How to Resheet Over Plank or Spaced Board Decking Before Installing Shingles
Old cedar roofs sit on boards with gaps between them. Asphalt shingles need a solid surface, so the boards get covered with new sheathing.
- Advanced
- 1 to 2 days for a typical roof
- Updated
The short answer
Resheet over plank decking by stripping the old roofing, pulling or hammering flat all old nails, replacing rotten boards, and covering the boards with 1/2 inch plywood or 7/16 inch OSB laid across them. Fasten into the boards and through into the rafters with nails long enough to penetrate the framing, leave a 1/8 inch panel gap, and check that the attic still has intake and exhaust ventilation.
Cedar shake and shingle roofs were traditionally nailed to spaced boards, often 1 by 4 or 1 by 6 lumber with gaps of several inches, sometimes called skip sheathing. The gaps let air move under the cedar so it dried quickly. That worked for cedar. It does not work for asphalt shingles, which need a solid, continuous surface for their nails to hold, for the shingles to lie flat, and for the warranty to apply.
Across the Fraser Valley many older houses, including in Chilliwack, Hope and Agassiz, were built with cedar roofs on plank decks. When a cedar roof is replaced with asphalt shingles or metal, the homeowner has a choice: take the boards off and replace them with new sheathing, or cover them with plywood or OSB. Covering is faster and keeps the existing structure, as long as the boards are sound and the fasteners reach the framing.
This guide covers the resheeting. For replacing individual panels, see how to replace rotten roof sheathing. For a metal roof, which can go on strapping in some cases, see how to strap a roof for metal. For replacing the cedar itself, see how to replace cedar shake, and for why cedar was laid on spaced boards in the first place, see how to install cedar shakes.
Tools
- Circular saw
- Pry bar and cat's paw
- Coil nailer or framing hammer
- Chalk line
- Reciprocating saw
- Hammer
- Speed square
- Level
- Wire brush or scraper
Materials
- Plywood or OSB roof sheathing, 7/16 to 1/2 inch
- H-clips
- Ring-shank nails, 2-1/2 inch or 3 inch, depending on the board thickness
- Replacement 1 by 6 or 1 by 8 boards where needed
- Ice and water shield and underlayment
- Vent products for added ventilation
Safety first
- Spaced boards are not a solid surface. A worker can put a foot between boards and go through, so keep tied off and stay on the boards directly over rafters.
- Old cedar roofs may have moss, rot and loose shakes. Wear sturdy boots, eye protection and a dust mask.
- Older roofs may have asbestos-containing felts or other materials. Have suspect materials tested before disturbing them.
- Plan the loads. Stacks of plywood bundles are heavy. Distribute them over rafters and keep the pile away from the edge.
Step by Step
Step 1: Strip the cedar and everything attached to it
Cedar shakes come off with a shovel or a flat bar, and the tear-off technique in how to tear off an old shingle roof applies. Expect old nails everywhere, often rusted in place. Pull them or hammer them flat, because raised nail heads will lift the new plywood and make noise and dips.
Remove old metal valleys, flashings and vent pipes. Keep the ground tarped and swept with a magnet afterward, since the old roof throws nails and cedar debris widely.
Step 2: Inspect every board and the rafters
Plank decks rot at the eaves, around chimneys and below valleys and any old leak. Press a screwdriver into dark or suspect boards. Replace anything that is soft, split along the nail line or so cupped that a plywood panel will rock on it. A replacement board is 1 by 6 or 1 by 8 nailed to the rafters at both ends.
Look at the spacing. Boards on 16 inch rafter centres should land on the rafters, and gaps between them may be 1 to 6 inches. Note any rafters that are cracked or sagging, and see how to inspect a roof deck for rot for the full checks.
Step 3: Fill large gaps and level high or low boards
Plywood laid over a wide gap sags there and tends to crack the shingles above. If gaps are more than about 6 inches, or if there are missing boards at the eaves, rakes or ridge, fill them with lumber so every panel edge has something under it. Check that a long straightedge laid across the boards does not rock.
If boards have cupped edges, nail them down or plane them flat. If the deck has several different thicknesses of board, shim the thin ones so the surface is continuous.
Step 4: Choose the sheathing and lay it across the boards
Because the boards underneath carry some of the span, many roofers use 3/8 to 1/2 inch plywood over board decking, but 1/2 inch is the common choice, and the thickness is governed by the local code and the roofing manufacturer, so confirm it. Install the panels with their long edge across the boards and rafters, end joints centred on a board, and staggered between rows.
Leave a gap of about 1/8 inch at the edges and ends of each panel. Set H-clips between rafters on the long edges where the panel edge lies over a gap.
