What Is Roof Sheathing?

What roof sheathing is

A sloped roof is built from the bottom up. First come the rafters or trusses, the sloped wood beams that give the roof its shape. Next comes the sheathing, a skin of wood nailed across those beams. On top of the sheathing go the underlayment and then the roof covering, such as shingles or metal.

The sheathing does two jobs. It spreads the weight of snow, roofers and roofing across the beams. It also gives the shingle nails something solid to bite into. The code says asphalt shingles must be fastened to solidly sheathed decks ². Without a sound deck, the shingles cannot be held down.

Roofers, insurers and supply houses use several names. Roof deck, decking and sheathing all mean the same layer. The code uses the word sheathing in Section R803 ¹. Quotes often say decking or deck boards. When a roofer talks about replacing sheets or boards, this is the layer they mean.

How it works

Think of sheathing as a bridge. Each panel or board spans from one rafter to the next. Weight on the roof bends the panel a little between supports. The wider the gap between rafters, the stiffer the sheathing must be. That is why the code ties thickness to rafter spacing ¹.

Panels are laid with their long side running across the rafters. The code table assumes the panels run over two or more spans this way ⁴. Running them across the framing makes the deck stiffer than running them the same way as the rafters.

The panel edges between rafters are a weak point. They can sag or move against the next panel. The code table lets a panel span farther when those edges are supported.

Edge support can be wood blocking, metal panel edge clips, or tongue-and-groove edges that lock together ⁴. Roofers often call the clips H-clips because of their shape.

Sheathing also ties the roof frame together. Nailed tight to every rafter, the panels keep the frame from racking, which means leaning out of square. That is one reason the code sets nail sizes and spacing for the deck, not just for the shingles.

Types of roof sheathing

Three kinds of decking show up on homes. The code covers all three ¹.

The code says plywood and OSB must carry a grade mark from an approved agency showing the grade, bond class and performance ¹. That stamp is how an inspector or roofer tells what a panel is.

Sources: [1]

Plywood vs OSB

Plywood and OSB are both wood structural panels in the code. Both can be used for roof sheathing if they carry the right grade mark and span rating ¹. A panel with a given span rating is built to carry the load that rating stands for, whether it is plywood or OSB.

The difference shows up with water. The roofing contractors' trade group, NRCA, says it prefers plywood. Its technical article says OSB panels are subject to changes in size, ridging and nail back-out when moisture changes ⁶. Ridging is a raised line in the shingles along the panel edges.

OSB is widely used because it is made in large sheets from smaller trees. In practice, the choice often comes down to what is on the truck.

What matters more is that the panel is the right thickness, is kept dry, is gapped, and is nailed right. If your quote names OSB, ask the roofer what thickness and span rating it is.

Plank decking and gaps

Older homes often have solid boards for a deck. Lumber sheathing is allowed. The code requires boards at least 5/8 inch net thickness when the rafters are 24 inches apart, and thicker tongue-and-groove planks for wider spacing ¹.

Boards were sometimes laid with gaps, either on purpose for wood shingles or because they shrank over the years. The code allows spaced boards under wood shingles and shakes.

Those boards must be at least 1 by 4 inch nominal and spaced at the same distance as the shingle exposure ².

In places where the average January temperature is 25°F or less, the part of the roof that needs an ice barrier must be solid sheathing ².

Asphalt shingles are different. They need a solid deck ². Wide gaps leave the shingles without support and give the nails nothing to hold.

When a roof that once had wood shakes is redone in asphalt, the roofer usually has to fill the gaps or lay new panels over the old boards.

How wide a gap is too wide depends on the shingle maker's own instructions, so ask the roofer which rule they follow.

The code also bars spaced lumber sheathing for wood shingles in Seismic Design Category D2, which covers some high-earthquake areas ¹.

Worked example: reading a panel stamp

Say the trusses on your house are 24 inches apart and the stamp on a sheet in the attic reads 24/16, 7/16 inch. Here is how that lines up with the code table ⁴.

