Wood shake and wood shingle roof installation field guide

What a wood shake and shingle roof is, and the two things that govern it

A wood shake or shingle roof is a covering of individual natural-wood pieces, almost always Western red cedar, laid in overlapping courses so each one laps the pieces below and water sheds down the slope. It gives a look nothing manufactured quite matches and lasts a long time when it is built right. Built wrong, it is a roof with a short fuse and a long list of callbacks.

Two things govern whether a cedar roof lives or dies, and both are different from anything an asphalt roof has to worry about. The wood has to breathe, meaning it has to dry from both faces, because cedar that stays wet on the underside rots and the nails bleed. And the wood has to resist fire, because untreated cedar is combustible, the code cares about that more than almost anything else, and many wildfire areas now restrict or ban wood roofs outright.

Everything else in this guide hangs off those two governors. The spaced sheathing, the interlayment, the stainless fasteners, and the keyway gaps all exist to let the wood breathe and move. The fire-retardant treatment and the Class A assembly exist to keep it from being the thing that burns the house down. The shingle system that handles slope, flashing, and valleys the same way a cedar roof does is covered in the steep-slope asphalt shingle guide, and the felt and self-adhered layers under any roof are covered in the underlayment guide. Read those alongside this one.

Why does a wood roof have to breathe and resist fire?

A wood roof has to breathe and resist fire because cedar is a natural, porous, combustible material, and ignoring either fact is how the roof fails. These are the two governors, and they pull the install in directions an asphalt or metal roof never deals with.

Breathing comes first because it is the slow killer. Cedar takes on water in the rain and gives it back as it dries, and it has to give it back from both faces. Trap the underside against a solid, sealed deck with no airflow and no way to dry, and the wood stays damp, holds rot, and the fasteners corrode in the wet. A cedar roof that cannot dry from below is rotting from the day it goes on, and you will not see it until the deck is soft.

Fire is the fast killer and the one the code is built around. Untreated cedar burns, and a burning cedar roof throws firebrands that carry the fire to the next house. That is why fire-retardant treatment, the roof's fire class, and the local wildfire rules drive the whole job before the look or the budget enters the conversation. Get the breathing assembly and the fire rating right and the rest of the roof is detailing. Get either one wrong and the detailing does not matter.

What is the difference between a wood shake and a wood shingle?

A wood shake is split, and a wood shingle is sawn. That single difference in how the piece is made drives everything else about how it looks and how it goes on. A shake is split from the log, by hand or by machine, so it follows the grain and comes out thick, rugged, and irregular, with a rough split face that throws a deep shadow line across the roof. A shingle is sawn on both faces, so it comes out smooth, thinner, and uniform, with a tailored, even look.

The thickness is the working difference. Shakes run heavy at the butt, commonly 1/2 in to 3/4 in or more, while shingles are thinner, often in the range of 0.40 in to 0.45 in at the butt, and taper to a fine edge. The thick, uneven shake is the rustic look people picture on a lodge or a historic home. The smooth shingle is the cleaner, more formal look, and it lays tighter and flatter to the roof.

The install differs because of it. Shakes are irregular, so they need an interlayment, a felt baffle laid between the courses, to stop wind-driven water from getting under the uneven pieces. Shingles lay tight and uniform and do not use that interlayment. Both shed by overlap and both need to breathe, but the shake roof carries that extra layer between every course and the shingle roof does not. Confirm the product and the method against the Cedar Shake and Shingle Bureau and the manufacturer, because the two are detailed differently for a reason.

The wood and the grades

Western red cedar is the standard for a reason: it is dimensionally stable, naturally resistant to rot and insects from its own oils, and it splits and saws cleanly. Alaskan yellow cedar shows up as a harder, blonder alternative, and redwood and Eastern white cedar appear in some regions and grade systems. The species matters because the natural decay resistance and the way the wood moves both ride on it, so do not treat one cedar as a drop-in for another without checking.

Grade is where the money and the life of the roof live. The Cedar Shake and Shingle Bureau grades Western red cedar shingles as No. 1, No. 2, and No. 3, and the premium roofing grade is No. 1, the Blue Label. The best material is clear heartwood, edge-grain (also called vertical grain), and free of defects, because heartwood resists decay far better than sapwood, and edge-grain wood cups and checks less than flat-grain. A premium No. 1 shingle is commonly described as 100 percent heartwood, 100 percent clear, and 100 percent edge-grain.

