Most homeowners never see their roof sheathing. It sits under the shingles, gets covered the day the roof goes on, and only comes back into view when a roofer strips the old roof off. Yet the roof sheathing is what every shingle is nailed into and what decides whether a new roof lies flat or shows every dip in the framing. If you are planning a re-roof this autumn, knowing what is under your shingles will help you read a quote and avoid surprises on tear-off day.
This guide covers what the deck does, how plywood and OSB compare, common thicknesses, what a roofer finds once the old shingles are gone, and how replacement is priced. Every check described here is done by a professional roofer. Your part is noticing signs inside the house and asking the right questions.
What Roof Sheathing Actually Does Under Your Shingles
Roof sheathing, often called the roof deck, is the layer of structural panels or boards fastened across the rafters or trusses. Asphalt shingles have almost no strength of their own. They rely on a solid, continuous surface to hold their nails, stay flat and carry snow, wind pressure and the crew that installs them.
- Nail holding. When the deck is soft or crumbly, nails back out, shingles lift in wind, and the shingle manufacturer’s installation requirements are no longer met.
- Load spreading. Panels spread snow load and foot traffic across the framing instead of letting it press down between rafters.
- Stiffening the structure. Fastened panels tie the framing together so the roof acts as one assembly against wind.
- A base for waterproofing. Underlayment, ice and water membrane at the eaves and valleys, and metal flashings all need a clean, flat, dry surface.
When a roof looks wavy, the cause is very often the deck, not the shingles. New shingles take the shape of whatever is underneath, so a re-roof is the one practical chance to correct a tired deck before it is covered for another couple of decades. That is why a careful roof replacement that includes a full deck assessment treats the sheathing as part of the job, not an afterthought.
Board Sheathing, Plywood and OSB: What Is Under GTA Roofs
The age of your house usually predicts what the roofer will find. Many older Toronto-area homes have board sheathing: solid lumber boards nailed across the rafters, sometimes with gaps that once suited wood shingles. Asphalt shingles need a continuous surface, so spaced boards are normally covered with new panels during a re-roof or removed and replaced. Homes from roughly the post-war decades into the 1980s often have plywood, while oriented strand board (OSB) became more common in later construction.
Plywood is made of thin wood veneers glued with the grain of each layer at right angles to the next, which makes it stiff and dimensionally stable. It absorbs water fairly quickly but also dries quickly and usually recovers its thickness. Its weak point is repeated wetting over years, which weakens the glue lines until the plies separate, called delamination.
OSB is made from cross-oriented layers of wood strands pressed with resin and wax. It is uniform, has no hidden voids and generally costs less. Water penetrates it slowly, but once soaked it dries slowly, and the edges are the weak spot: soaked OSB edges can swell permanently and show up as faint straight ridges across a finished shingle roof.
Plywood vs OSB Roof Sheathing: An Honest Comparison
Both products are accepted as structural roof sheathing when they carry the proper grade stamp and span rating, and both are acceptable decks for the major shingle manufacturers when installed correctly. The differences are in moisture behaviour and cost.
| Factor | Plywood | OSB |
|---|---|---|
| Short wetting during a job | Absorbs quickly, dries quickly, usually recovers | Absorbs slowly, dries slowly, edges can swell |
| Long-term moisture damage | Plies delaminate and the face veneer blisters | Panel softens, edges crumble and strands loosen |
| Consistency | Can have knots and voids in inner plies | Very uniform, no voids |
| Material cost | Typically higher, and the gap moves with lumber markets | Typically lower |
| Nail holding | Very good when dry and sound | Very good when dry and sound |
| Where it often makes sense | Roofs with past moisture issues or slow-draining sections | Most standard re-roofs with good weather protection during the job |
In practice, installation matters more than the product. A well-ventilated attic, panels kept dry during the job, correct gapping and proper fastening give a long service life with either. Plywood is worth considering when attic condensation is still being corrected or when you want the panel that tolerates an unexpected soaking better. Ask your roofer which panel they plan to use and why. Some manufacturers also make OSB with enhanced edge sealing intended to resist swelling; claims and warranty terms differ, so check the manufacturer’s current documents.