Now take a 24/0 panel, which is 3/8 inch thick. The table lets it span 24 inches only with edge support, and 20 inches without it ⁴.

On 24-inch trusses with no clips, that panel would be outside the table. A stamp you can read from the attic is a quick way to learn what your deck is made of.

Where it goes on the roof

Sheathing covers every rafter bay from the eave to the ridge. At the ridge, a gap is often left open so a ridge vent can let attic air out.

At the eaves, the deck edge is where the drip edge goes. The code requires drip edges to extend at least 1/4 inch below the sheathing ².

At gable ends, the sheathing can stick out past the end wall to form part of the overhang. The code limits that cantilever to 9 inches unless overhang framing supports it ¹.

The underside of the deck faces the attic. The code allows panels that are only exposed on the underside to be an Exposure 1 grade, which is an interior panel made with exterior glue ¹. Panels that will be out in the weather for good must be Exterior grade.

What a sound deck does for your roof

• Gives shingle nails a solid base so the roofing stays down in wind ²
• Spreads snow and foot traffic across the rafters
• Stiffens the roof frame so it stays square
• Gives a flat surface so the shingles lie flat and seal
• Makes the roof safer to walk on for the crew doing the work
• Lets the next roof go on without extra repairs if it stays dry

Limitations

• Wood swells and shrinks with moisture. Tight panels can buckle, which APA says is a looks or service issue, not a structural one ⁵.
• Sheathing is not waterproof. It needs underlayment and roofing over it.
• Thin panels need edge support to reach their full span ⁴.
• Damage is often hidden under the shingles until tear-off.
• Old board decks may not meet a new shingle maker's flatness or gap rules.

What drives the cost of deck work

We do not list prices. Deck repair is one of the biggest unknowns on a reroof, because no one can see the deck until the old roof is off. These are the things that move the number. For a full estimate, try our roof cost calculator.

• How much is bad. Most quotes price deck replacement per sheet (a 4 by 8 foot panel) or per foot of board, so the total depends on how many are replaced.
• Panel type and thickness. Thicker panels and plywood are usually priced above thinner OSB.
• Plank decks. Filling gaps or laying a full new layer of panels over old boards adds material and labor across the whole roof.
• Roof size. Measured in roofing squares; a full redeck scales with area.
• Pitch and access. Steep roofs and hard-to-reach areas take more time per sheet. See our roof pitch guide.
• Rot below the deck. If rafters or fascia are also damaged, that is a separate repair.

How deck repair fits into a reroof

A licensed roofer does this work. These are the steps you can expect and check.

• The old roofing comes off. The code says a roof replacement removes the old layers down to the roof deck ³.
• The roofer walks and looks at the bare deck for soft, rotten, broken or delaminated spots.
• Bad sheets or boards are cut out back to the rafters and replaced with sheathing of matching thickness.
• New panels are laid long side across the rafters, with about a 1/8-inch gap at edges and ends as APA recommends ⁵.
• Panels are nailed to the rafters. NRCA lists 8d common nails 6 inches apart at panel edges and 12 inches apart at middle supports as a minimum for typical panels ⁶.
• Underlayment goes on soon after, to keep the deck dry ⁷. Then the roofing.

Keeping the deck healthy

You do not service sheathing directly. You keep it dry. Water reaches the deck from two sides: leaks from above and moist attic air from below.

From above, fix missing shingles and failed flashing quickly. A small leak that runs for a season can rot a sheet. From below, the attic needs airflow.

The code requires at least a 1-inch space between insulation and the roof sheathing at the vents, and says insulation must not block the flow of air ⁸.

Look at the deck from the attic once a year with a flashlight, from the floor, without walking on the ceiling. Dark stains, frost on the nails in winter, or a musty smell are worth a call to a roofer.