Read the grade off the bundle, not the salesperson. CSSB Certi-label bundles carry a grade stamp that names the grade, the species, the certifying agency, the code standards met, and the mill. Lower grades carry more flat grain, more sapwood, and knots, and they belong on outbuildings and walls, not on the roof you want to last. The grade, the species, and the heartwood and grain content are the material decision, and they should match what the manufacturer and the CSSB specify for a roof application.

Can you put a wood roof in a fire zone?

Often you cannot, and this is the single most important question to answer before you bid a cedar roof. Untreated cedar is combustible, and in many wildfire and wildland-urban-interface jurisdictions a wood roof is restricted to fire-retardant-treated only, or banned entirely regardless of treatment. Check the adopted code and the AHJ first, because the answer can kill the job before any detailing starts.

Fire-retardant treatment for cedar is pressure-impregnated, where the chemicals are forced into the wood under pressure rather than brushed on, and it is the basis for a fire-rated cedar product such as the CSSB Certi-Guard line. The roof carries a fire class from the assembly test, Class A being the most resistant, then Class B, then Class C, rated under test methods like ASTM E108 or UL 790. The reality the brochures soften is that fire-retardant-treated cedar generally reaches Class B or C, and getting a cedar assembly to a full Class A usually takes more than the treated wood alone.

Two things make this harder than it looks. The treatment is not permanent, and it weathers down under sun and rain over the years, so a roof that tested at a class when new can drift below it unless it is re-treated. And many high-hazard zones have moved past treated-only to a flat prohibition on wood roofs, treated or not, because the firebrand risk is too high. Do not assume a class or a treatment satisfies the code. Confirm the required fire class, whether wood is allowed at all, and the re-treatment expectation against the adopted code, the AHJ, the CSSB, and the manufacturer before you commit.

The WUI and the local code

The wildland-urban interface, the WUI, is where development meets wildland fuel, and it is where wood roofs run into the hardest rules. In WUI and high fire-hazard zones the code commonly requires a Class A roof assembly, and in many of those zones wood shakes and shingles are restricted to treated products or prohibited outright, treated included. This is a check-before-you-bid item, not a detail you sort out during install.

California is the sharp example, where statewide WUI provisions and very-high fire-hazard severity zones have pushed many areas to ban wood roof coverings regardless of fire-retardant treatment, and recent wildfire rebuilds have tightened that further. Other states and counties adopt and amend their own versions, so the rule that applies on your job is the locally adopted one, not a national default. Two towns a few miles apart can land on opposite sides of whether cedar is even legal.

Treat the fire and WUI question as the first call you make. Pull the adopted building and fire code for the address, confirm the fire-hazard zone, and confirm with the AHJ whether wood is allowed, allowed only treated, or banned, and what fire class the assembly has to meet. Get that in writing before the proposal goes out. A cedar roof bid and built where the code prohibits it is a roof that fails inspection or has to come back off, and the code and the AHJ are the authority on every word of it.

Why does a wood roof need to breathe?

A wood roof needs to breathe because cedar has to dry from both faces, and trapped moisture under the wood is the number-one cause of a cedar roof rotting out early. The top face dries in the sun on its own. The underside only dries if air can move behind the wood, and that is the part the assembly has to provide.

Picture where the water goes. Rain wets the cedar, some of it works into the keyways and the laps, and the wood swells. When the weather clears, the top dries fast, but the underside sits against the deck where there is no sun and little air. Build that underside against a solid, sealed surface with no ventilation, and the wood stays damp on the bottom, the rot starts there, and the fasteners corrode in the standing moisture. The roof can look fine from the street while the underside is going soft.

So the build has to let the underside breathe. The traditional way is spaced sheathing, open boards with air gaps behind the wood. The other way is a solid deck with a ventilation layer, a battened or matted air space, between the deck and the cedar so the underside still gets airflow. Either approach exists for the same reason: give the wood a way to dry from below. Skip it, lay cedar tight on a sealed deck, and you have built a roof that cannot dry and will rot from the bottom up. Confirm the breathing assembly against the CSSB and the manufacturer, because this is the install rule the whole system depends on.