Thickness, Rafter Spacing and Why H-Clips Matter
Roof sheathing thickness depends on how far apart the rafters or trusses are, the snow load the roof carries, and the panel’s span rating printed in its grade stamp. Wider framing spacing needs a stiffer panel. Sizes commonly found on Toronto-area roofs include:
- 3/8 inch panels, mainly on older roofs and a frequent source of sagging between rafters.
- 7/16 inch OSB, very common on newer homes built with trusses.
- 1/2 inch plywood, often labelled 15/32 inch, a frequent re-roof choice.
- 5/8 inch panels, used for wider framing spacing, extra stiffness, or to match thicker surrounding boards.
The minimum acceptable thickness depends on framing spacing, the panel’s rating and current Ontario Building Code requirements, so your roofer should confirm the right product for your framing. If rafters need repair or reinforcement, check with your municipality’s building department about whether a permit is required.
What H-clips do
H-clips, also called panel clips, are small metal clips shaped like the letter H. They slip onto the long edges of two neighbouring panels between rafters, where no framing sits under the joint. They keep the edges level so one sheet cannot flex further than the next, and they set the small expansion gap panel manufacturers call for, commonly around 3 mm. Without that gap, panels butted tight can buckle upward in damp weather and the ridges telegraph through the shingles. Whether clips are needed depends on panel thickness, framing spacing and edge profile, since tongue and groove panels do the same job.
Installation details worth asking about
- Long side of each panel running across the rafters, with end joints staggered and landing on framing.
- Fasteners of the type and spacing set by the manufacturer and code, closer along edges than in the field, and driven into framing rather than missing beside it.
- OSB laid textured face up, as manufacturers direct.
- New panels matched in thickness to the surrounding deck so there is no step for the shingles to bridge.
What a Roofer Finds When the Old Shingles Come Off
No one can fully judge roof sheathing from inside a finished house, and homeowners should never try to check it themselves. A professional roofer can gather clues beforehand from the underside of the deck in the attic and from soft areas noticed while working on the roof, but the full picture only appears at tear-off, when every sheet is exposed.
| What the roofer sees | What it usually means | Common cause | Typical fix |
|---|---|---|---|
| Plies separating, blistered face veneer | Plywood delamination | Repeated wetting from leaks or attic condensation | Replace sheets and correct the moisture source |
| Swollen, fuzzy or crumbling panel edges | OSB edge swell or breakdown | Water entering at joints, wet installation, old leaks | Replace affected sheets |
| Dark staining and wood a probe sinks into | Rot | Long-term leak at flashing, a valley or the eaves | Replace sheets and check the rafters below |
| Spongy feel underfoot | Soft spot or weakened panel | Moisture damage or an undersized panel | Replace, adding clips or blocking where needed |
| Shallow dips between rafters | Sagging panels | Thin panels, missing clips, moisture or long-term loads | Replace with stiffer panels |
| Rusted nails and dark patches on the underside | Attic moisture problem | Poor ventilation and warm air leaking from the house | Replace damaged sheets and fix ventilation |
Where the damage concentrates
Deck damage is rarely even. It gathers where water has been getting past the shingles for years: around chimneys with tired flashing, along valleys, around older skylights, at plumbing vents, and along the eaves where ice dams back water up in winter. Rotted sheathing around a chimney usually means the flashing should be rebuilt as part of the job, which is where proper chimney flashing and repair work comes in. Rot along the lower edge often traces back to recurring ice dam problems, and an older unit that has leaked for years may be better replaced with new skylights while the roof is open. A roofer also checks the sheets beside any damaged one, because water often travels along joints into neighbouring panels.
Sagging and attic moisture
Sagging sheathing creates a gentle scalloped pattern between rafters, usually from panels too thin for the spacing, missing clips, panels that were covered while wet, or long-term attic moisture. A roofer separates a deck problem from a framing problem: if rafters have sagged or cracked, the fix may call for a structural assessment first. Attic moisture is its own culprit. Through the heating season, warm, moist air leaking into the attic condenses on the cold underside of the deck, leaving staining, mould and delaminated plies over several winters. New sheets will go the same way unless attic ventilation and air leakage are fixed too.
Signs You Might Notice From Inside the House
The signs worth noticing show up in everyday life, without anyone in the household going onto the roof or into the attic:
- Water stains on upstairs ceilings, especially near exterior walls or below a chimney or skylight.
- Bubbling paint or soft drywall in top-floor rooms or closets.
