than nothing.

The common code target for attic ventilation 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. Confirm the ratio against the adopted code. Good balanced ventilation keeps the deck cold and even, which is half the ice-dam defense in a cold climate.

A low-slope compact roof is the opposite case: there is usually no vent space at all. The insulation sits above the deck, the membrane sits right over it, and there is no gap between them to ventilate, so the assembly relies on the vapor retarder and the air barrier to keep moisture out of the buildup rather than on airflow to dry it. Low slope also makes any vent channel ineffective, because air moves poorly along a near-flat path. The moisture control on a low-slope roof is built into the layers, not vented out, which is why a wet compact roof is a serious problem with nowhere to dry. Ventilation is its own subject, covered by topic in the ventilation guide.

Drainage: the steep shed versus the low-slope drain

Drainage is how the water actually leaves, and the two families do it differently. A steep-slope roof drains by shedding: gravity runs the water down the face to the eave, into the gutters, and down the leaders, so the slope is the drainage and the only hardware is the gutter and the downspout. The steeper the roof, the faster it clears and the less time water has to back up. Drainage problems on a steep roof are usually undersized or clogged gutters and ice dams at the eave, not the field of the roof.

A low-slope roof has to be drained on purpose, because the water does not run off on its own. It drains to interior roof drains, to scuppers through the parapet, or to gutters at the edge, and the roof has to be sloped to those points, commonly with tapered insulation or a sloped structure, so the water actually reaches them. A flat roof with no positive slope ponds, and standing water ages the membrane, adds load, and finds any weak seam given enough time. Many warranties require the roof to drain within a set time and exclude damage in areas that pond.

The durable fix for ponding is slope and drains, not a tougher membrane. A membrane only waterproofs the puddle; it does not remove it. Size and place the drains and scuppers for the roof area and the rainfall, add overflow drainage so a clogged primary drain does not flood the roof, and build the slope to carry water to them. Drainage design is its own subject, covered by topic in the drainage guide. The slope-to-drain is part of the system, not an afterthought.

Green roofs and special low-slope assemblies

A green, or vegetative, roof is a planted assembly built on top of a low-slope waterproofing system, and it is a system layered on a system. Under the soil and the plants sits a waterproof membrane, a root barrier so the roots cannot reach the membrane, a drainage layer, and a growing medium, all carried by a structure designed for the added weight of saturated soil. The membrane below has to be the durable, redundant kind, often a multi-ply or a heavy single-ply, because once the garden is built you cannot get to the membrane easily to fix a leak.

Green roofs buy stormwater retention, insulation value, longer membrane life out of the sun, and usable amenity space, which is why they show up on urban and institutional buildings and where codes or incentives push for them. They cost more, weigh far more, and demand a structure and a leak-detection plan built for them. The weight is the gate, the same as it is for tile and slate on the steep side.

Other special low-slope assemblies follow the same logic of building over the membrane: protected-membrane roofs that flip the insulation above the waterproofing, plaza and amenity decks that carry pavers on pedestals, and ballasted systems that hold the membrane with stone. The shared idea is that the waterproofing goes down first and durable, and everything else is built on top of it. These assemblies are covered by topic in the green roof and amenity deck guides. The structural capacity and the future access to the membrane drive whether they fit.

How do you choose a roofing system?

You choose a roofing system by running the conditions in order, and the slope comes first because it decides the family before anything else. Once the slope sets steep-slope or low-slope, the building type, the budget, the wanted life, the climate, the foot traffic, the energy and code requirements, and the look narrow it to a covering. Run them in that order and the roof largely specifies itself. Start from the aesthetics or a brand preference and you fight the building the whole way.

Residential and light-commercial pitched roofs are steep-slope, and the call is usually shingle for cost, architectural shingle as the default, with metal, tile, or slate where the owner pays for life and look and the structure can carry the weight. Commercial, industrial, and flat-roofed buildings are low-slope, and the call is usually single-ply for most ordinary roofs, PVC where grease or chemicals are present, BUR or mod-bit where multi-ply redundancy or a trafficked surface matters, and SPF where insulation value and recover-without-tear-off lead. The two deep guides carry these decisions in full.

The conditions that flip a decision are specific, not vague. Grease exposure forces PVC. A cool-roof mandate forces a reflective surface. Heavy foot traffic or hail forces a tough cap or a cover board. A structure that cannot carry weight rules out tile and slate. A coastal wind zone forces a fully adhered or heavily fastened attachment. Match the system to the building's real conditions, and weigh installed cost against the warranted life, not the first cost alone.

Service life, cost tier, and warranty by system

Roofing systems span a wide range of service life and first cost, and the two track together: the longer-life coverings cost more up front and lean on the structure harder. Treat the figures below as planning ranges, not quotes, because the real life of any roof depends more on the install quality, the maintenance, and the climate than on the material, and the cost varies by region, roof complexity, and the assembly under the covering. Confirm the warranted term against the specific product.

On the steep side, asphalt shingle is the lowest cost and the shortest life, commonly 15 to 30 years by grade. Metal runs 40 to 70 years at a higher cost. Tile reaches 50 to 100 years and slate 75 to 200, both at premium cost and heavy weight. On the low-slope side, single-ply, BUR, and mod-bit commonly run 20 to 30 years, and SPF can run 20 years and beyond when the coating is maintained. The warranty matters as much as the chemistry: a manufacturer's no-dollar-limit warranty on a low-slope system, and the wind and material warranties on a steep roof, only issue and only hold if the system went on to the printed instructions. The install drives the warranty more than the product does.

The honest read is that the cheapest covering on the data sheet is rarely the cheapest roof over its life. A shingle roof replaced twice in the time a metal roof lasts once can cost more in the long run, and a low-slope roof that loses its warranty to a thin membrane or a bad seam costs the owner the repair the warranty would have covered. Weigh the installed cost against the warranted life, and read the warranty's conditions before you treat the term as a promise. Warranty by system is covered by topic in the roof warranty guide.