Spaced sheathing versus a solid deck

Spaced sheathing, also called skip sheathing or open lath, is the classic cedar substrate: dimension lumber boards, commonly 1x4 or 1x6, run across the rafters with gaps between them so air reaches the underside of the wood. The boards are spaced so their on-center spacing matches the weather exposure of the shakes or shingles, which puts a nailing board exactly where each course needs to be fastened. The gaps are the breathing.

The boards do two jobs at once. They give the fastener solid wood to bite, and the open gaps between them let the underside of the cedar dry after every rain. That is why a spaced-sheathing cedar roof can shed water and still dry out, where the same cedar laid tight on a sealed deck would stay damp underneath and rot. Confirm the board size and the spacing against the exposure and the CSSB, because the spacing is set by the course exposure, not by habit.

A solid deck is the alternative, and it is common where the code or the structure wants continuous sheathing, but a solid deck under cedar needs help to breathe. The fix is a ventilation layer between the deck and the wood: a battened space on furring strips, or a three-dimensional breather mat that holds an air gap behind the cedar. The deck below that, its type, thickness, and the moisture problem with new sheathing, is its own subject covered in the roof deck guide. The point here is that cedar on a solid deck without a ventilation layer is cedar that cannot dry, and that is a rot problem you are building in on purpose.

The underlayment under a cedar roof

Underlayment on a cedar roof is the secondary water barrier over the deck or sheathing, the same backup role it plays under any covering, but the cedar version has a twist: it cannot trap vapor. The wood has to dry downward into a ventilated assembly, so the underlayment package is chosen to back up the water without sealing the underside of the cedar into a wet pocket. The felt and synthetic types, their weights, and their laps are covered in the underlayment guide; read it alongside this for the material detail.

Asphalt-saturated felt is the traditional underlayment here, commonly a No. 30 felt to standards like ASTM D226 or ASTM D4869, and at the eaves and in cold climates a self-adhered ice-and-water membrane goes in for the backup where water stands and ice dams form. The eave membrane detail is the same one spelled out in the shingle and underlayment guides, and the cedar roof uses it for the same reason.

The cedar-specific warning is the vapor trap. Run a fully impermeable self-adhered membrane across the whole deck under cedar, with no ventilation layer above it, and you have sealed the bottom of the wood against a surface it cannot dry into, which fights the breathing the whole roof depends on. That is why the full-deck self-adhered approach belongs with a ventilated air space, not laid tight against the cedar. Confirm the underlayment, the eave membrane, and the assembly against the CSSB, the manufacturer, and the adopted code, and keep the breathing path in mind when you pick it.

The shake interlayment: the felt baffle between courses

Interlayment is the felt strip laid between the courses of a shake roof, and it is the detail that separates a shake install from a shingle install. The CSSB recommends an 18 in wide strip of No. 30 felt, to a standard like ASTM D226 Type II or ASTM D4869 Type IV, laid over the top portion of each course of shakes and extending up onto the sheathing above. It is a baffle, not a continuous underlayment.

It exists because shakes are split and irregular. The rough, uneven shake does not lay tight to the course below the way a smooth sawn shingle does, so wind-driven rain can drive up and under the gaps. The interlayment strip sits in the path of that water, over the top of each shake course, and blocks it from reaching the deck through the irregular laps. Position the bottom edge of each strip above the butt of the shake at a distance equal to twice the weather exposure, so the felt lands in the right spot to baffle the water without showing in the keyways below.

Shingles do not use interlayment. They are sawn smooth and lay tight, so they shed by their own overlap and the felt strip between courses is a shake-only detail. Putting interlayment in a shingle roof is wasted material and can interfere with the lay. Get the felt weight, the strip width, and the position right against the CSSB and the manufacturer, because a shake roof without its interlayment leaks under wind-driven rain, and a shingle roof does not need it at all.

What fasteners are used on a cedar roof?

Use stainless steel or hot-dip galvanized nails on cedar, and nothing else. Cedar is full of natural tannins and oils, and those tannins corrode the wrong metal fast, eating an electro-galvanized or bright nail down until it loses its hold, and bleeding black stains down the roof where the corroding nail meets the wood. This is the number-one fastener rule on a cedar roof, and it is not a place to save money.

The right fasteners are stainless steel, Type 304 or Type 316, or hot-dip galvanized to a standard like ASTM A153, commonly ring-shank for holding power. Within about 15 miles of salt water, the call tightens to Type 316 stainless only, because the salt air will get past the others. Electro-galvanized and non-galvanized nails are explicitly out, because their coating is too thin to survive the tannins, and a roof nailed with them is a roof that stains and works loose long before the wood is done.

