Roof Tear-Off, Decking and Underlayment: The Work Under the Shingles
10 step by step guides
A roof is only as good as what it is fastened to. The shingles or metal panels get the attention, but the deck under them and the membranes between the two decide whether the roof lasts 10 years or 30. Most failures that get blamed on roofing material are really a soft deck, a wrinkled underlayment, or a detail left out at the eave.
The work runs in a clear order. Plan the tear-off first: bins, tarps, weather, the neighbours and the landscaping. Then strip the old roof, and inspect the bare deck before anything goes back on it. Rot gets cut out and replaced with new sheathing, and old spaced boards get resheeted.
Then the roof is dried in. Ice and water shield goes on the eaves and the vulnerable spots, and the field gets synthetic or felt underlayment. Since the Fraser Valley can change from sun to a downpour in an afternoon, every tear-off needs a plan to dry in the roof overnight.
Tear-Off, Decking and Underlayment Guides
- Guide 1How to Plan a Roof Tear-OffPlan a tear-off by sizing the bin to the roof (about 1 ton of debris per 7 squares of single-layer shingles), placing it under the eave where you can throw or chute straight in, covering landscaping and vents, checking older roofs for asbestos before disturbing them, and only stripping as much roof as can be dried in before nightfall. Check the forecast and warn the neighbours.AdvancedHalf a day of planning and setup
- Guide 2How to Tear Off an Old Shingle RoofTear off a shingle roof by working from the ridge down with a shingle shovel or fork, sliding the tool under the shingles to break the nails loose and throwing debris into a bin below. Remove the underlayment, drip edge and old flashing, pull or hammer flat every leftover nail, and sweep the deck and the ground with a magnet. Strip only what can be covered the same evening.Advanced4 to 8 hours for a typical roof
- Guide 3How to Inspect a Roof Deck for RotInspect a roof deck after tear-off by walking every panel, pressing with a screwdriver and your weight to find soft spots, checking the underside from the attic for stains and daylight, and looking for delaminated plywood, crumbling edges, deflection between rafters and loose or rusted fasteners. Replace any panel that is soft, delaminated or stained through, cutting back to the centre of a rafter.Advanced1 to 2 hours for a typical roof
- Guide 4How to Replace Rotten Roof SheathingTo replace rotten roof sheathing, cut the damaged panel out along the centre of the rafters, pull nails, and install a new panel of the same thickness with its long edge across the rafters. Leave a 1/8 inch gap at panel edges, add H-clips between rafters, and nail every 6 inches at edges and 12 inches in the field with 2-1/2 inch nails. Common thicknesses are 7/16 OSB or 1/2 inch plywood.Advanced1 to 4 hours for a few panels
- Guide 5How to Resheet Over Plank (Board) DeckingResheet 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.Advanced1 to 2 days for a typical roof
- Guide 6How to Install Ice and Water ShieldInstall ice and water shield on a clean, dry deck, peeling the release film in sections and pressing the membrane flat without wrinkles, starting at the eave over the drip edge. Where the BC Building Code requires eave protection on a shingle roof, it runs at least 900 mm (about 3 feet) up the slope and to a line at least 300 mm (about 12 inches) inside the inner face of the exterior wall. Common laps are 3 inches at the sides and 6 inches at the ends.Advanced2 to 4 hours for a typical roof
- Guide 7How to Install Synthetic UnderlaymentInstall synthetic underlayment over clean, dry sheathing by starting at the eave and rolling horizontally with the printed side up, lapping each course over the one below by the marked line (commonly 4 inches at sides and 6 inches at ends), and fastening with cap nails or cap staples about every 12 inches along laps and in the field as the manufacturer specifies. Cover it with roofing within the UV exposure limit on the roll.Advanced2 to 5 hours for a typical roof
- Guide 8How to Install Felt UnderlaymentInstall asphalt felt by rolling it horizontally from the eave up, lapping each course at least 2 inches over the one below and ends at least 4 inches (check the product and code), and fastening with cap nails or roofing nails with tin caps just enough to hold it flat. Use 15 lb for standard pitches and 30 lb where extra strength is wanted. Cover it with roofing promptly, because wet felt wrinkles.Intermediate2 to 5 hours for a typical roof
- Guide 9How to Dry In and Tarp a Roof OvernightDry in an open roof by covering bare deck before nightfall with self-adhered membrane or properly lapped underlayment, taped and capped at the edges, or with heavy tarps over the ridge and fastened to lumber strips, not through the tarp alone. Overlap tarps from the ridge down, weigh and batten them against wind, close off open edges and penetrations, and check the forecast before stripping any section.Advanced1 to 3 hours per section
- Guide 10How to Fix a Sagging Roof DeckFind out whether a sag is in the sheathing or the framing by checking from the attic. A dip between rafters is fixed by replacing or re-fastening the panel and adding H-clips or blocking. A sag in the rafters or ridge is structural and is fixed by sistering with full-length lumber or by a permanent brace designed by an engineer. Never just build up the shingles over a sag.ProfessionalHalf a day for sheathing, 1 to 3 days for framing
Also on this site
Tear-Off, Decking and Underlayment: Common Questions
What happens during a roof tear-off?
The old shingles, underlayment and flashing are stripped down to the bare roof deck, usually in sections so that no more roof is opened than can be dried in before nightfall. The deck is then inspected, any rotten sheathing is replaced, and new underlayment, eave protection and roofing go on. Nails, debris and old materials are cleared into a bin and swept with a magnet.
Can you roof over old shingles instead of tearing off?
Sometimes a second layer is allowed by code, but it hides the deck from inspection, adds weight, and usually shortens the life of the new roof because old curled shingles telegraph through. Wet or rotten decking under a second layer goes unnoticed until it fails. A full tear-off is the only way to see and fix what the roof is fastened to.
How long does roof underlayment last?
Underlayment is a backup layer, not the finished roof. Synthetic sheets are rated for a limited period of exposure before the shingles go on, often 30 to 180 days depending on the product, and felt is much less tolerant of weather. Under a finished roof it can last as long as the roof itself, provided the roofing above it stays intact.
Should roof sheathing be replaced during a reroof?
Only the sheathing that is damaged. Soft, delaminated, stained or badly nail-sick panels are cut out back to the rafters and replaced. Solid, dry sheathing can stay. The bare deck after tear-off is the one chance to check every panel, so it is worth walking the whole roof before any underlayment goes on.
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