Roof decking replacement cost

Replacing roof decking costs $150 to $400 per 4x8 sheet installed, typically about $250. A roof needing ten sheets adds $1,500 to $4,000 to the job, and a full deck replacement on a 20 square roof runs $8,000 to $16,000.

This is the line that most often makes a final invoice beat the estimate, and it is the one nobody can quote honestly in advance. The deck is under the shingles. Until they come off, everyone is guessing.

What it costs

One 4x8 sheet covers 32 square feet, so a square of roof takes just over three sheets.

Get the per-sheet rate in writing before work starts

This is the single most useful thing in this article.

A roofing contract should state what a replacement sheet costs, before anyone is on the roof. Then when the crew calls down that they have found eight bad sheets, the arithmetic is already agreed and the conversation is about the count rather than the price.

Without that clause you are negotiating from a position where half your roof is open and the crew is standing on it. Contractors who work fairly put the rate in the contract as a matter of course. One who will not name it beforehand is asking you to sign a blank cheque, and you should say so.

Ask also to be shown the bad sheets, or photographs of them, before they are carted away.

Why decking goes bad

A leak nobody knew about. Usually at a flashing, a valley or a pipe boot. The wood stays damp, softens and eventually delaminates. This is the most common cause and it is localised: a few sheets around one penetration.

Bad ventilation. Condensation on the underside of the deck through winter after winter. This one is not localised; it affects whole slopes, and it is why inadequate ventilation turns a decking repair into a re-deck.

Age and the wrong original material. Some 1970s houses were sheathed in particleboard or thin OSB that fails as soon as it gets damp once.

Plank decking with wide gaps. Older houses have solid timber boards rather than sheets. They are often sound but too widely spaced for modern shingle nailing, which needs continuous support. Overlaying with 3/8 inch sheet is the usual fix.

What thickness, and why it matters

Building codes generally require 15/32 inch — sold as half inch — sheathing for rafters at 24 inch centres, and 3/8 inch is acceptable at 16 inch centres. Plenty of older roofs have 3/8 inch on 24 inch rafters, which sags visibly between them.

If the deck is being replaced anyway, going to 5/8 inch on wide-spaced rafters costs a little more per sheet and gives a flatter roof that holds nails better. On a roof taking tile or slate it is not optional.

Plywood or OSB. OSB is cheaper and, dry, performs the same. Wet, it swells at the edges and does not recover; plywood dries out and mostly does. On a roof in a wet climate, or under a metal roof where condensation is possible, plywood is worth the difference.

When the whole deck goes

Full replacement rather than patching when:

• More than about a third of the sheets are soft
• The decking is particleboard, which fails as a category
• There is a third layer of shingles being removed and the deck has been carrying the weight for decades
• The roof has visible sag between rafters across whole slopes
• The covering is changing to something heavier

Full re-decking on a 20 square roof adds $8,000 to $16,000 to the job, which usually makes it the largest single line after the shingles themselves.

Frequently asked questions

How much decking will my roof need?

Nobody knows until it is open. A rough planning figure is 5% to 15% of sheets on a roof of ordinary age with no known leaks, and more where there is a history of them. Budget for ten sheets and be pleased if it is three.

Can I see the deck before the work?

Partly, from inside the attic on a bright day. Daylight through the boards, dark staining, or a spongy feel when pressed from below all point to problems. It will not show you the parts above insulation or over finished ceilings.

Is decking covered by insurance?

If it was damaged by a covered peril, such as a tree or a storm, usually yes. If it rotted slowly from a leak or condensation, no — that is wear, and it is excluded.

Can bad decking be repaired rather than replaced?

Not really. Delaminated or rotten sheet material has lost its structure and cannot be dried back into service. Sistering support underneath is a temporary measure, not a repair.

Does new decking need a permit?

Structural sheathing usually falls under the roofing permit rather than needing its own. Where a large proportion of the deck is being replaced, some jurisdictions treat it as structural work and inspect it separately.