Placement and drive matter as much as the metal. The standard is two nails per shake or shingle, set roughly 3/4 in to 1 in in from each edge and about 1 in to 2 in above the butt line of the course that will lap over them, so the next course hides the heads and each fastener stays out of the weather. Drive the nail flush, snug to the surface, not over-driven, because an over-driven head crushes into the soft cedar, splits it, or sinks below the surface and loses its grip. The fastener has to be long enough to penetrate the sheathing, commonly at least 3/4 in into the wood. Confirm the type, length, and placement against the CSSB and the manufacturer for the product and the exposure.

The keyway, the gaps, and the offset

The keyway is the gap left between adjacent shakes or shingles in a course, and it is there because wood swells. Cedar takes on water in the rain and expands across its width, and if there is no room between the pieces, the swelling wood has nowhere to go but up, and the roof buckles, cups, and fishmouths. The keyway is the expansion joint, and getting it too tight is a classic install failure.

Leave the gap. For shakes the common keyway is 3/8 in to 5/8 in, and for shingles it is often 1/4 in to 3/8 in, set so the wet wood can expand without lifting its neighbor. Too tight and the roof buckles when it rains. Too wide and you have opened a gap that leans harder on the course below and the interlayment to keep water out. Confirm the keyway width against the CSSB and the manufacturer for the product you are running.

The offset is the other half of the rule, and it is about where the gaps land course to course. The keyways must not line up between courses, because aligned gaps make a straight channel for water down to the deck. The standard is to offset the joints by at least 1-1/2 in from the course below, keep the keyways in any two adjacent courses from lining up, and avoid having joints in any three consecutive courses fall in a single line. A roof with stacked keyways leaks straight through, no matter how good the wood is. Stagger every course and check it as you go up.

Exposure: how much of each course shows

Exposure is how much of each shake or shingle is left to the weather after the next course laps over it, and it sets both the coverage and the overlap. Run it long to stretch the material and you thin the overlap until water reaches the fasteners and the deck. Run it short and you waste wood and pile up more courses than the roof needs. The right exposure depends on the length of the piece and the slope of the roof.

The published figures give the shape of it, and the manufacturer and CSSB control the exact number. On a roof of 4:12 and steeper, common shingle exposures run about 5 in for a 16 in shingle, 5-1/2 in for an 18 in shingle, and 7-1/2 in for a 24 in shingle, while a 24 in shake commonly runs around a 10 in exposure. Drop to a reduced slope between 3:12 and 4:12 and the exposures shorten, because the flatter roof sheds slower and needs more overlap to stay ahead of the water.

The reason exposure ties to slope is the same shed-not-seal logic the shingle guide spells out. A steeper roof moves water off faster, so it tolerates more exposure and less overlap, and a flatter roof needs the courses closer together so the overlap does more work. Match the exposure to the piece length and the slope from the manufacturer's table, set a chalk line for each course, and hold it, because exposure that wanders leaves the roof with uneven coverage and laps that do not protect the fasteners the way the design assumed.

The minimum slope for a cedar roof

Cedar sheds water by overlap, so it needs slope, and the common minimum for a shake or shingle roof is 4:12, with reduced exposures allowing application down to about 3:12. Below that the roof sheds too slowly for a wood covering and the water has time to back up under the courses. The exact floor and the reduced-slope exposures come from the CSSB and the manufacturer, with the adopted code on top.

Steeper sheds better, and on a cedar roof that is also a durability point, not just a leak point. A steep cedar roof drains and dries fast, which keeps the wood from staying wet, holds back the moss, and stretches the life of the roof. A low cedar roof near the minimum stays wetter longer after every rain, so it rots and grows moss sooner, which is why the shaded, low-slope sections are the first to fail.

Where the slope drops below the cedar minimum, you do not push wood past where it works. That is membrane territory, a different system covered in the low-slope and underlayment guides, built to hold water rather than shed it. Confirm the minimum slope and the reduced-slope detailing against the CSSB, the manufacturer, and the adopted code before you commit a wood roof to a marginal pitch.

Valleys: open metal, lined and wide

Valleys on a cedar roof are open metal, lined, and wide, because wood does not lap a valley. Where two planes meet, the combined runoff of both slopes concentrates in the valley, and you carry that water on a metal channel down the center, not on cut cedar edges. The shakes or shingles from each plane are trimmed back to a line on each side and the water runs on the metal between them.