- A musty smell upstairs that gets stronger after a few days of autumn rain.
- A roofline that looks wavy or dipped when you happen to see it from the yard or sidewalk.
- More shingle granules washing out at the downspouts than in past years, a sign the shingles are wearing out.
None of these confirms deck damage on its own; they are reasons to have a professional look. A fresh ceiling stain is best handled with a prompt roof leak repair visit, so a small leak does not turn into several rotted sheets.
Why Autumn Rain Matters When the Deck Is Exposed
October is a popular time to re-roof in the GTA, but it brings frequent rain, heavy dew on cooler nights and shorter days. Once the old shingles come off, the roof sheathing is the only thing between the weather and your attic, and bare OSB edges in particular do not like to be soaked. A careful crew plans for it:
- Check the forecast each morning and strip only what can be closed in that day.
- Replace damaged sheets and install eave membrane and underlayment the same day an area is opened.
- Keep tarps on site and secure them over anything left exposed overnight.
- Let a damp deck dry before covering it, since self-adhered membranes need a clean, dry surface and manufacturers set minimum application temperatures.
- Store new panels off the ground and covered.
Shingling over wet sheathing traps moisture, feeding mould and swelling and keeping membranes from bonding. Cooler temperatures also slow the sealing strips on shingles, so on late-season jobs roofers follow the manufacturer’s guidance, which may include hand-sealing. Ask how the crew handles a downpour mid-job.
How Sheathing Replacement Is Quoted on a Re-Roof
Because the deck cannot be fully assessed until tear-off, contractors usually quote roof sheathing one of three ways: a per-sheet price applied to the actual count, an included allowance of a set number of sheets with a rate beyond that, or a full re-deck priced by area when the deck is spaced boards, too thin or widely damaged. A standard sheet is 4 by 8 feet, or 32 square feet. The ranges below are broad estimates for Toronto-area homes and vary with access, pitch, storeys, panel prices and scope; your written quote is what counts.
| Item | How it is usually priced | Typical estimate (varies by house and scope) |
|---|---|---|
| Replacing a damaged OSB sheet | Per 4 by 8 sheet, installed | Often in the range of $75 to $130 per sheet |
| Replacing a damaged plywood sheet | Per 4 by 8 sheet, installed | Often in the range of $90 to $150 per sheet |
| Full re-deck over or in place of old boards | Per square foot, added to the re-roof | Often around $2 to $4.50 per square foot |
| H-clips with new panels | Usually included with the panel price | Small cost, often included |
| Rafter repair found during tear-off | Quoted separately once seen | Varies widely, priced on site |
Overlay or replacement is part of the same decision. Sound, flat, well-fastened boards can be overlaid with new panels fastened through into the rafters, though the added thickness means drip edge, gutters and wall and chimney flashings must be adjusted. Rotted, cupped or loose boards should come off. When damage or sagging is spread across most of a panel deck, a full re-deck usually gives a flatter result than a patchwork of old and new.
Deck questions to settle before signing
- What is the per-sheet price, for which panel and thickness, and are any sheets included?
- Will you photograph damaged sheathing and share the count before the final invoice?
- How are partial sheets charged?
- Will you use H-clips and keep the gaps the manufacturer calls for?
- If you find board sheathing, will you overlay or replace it?
- How will you protect the open deck if it rains?
A per-sheet price in the contract protects both sides: you know the cost in advance, and the roofer can replace what is genuinely needed without stopping to negotiate. Be cautious with a quote that never mentions the deck. Our roofing FAQ answers more common questions about re-roof quotes.
Is plywood or OSB better for roof sheathing?
How do I know if my roof sheathing needs replacing?
Can new shingles go over damaged decking?
Do I need H-clips on my roof?
How much does it cost to replace roof sheathing?
Is it safe to re-roof during autumn rain?
Get Your Roof Sheathing Assessed Before Your Next Re-Roof
A new roof is only as flat and secure as the deck underneath it. Whoever you hire, ask for the per-sheet price in writing before work begins, and ask to be shown any sheets that need replacing and why. If you are planning a re-roof, The Roof Technician can explain how the deck on your house is likely to be assessed and quoted.
Call us today at (416) 826-0040 or request a free estimate.
What sits under the shingles matters on every Toronto and GTA roof.