Re-roof: recover or tear off?

On a re-roof you choose between tearing the old roof off to the deck and recovering over it, and the right call depends on the system, the deck, and the code. A tear-off strips everything and lets you see and fix the deck, the underlayment, the ice-and-water membrane, and the flashing, all the layers a recover buries and trusts blind. A recover leaves the existing roof in place and installs the new system over it, which saves the tear-off cost and the debris and keeps the building dry during the work.

The code limits how many layers a building can carry, commonly a maximum of two, so a roof already at two layers must be torn off, and a wet or failing existing roof has to come off regardless of the count. On the steep side, a curled or uneven old roof telegraphs through new shingles and rules out an overlay. On the low-slope side, the new membrane has to be compatible with what it touches, and some combinations need a separator, PVC over asphalt being the signature example.

Recover is genuinely attractive on the right roof, and some systems are built for it. SPF bonds over many existing substrates, and a single-ply or coating recover over a sound, dry, in-limit existing roof can be a reasonable call. The honest case against recover is what it hides: you cannot inspect the deck for the rot that often caused the old roof to fail. A new roof over a failing deck leaks at the same details the old one did. Where the deck is sound and the layer count allows it, recover can win. Where any of that is in question, tear off. The recover-versus-replace decision is covered by topic in the re-roof guide.

What to document

The system, the slope, and the assembly under the covering get buried the day the crew finishes, so the record made while the roof was open 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. Capture the system and why it was chosen, the measured slope, the deck, the insulation and barriers, the attachment, the flashing, the drainage, and the warranty term.

Common mistakes

• Putting a shedding material on too flat a roof, like shingles below their slope minimum, so wind-driven rain and ice backup leak the laps.
• Putting a low-slope membrane on a steep roof, paying for the wrong system where overlapping materials would shed for less.
• Treating the covering as the whole roof and ignoring the deck, the insulation, the barriers, and the flashing where the roof actually fails.
• Choosing the system by aesthetics or brand habit before the slope, the structure, and the climate have narrowed it.
• Specifying tile or slate without verifying the structure can carry the dead load, then finding out after the order.
• Skipping the drainage design on a low-slope roof, so the field ponds and the standing water ages the membrane and voids the warranty.
• Confusing the ventilation of a vented attic with a compact low-slope roof, and trying to vent an assembly that has no vent space.
• Picking a system for the wrong foot traffic or use, like a delicate cap on a roof that crews walk daily.
• Recovering over a failing or wet deck instead of tearing off, so the new roof leaks at the same details the old one did.
• Comparing only first cost and ignoring the service life and the warranty, so a cheap covering costs more over the building's life.

Field checklist

Standards and references

The manufacturer's printed system and warranty govern the specific roof, full stop. They set the approved assembly, the slope minimum, the attachment, the compatibility rules, and the conditions the warranty requires, and the closeout inspection runs against them. Everything below is the framework around that controlling document, so confirm the specifics against the actual product.

The NRCA Roofing Manual is the practical reference for both families, published in separate steep-slope and low-slope volumes that carry the slope guidelines, the system definitions, and the recommended practices. NRCA's recommended slope minimums are best-practice guidance and generally sit steeper than the code minimums, so the two can differ and both matter. The ASTM material standards define what each covering and membrane has to be, and the product literature cites the ones that apply. Cite the standard that controls the point, and confirm the current designation and edition before putting it on a submittal.

Around the materials sits the rest. Wind uplift runs under ASCE 7 for the loads, with FM Global data sheets adding requirements on insured roofs and UL listings carrying the fire and assembly classifications. The building code, the IBC for commercial and the IRC for residential, carries the enforceable slope minimums, the layer limits, the ice-barrier and underlayment requirements, the ventilation ratios, and the drainage and overflow rules, with the IECC carrying the insulation and energy requirements and cool-roof reflectance. Reflectance ratings come from the Cool Roof Rating Council and ENERGY STAR, and fall protection from OSHA. 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 code override any rule of thumb here.

Units, terms, and conversions

Roofing carries its own vocabulary, and the same idea 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. Roof area is measured in squares, where one square is 100 sq ft of finished roof. Membrane thickness is in mils, thousandths of an inch, commonly 45, 60, and 80 mil. Insulation is rated in R-value per inch and as a total assembly R-value, using average thickness for tapered layers. Reflectivity is rated by solar reflectance and solar reflectance index. Net free area is the actual open vent area for attic ventilation, sized against the attic floor area by a ratio like 1 in 150 or 1 in 300.

Slope / pitch

Rise over run in inches per foot, written like 4:12; the slope decides the steep-slope versus low-slope family

Steep-slope

A roof that sheds water with overlapping materials, commonly above roughly 3:12 to 4:12

Low-slope

A roof too flat to shed reliably, commonly under roughly 2:12, waterproofed with a continuous membrane

Single-ply

A low-slope membrane in a single sheet, TPO, EPDM, or PVC, seamed welded or taped into a continuous skin

BUR

Built-up roofing, the multi-ply tar-and-gravel low-slope system of alternating bitumen and reinforcement

Modified bitumen

Polymer-modified asphalt low-slope sheet, applied by torch, mop, cold adhesive, or self-adhered

SPF

Spray polyurethane foam, a sprayed monolithic low-slope roof that insulates and waterproofs, protected by a coating

Compact roof

A low-slope assembly with insulation above the deck and no vent space below the membrane

Cool roof

A reflective roof surface that lowers surface temperature and cooling load, rated by solar reflectance index

Square

100 sq ft of finished roof area, the unit roofing is sold and measured by

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