Signs of a bad roof deck

• A sag or dip in the roof line seen from the street
• Water stains, dark spots or mold on the underside of the deck in the attic
• Light coming through from the attic where there should be none
• Soft or spongy spots a roofer feels when walking the roof
• Shingles with raised lines along panel edges, which can point to ridging or buckling ⁶
• Nails backing out and pushing up through shingles

These are signs, not proof. A roof inspection can find the cause. Do not walk on the roof yourself to check.

What the code requires

These rules come from the 2021 International Residential Code. Your state or city may use a different edition or change parts of it, so check with your building department.

Sources: [1] [2] [4]

Sheathing vs related roof layers

Common misconceptions

Myth A new roof always comes with a new deck.

Reality Most reroofs only replace the sheets that are bad. A full redeck is a separate choice or need.

Myth OSB is not allowed on roofs.

Reality The code allows OSB that meets DOC PS 2 and has the right span rating ¹. NRCA prefers plywood, but that is a preference ⁶.

Myth Thicker is the only thing that matters.

Reality The span rating, edge support, gaps and nailing all matter. The code table is built on span ratings ⁴.

Myth Board decks must be torn out.

Reality Lumber sheathing is allowed ¹. Board decks with wide gaps may need filling or an overlay for asphalt shingles.

Myth Buckled panels mean the roof is failing.

Reality APA says panel buckling is a looks or service issue, not a structural flaw ⁵. A roofer should still find the cause.

Local rules and climate

The rules above come from the model code. Each state or city adopts its own edition and may change it. Your local building department decides which rules apply to your address and whether a reroof permit includes a deck check.

Climate changes the details. Heavy snow areas may need thicker panels or closer framing, since the table assumes set loads ⁴.

Cold areas that require an ice barrier need solid sheathing under it, even on wood shake roofs ². High-wind areas often have tighter deck nailing rules set locally. Ask your roofer or building department which apply.

What to ask your roofer

• What is the price per sheet (or per foot of board) for deck replacement, and is any included in the quote?
• Will you photograph the bad deck before you replace it?
• What thickness and span rating will you use, and will it match what is there?
• Do we have board decking, and does it need filling or an overlay for shingles?
• Will you renail the existing deck to current rules where needed?
• Do our nails reach 3/4 inch into the deck or go fully through?

Keep going

• Roof replacementDeck repairs happen at tear-off
• How a roofing project worksWhere the deck check fits in the job
• Roofing cost guideWhat moves the price of a new roof
• Choosing a rooferCompare quotes line by line, including deck extras

Related terms

Roof Underlayment

Roof underlayment is a sheet laid over the whole roof deck before the shingles or other roof covering go on.

Roof Rafter

A sloped framing member that runs from the ridge to the wall and supports the roof deck. Rafters are cut and assembled on site, unlike trusses.

Roof Truss

A roof truss is a frame of wood pieces joined into a set of triangles, usually in a factory, that holds up a roof.

Roofing Nails

Roofing nails are short nails with a wide, flat head that hold shingles to the roof deck.

Roof Rot

Decay in the wooden deck, fascia or framing after long exposure to moisture. Rotten deck must be replaced before new roofing goes on.

Roof Sagging

A visible dip in a roof line or plane. It can mean a wet or failed deck or a structural problem and needs a professional look.

Reroofing

Reroofing is the work of putting a new roof covering on a house that already has one, either by tearing the old covering off down to the deck (a replacement) or by laying the new covering over the old one (a recover, often called an overlay).

Roof Ventilation

Roof ventilation is the system of vents that lets outside air enter an attic low, near the eaves, and leave high, near the ridge, so heat and moisture do not build up under the roof deck.

Also referenced from: Roofing System, Low-Slope Roof, Slate Roofing, Clay Tile Roofing, Wood Shakes, Roof Truss, Roof Underlayment, Ice and Water Shield, Drip Edge, Roofing Nails, Ridge Vent, Soffit Vent, Roof Wind Uplift, Hurricane Roofing, Reroofing, Roofing Square, Roof Estimate