Line the valley first, then set the metal. A self-adhered membrane or felt goes down the full length of the valley before the metal, as the backup under the channel. The valley metal is corrosion-resistant sheet, commonly a painted or coated galvanized steel, aluminum, or copper, run in lengths that lap downslope so water cannot drive under a seam. The metal is wide, commonly on the order of 20 in or more in total width, and many cedar valleys use a W-profile with a center crimp that keeps water from washing across to the other plane.

Keep the cedar out of the water. Trim the courses back to the valley line with a clean edge, hold them off the center of the channel so debris and ice do not dam against the wood, and do not run shakes or shingles across the valley. The open metal valley is the durable detail because the water runs on metal, sheds debris, and handles the ice that a closed valley would trap against the wood. Confirm the metal type, the width, and the trim line against the CSSB and the manufacturer.

Hips and ridges

The hips and the ridge are capped to close the top of the roof where the planes meet, and on a cedar roof the cap is either a factory-assembled hip-and-ridge unit or a site-built woven cap. Both cover the cut top ends of the field courses from each side so wind and water cannot get into the gap at the peak, and both are laid with the exposure matched to the field of the roof.

A site-built cap, sometimes called a Boston ridge, alternates the cedar pieces over the peak so the laps shed the prevailing weather, with each piece fastened down through the cap into solid wood. Factory cap units come pre-made to speed the work and keep the exposure even. Either way, the nails have to be long enough to reach through the cap and the build-up below into the sheathing, commonly an 8d or longer nail, and they have to be the same stainless or hot-dip fasteners the rest of the roof uses, because the ridge sees the same tannins and the same weather.

Lay the caps so the laps run away from the prevailing wind, set the exposure to match the field so the roof reads even from the peak to the eave, and keep the fastener heads covered by the next cap. The ridge is where uplift is highest, so a cap that is short-nailed or fastened with the wrong metal is the first thing to leave in a wind. Confirm the cap detail, the exposure, and the fastener length against the CSSB and the manufacturer.

The starter course and the eave

The first course at the eave is doubled, sometimes tripled, because the bottom edge of the roof has no course below it to back up the keyways. The starter layer under the first visible course covers the gaps so water cannot run straight through the joints to the deck at the most exposed edge of the roof. Skip the doubled starter and the eave leaks through the first keyway it finds.

Set the eave to throw water clear. The cedar overhangs the fascia or the drip edge by a small reveal, commonly around 1 in to 1-1/2 in, so the water drips off the wood into the gutter instead of wicking back onto the fascia and rotting it. Many cedar eaves use a cant strip or a doubled starter to kick the first course up to the right angle so it lays at the same plane as the courses above. The drip edge and the eave membrane detail are the same ones the shingle and underlayment guides cover, and the cedar roof uses them for the same backup.

Keep the starter keyways offset from the first visible course, the same offset rule that governs the field, so the doubled layer actually backs up the gaps instead of lining up with them. Confirm the eave overhang, the starter, and the cant detail against the CSSB and the manufacturer, because the eave is where wind uplift and standing water both concentrate, and a weak starter there shows up as a wet fascia and a rotted deck edge.

Why do wood roofs fail?

Wood roofs fail in a short list of ways, and almost all of them trace back to the wood not being able to breathe, move, or hold its fasteners. Knowing the failure modes tells you what to look for on an inspection and what to get right on the install.

Rot is the big one, and it comes from trapped moisture. A cedar roof that cannot dry from the underside, laid tight on a sealed deck with no ventilation, stays damp on the bottom and rots from there, often invisibly until the deck goes soft. Moss and algae are the shade-and-moisture failure, worst on north slopes and under overhanging trees, where the roof stays wet, the moss holds more water against the wood, and the rot accelerates. Cupping and splitting are the weathering failure, where sun and wet-dry cycling curl and check the wood, and a too-tight keyway makes it worse by giving the swelling wood nowhere to go but up into a buckle.

The fasteners fail their own way. Electro-galvanized or bright nails corrode in the tannins, bleed black stains, and lose their grip, so the wood works loose and lifts in the wind. A keyway set too tight buckles the roof when the wood swells, and aligned keyways with no offset leak straight to the deck. None of these is exotic. They are the predictable result of cheating the breathing, the keyway, or the fastener, which is why those three are where the install has to be right.