Step 5: Fasten into the boards and through into the rafters
For 1 inch boards (actual 3/4 inch) plus 1/2 inch plywood, a 2-1/2 inch nail goes through both and about 1-1/4 inch into the rafter, and a 3 inch nail gives a deeper bite. Fasteners should hit the rafters where possible, with the boards adding extra holding. Use the same 6 inch edge and 12 inch field spacing as for normal sheathing, and confirm the schedule for your code and panel.
Do not nail through the plywood into a gap with nothing behind it. That gives no holding power and leaves a bump. Use a chalk line to mark the rafters so you know where to drive.
Tip: Snap lines for both rafters and boards. The plywood hides them, and once a panel is down you cannot find the framing again.
Step 6: Check that the new deck is flat, tight and continuous
Lay a long level across the new panels in several places. Sand or plane high edges, shim low ones, and add fasteners to any panel that moves under foot. A plank deck resheeted well is stiffer than a standard deck because of the double layer.
Step 7: Re-check the attic ventilation
A cedar roof on spaced boards ventilated itself. A solid deck does not, so the attic needs proper intake and exhaust, or moisture will build up under the new roof. Check the attic ventilation basics and add soffit intake vents or a ridge vent where the attic lacks them.
Step 8: Dry in the roof and install the eave membrane
New sheathing is exposed wood and should be covered promptly, especially OSB. Install ice and water shield at the eaves, then underlayment per install synthetic underlayment.
Resheet or Replace the Boards?
Covering boards with plywood is usually faster and adds stiffness, but it adds weight, cost of material and a second layer of wood. Removing the boards and replacing them with full sheathing is cleaner when the boards are rotten, uneven or hard to fasten to. A mix is common: replace bad boards and cover the rest.
Why Shingles Need a Solid Deck
- Nails need wood behind them along the full course. A nail over a gap holds nothing.
- Asphalt shingles sag into gaps and crack at the edges.
- Shingle warranties call for a continuous deck in good condition.
- Wind uplift resistance depends on the fastening into solid wood.
Common Mistakes
- Laying shingles directly on spaced boards. Nails miss the wood, shingles sag into the gaps and blow off in wind.
- Nailing plywood into the gaps. The nails do not hold and leave bumps. Find the rafters and the boards.
- Covering rotten boards. The new sheathing hangs on rotten wood and moves under load.
- Using nails too short. They stop in the board and do not reach the rafter, leaving the deck loose.
- Forgetting ventilation. A cedar roof breathed through the gaps. A solid deck traps moisture unless vents are added.
- Leaving gaps in the new panels touching. Panels without the 1/8 inch gap buckle after wet weather.
When to Call a Professional
If the boards are rotten over large areas, the rafters are damaged, the roof is steep, or the attic has no ventilation, call a roofing contractor. Adding a second layer of sheathing also adds weight, and on an older house the framing may need to be checked.
A contractor can tell whether replacing the boards or covering them is the better choice, and can also handle the new ventilation and the eave details in the same job.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Replacement service or the towns we cover.
How to Resheet Over Plank (Board) Decking: Questions
Can you put asphalt shingles on spaced boards?
No. Asphalt shingles need a solid, continuous deck so every nail hits wood and the shingles lie flat. Over gaps the nails do not hold and the shingles sag and crack. Spaced boards from an old cedar roof must be covered with plywood or OSB, or replaced with sheathing, before shingles go on.
What thickness plywood goes over plank decking?
Half inch plywood or 7/16 inch OSB is the common choice, with 3/8 inch sometimes used where the boards are tight. The right thickness depends on the BC Building Code, the board spacing and the roofing manufacturer, so confirm it. Fasteners need to be long enough to go through the panel and the board and into the rafter.
Do you have to remove the old roof boards?
Not always. Sound boards can stay and be covered with new sheathing, which saves labour. Boards that are rotten, split, badly cupped or too widely spaced are replaced or removed. The decision is made after the old roof is off and the boards are inspected, so have a price per sheet and per board agreed in advance.
Why does an old cedar roof have gaps between the boards?
Cedar shakes were nailed to spaced boards so air could move under them and dry the wood from both sides. That extended the life of the cedar in wet climates. The gaps are a problem for asphalt shingles, which need a solid deck. When switching from cedar, the gaps must be closed.
Does a solid roof deck change attic ventilation?
Yes. The gaps in a plank deck let some air and moisture move, and a new solid deck seals them. The attic then relies entirely on its intake and exhaust vents. Check that soffit intake and ridge or roof exhaust are working and add vents if the space has none, to avoid moisture and mold under the new roof.
Related Guides
All Tear-Off, Decking and Underlayment guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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