Learning Center · Framing
Rafter and Roof Deck Repair: Sistering, Decking and What the Code Requires
The wood under a roof is found, not inspected. Most rafter and deck problems show up the morning the old shingles come off. This guide explains what sistering a rafter means, when a deck sheet gets replaced, what the code requires of the new wood, and how the work is priced so it does not become a surprise.
Updated October 12, 2026
Read This First
The short version
- A damaged rafter is usually sistered: a new board is fastened alongside it so the pair carries the load, not cut out and replaced.
- Deck sheets come out for rot, delamination, soft spots and damage around old penetrations; GAF requires at least 3/8 inch panels with gaps no wider than 1/4 inch.
- Trusses are different: the code bars cutting or altering a truss without a design professional’s approval.
- Decking is priced per sheet: the first two sheets are included and each sheet beyond that is $80.
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Direct Answer
How are rafters and roof decking repaired?
Rafters are sistered, decking is replaced sheet by sheet, and both are photographed before they are covered.
Rafters
A bad rafter gets a partner. Sistering means a new board of the same depth is set tight against the damaged rafter, overlapping well past the bad section onto sound wood at both ends, and fastened through so the two act as one. The roof load moves into the new wood without opening the ceiling below or rebuilding the roof. A rafter rotten at its tail, the end at the eave, is the most common case, and the sister runs from the wall plate up past the rot.
Decking
Decking is replaced one sheet at a time. A sheet comes out when it is rotten, has come apart in layers, gives underfoot, or has been cut and patched so many times around old vents that it no longer holds a nail. The new sheet is cut to land on the rafters, fastened on the schedule the code gives, and photographed before underlayment covers it. Our roof deck and underlayment guide covers the whole layer.
Framing
Sistering a rafter, and when sistering is not enough
Sister a rafter damaged in one spot; replace it or call an engineer when damage runs its length or the roof sags.
When a sister does the job
Local damage, sound wood on both sides. Rot at the tail from a leaking edge, a split from an old nail line, a crack from a dropped load during a past job: each has good wood above and below it, and a sister bridging the bad section restores the member. The sister is the same lumber depth as the rafter, long enough to bear on sound wood at both ends, and fastened on a regular pattern along its length.
When it is not
Length and sag change the answer. A rafter rotten along most of its length, a whole slope that has sagged between the ridge and the wall, or rafters that have spread the walls outward are not sistering jobs. They need a replacement member, added support, or an engineer’s design, and the code’s span tables in R802.4 are what the engineer sizes against. Our framing service page explains how structural work and the city’s requirements fit together.
Trusses are not rafters
Do not cut a truss. IRC R802.10.4 says truss members may not be cut, notched, drilled, spliced or otherwise altered without the approval of a registered design professional, and adding load such as equipment beyond the truss design needs verification. A damaged truss chord is repaired to an engineer’s detail, not sistered on the roofer’s judgment. R802.7 limits notches and holes in sawn rafters as well.
Deck Standards
What a roof deck has to be before shingles go on
At least 3/8 inch, no gaps over 1/4 inch, no spaced sheathing, solidly sheathed, fastened on the code schedule.
The manufacturer’s minimums
GAF sets the floor in its substrate bulletin TAB-R-101: plywood at least 3/8 inch thick, no gaps between deck boards or planks wider than 1/4 inch, and no installation over existing spaced or skip sheathing, the old boards with open gaps between them. The code agrees on the last point: IRC R905.2.1 says asphalt shingles are fastened to solidly sheathed decks, so spaced boards get solid sheathing before a shingle roof goes on.
The code’s fastening schedule
Nails, not staples, on a set pattern. IRC Table R602.3(1) calls for 8d common nails, 2 1/2 inches long, at 6 inches on center at panel edges and 12 inches at intermediate supports for roof sheathing from 3/8 to 3/4 inch, with 6 inch spacing at intermediate supports within 48 inches of ridges, eaves and gable ends in higher wind areas. R803.2.1 requires the panels to carry a grade mark, and the new sheet has to land on framing at its edges, which is why a replacement is a full sheet or a piece that reaches the rafters, not a scrap screwed to the old deck.
What a replaced sheet should look like
Flush, fastened and photographed. The new panel sits level with its neighbors, its edges bear on rafters, the nail heads are seated, and there is a photo of it before underlayment goes on. A deck replaced without photos is a line on the invoice you have to take on faith.
Discovery
Why the count is unknown until the roof is off
Soft spots are found by walking the deck after tear-off, so an honest estimate uses an allowance.
What the attic can and cannot show
From below you see some of it. Staining around an old leak, daylight at a vent, a rafter tail that is dark and crumbly at the eave: all visible from the attic with a flashlight. What you cannot see from below is a sheet that has gone soft on top or come apart in its layers. Those are found on the roof, by walking the bare deck after the shingles and underlayment are off.
Where to look hardest
Water tells you where. Valleys, around chimneys and vent pipes, along the eaves where water has sat behind a bad drip edge, and under any place a past repair was made. Rafter tails rot where fascia rotted, so a fascia repair and a rafter check go together; see our soffit and fascia guide.
On the Estimate
How decking and framing repair are priced
Decking is an allowance with a per sheet price; framing repair is priced from the photos, per member.
Decking, per sheet
The honest way is an allowance. Our roof estimates include the first two 4 by 8 sheets of decking, and each sheet beyond that is $80, stated on the estimate before the job starts. If tear-off finds six bad sheets, the four beyond the allowance cost 4 times $80, with a photo of each one and a count on the final invoice that matches the photos. An estimate that is silent about decking leaves both the count and the price to a change order after the roof is open.
Framing, per member
Rafter work is priced when it is found. A sistered rafter tail is a small line; a run of damaged rafters or anything needing an engineer is a written change order with photos and the fix described, approved before the roof is closed. No honest number exists for framing sight unseen, and a contractor who quotes one has not looked.
Keep the record
Everything under the shingles is covered for good the day the roof goes on. Photos of every replaced sheet and every sistered rafter, with the count, belong in your file with the invoice, because they are the only proof the work was done.
Common Questions
Questions homeowners ask
What does it mean to sister a rafter?
A new board of the same depth is fastened alongside the damaged rafter, reaching past the bad section onto sound wood at both ends, so the pair carries the load. It avoids removing the rafter or opening the ceiling.
When does roof decking need to be replaced?
When a sheet is rotten, delaminated, soft underfoot, or so cut up around old penetrations that it will not hold a nail. GAF requires at least 3/8 inch panels with gaps no wider than 1/4 inch, and the code requires a solidly sheathed deck under shingles.
How much does roof decking cost to replace?
Our estimates include the first two sheets, and each sheet beyond that is $80, stated before the job. The count is known only after tear-off, which is why it is written as an allowance with photos of every sheet replaced.
Can a roofer cut a roof truss?
No. IRC R802.10.4 bars cutting, notching, drilling or altering a truss without the approval of a registered design professional. A damaged truss is repaired to an engineer’s detail.
How are new deck panels nailed?
IRC Table R602.3(1) calls for 8d common nails at 6 inches on center at the panel edges and 12 inches at intermediate supports for 3/8 to 3/4 inch roof sheathing, closer near ridges, eaves and gable ends in higher wind areas, with the panel edges landing on framing.
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