Maintenance over the life of the roof

A cedar roof is a maintained roof, and the owner who treats it like an asphalt roof and ignores it gets a short life out of it. The maintenance is not heavy, but it is regular, and most of it comes down to keeping the wood dry and clear so it can do the breathing the install set it up for.

Keep the roof clean and dry. Blow or sweep the debris out of the keyways and the valleys, because trapped leaves and needles hold water against the wood and start the rot. Pull the moss and algae off the shaded slopes before they mat up and hold moisture, and trim back the overhanging branches that keep the roof in the wet shade. Replace the split, cracked, and cupped shakes as they show up, one at a time, before the gaps they leave turn into a leak.

Then there is the treatment cycle, which is where a cedar roof differs from anything else. The preservative that slows the rot and the fire-retardant that holds the fire class both weather down over the years, so a roof that started treated needs the preservative and, where the code requires a fire class, the fire-retardant re-applied on a cycle to keep doing their job. A cedar roof in a fire zone with a treatment that has weathered off is a roof that may no longer meet the class it was approved at. Confirm the re-treatment interval and the products against the manufacturer, the CSSB, and the AHJ, because the fire side of that is a code matter, not just upkeep.

Lifespan and re-treatment

A cedar roof commonly lasts 30 to 40 years or more when it is built to breathe, fastened with the right metal, and maintained, and a lot less when it is not. The number is not a property of the wood alone. It is a property of the assembly, the slope, the exposure, and whether anyone kept it clean and re-treated.

Slope and sun drive the spread. A steep, sunny cedar roof that drains and dries fast lives toward the long end, while a low, shaded, tree-covered roof that stays wet rots and grows moss and lands well short of it. The breathing assembly is the foundation of the long number: a roof that can dry from both faces outlasts one that cannot, every time, regardless of the grade of the wood on top.

The treatment is what holds the number where the conditions or the code demand it. Preservative slows the decay, fire-retardant holds the fire class, and both weather off, so the realistic plan for a treated roof includes re-applying them on a cycle rather than treating once and forgetting it. Set the expectation with the owner up front: a cedar roof is a long-lived roof only if it is maintained and re-treated, and the interval comes from the manufacturer, the CSSB, and, on the fire side, the AHJ.

Cost, value, and insurance

A cedar roof costs more than asphalt, in both material and labor, and the case for it is the look and the longevity, not the price. The thick split shake and the clean sawn shingle give a roof nothing manufactured matches, which is why they hold their place on historic homes, lodges, and high-end work where the appearance carries the cost. The premium is real, and so is the upkeep that comes with it.

The value side is honest only if the owner knows the whole picture. A maintained cedar roof lasts decades and looks the part the whole time, but it asks for the cleaning and the re-treatment that an asphalt roof does not, and an owner who will not do that maintenance is buying a roof that will underperform its price. Sell the look and the life, but sell the maintenance with it.

The one that surprises people is insurance. Some insurers decline wood roofs outright, and others surcharge them or require a fire-retardant class, especially in or near fire-prone areas, because the wildfire risk a wood roof carries is exactly the risk they are pricing. Check the insurability before the owner commits, the same way you check the fire and WUI code, because a roof that is legal to build but hard to insure is still a problem the owner did not see coming.

What to document

A cedar roof's most important details disappear the day the courses cover them, so the record is the only proof of how it was built. When a question comes up years out, whether it is a leak, a warranty claim, or a fire inspection, the answer turns on the grade, the fire treatment, the assembly, and the fasteners, and the only evidence is what was written down while the roof was open.

Capture the species and grade off the bundle label, the fire-retardant treatment and its class with the documentation, the breathing assembly used, the underlayment and the interlayment, the fastener type and length, the keyway and offset, the exposure, and the valley, hip, and ridge details. Photograph the assembly, the eaves, and the valleys before the cedar covers them. The fire treatment record matters most in a fire zone, because that is the document the AHJ and the insurer will ask for, and a roof with no record of its class has no defense.

Common mistakes

• Installing a wood roof where the WUI or fire code prohibits it, or leaving off the required fire-retardant treatment and fire class.
• Laying cedar tight on a solid, sealed deck with no ventilation, so the wood cannot dry from below and rots from the underside.
• Using electro-galvanized or bright fasteners, which the cedar tannins corrode, bleeding black stains and working the wood loose.
• Setting the keyways too tight so the swelling wet wood buckles and cups, or aligning the keyways course to course so water runs straight to the deck.
• Leaving the interlayment felt out of a shake roof, so wind-driven water gets under the irregular split shakes.
• Over-driving the nails so the heads crush into the soft cedar and lose their hold, or short-nailing the hip and ridge caps.
• Running the exposure long to stretch the material, which thins the overlap and exposes the fasteners and the deck.
• Skipping the doubled starter at the eave or the open metal valley, so the most exposed details leak first.
• Assuming a fire class or a treatment is permanent and never re-treating, so the roof drifts below its approved class.

Field checklist

Standards and references

The Cedar Shake and Shingle Bureau, the CSSB, is the trade authority for cedar roofing, and its grade rules and installation manuals govern the grades, the exposures, the keyways, the interlayment, and the fasteners. The CSSB Certi-label grades the wood, and lines like Certi-Guard for fire-retardant treatment and Certi-Last for preservative cover the treated products. Confirm the grade, the exposure, the assembly, and the treatment against the CSSB and the specific product literature, because the printed manual and the manufacturer's instructions are the authority and they can be stricter than any figure here.

The fire side belongs to the code and the test standards. A roof's fire class, Class A, B, or C, comes from assembly testing under ASTM E108 or UL 790, and fire-retardant-treated cedar generally reaches Class B or C rather than a standalone Class A. The wildland-urban-interface provisions in the adopted building and fire code, the local fire-hazard severity zone, and the AHJ decide whether wood is allowed at all, allowed only treated, or banned. That is the first thing to confirm on any cedar job, and it is the code and the AHJ that control it, not a rule of thumb.

The rest sits with the code and the manufacturer. The International Residential Code and International Building Code carry the enforceable wood-shake-and-shingle provisions, the slope and underlayment requirements, the fastener corrosion-resistance and penetration rules, and the fire-classification requirements, and the felt standards ASTM D226 and ASTM D4869 cover the underlayment and interlayment. The code is adopted and amended by jurisdiction, so confirm every requirement against the adopted edition and the local amendments, and let the CSSB, the manufacturer, the code, and the AHJ override anything general written here, especially on fire, the breathing assembly, the fasteners, and the keyways.

Units, terms, and conversions

Cedar roofing carries its own vocabulary, and the same idea reads differently across a CSSB manual, a manufacturer sheet, and a code section, so the terms below are the ones that cause confusion on the job.

Coverage is measured in squares, where one square is 100 sq ft of finished roof. Slope is rise over run in inches per foot, written like 4:12. Exposure is the inches of each shake or shingle left to the weather, set by the piece length and the slope. Shakes are sold by length, commonly 18 in and 24 in, and shingles by length as well, commonly 16 in, 18 in, and 24 in. Fastener metals are called out by type, Type 304 and Type 316 stainless, and hot-dip galvanized to ASTM A153.

Wood shake vs wood shingle

A shake is split, so it is thick, rough, and irregular; a shingle is sawn on both faces, so it is thinner, smooth, and uniform

Western red cedar / grade

The standard cedar species; CSSB grades it No. 1 (Blue Label, clear heartwood, edge-grain), No. 2, and No. 3

Fire-retardant treatment / WUI

Pressure-impregnated chemicals that raise the fire class; the wildland-urban interface where wood roofs are often restricted or banned

Spaced sheathing / breathing assembly

Open 1x boards or a ventilated air space that lets the underside of the cedar dry, so the wood does not rot

Interlayment

The 18 in No. 30 felt strip laid over each course of shakes as a baffle against wind-driven water; shakes only, not shingles

Keyway / offset

The gap between adjacent pieces for the wood to swell, and the side-to-side stagger of those gaps course to course so they do not align

Exposure

The portion of each shake or shingle left to the weather, set by the piece length and the slope per the manufacturer

Stainless / hot-dip fastener

Type 304 or 316 stainless, or hot-dip galvanized nails, the only metals that resist cedar tannin corrosion; never electro-galvanized

Related tools

Calculators and readiness checks for this work

Compare your options

FAQ

People also ask

• What is the minimum slope for asphalt shingles?
• What is roofing underlayment?
• What is a ballasted roof?
• What is a built-up roof?
• How much does a tile roof weigh?