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Roof Tear-Off and Decking Replacement Cost: What Is Under the Shingles, and What It Costs When It Is Rotten
Roofing
Tear-Off and Decking: The Part of the Roof You Only See for One Morning
Between the old shingles coming off and the new underlayment going on there is a window of about two hours when the roof deck is bare, and that is when every roofing job either stays on budget or does not. Soft plywood, gaps in old boards, a second layer nobody mentioned, the rot around the chimney: all found at 9 a.m., all priced by whoever is standing there unless the price was agreed before. This is what is under there and what each thing costs.
Tear-off
| What is on the roof | Cost | What it involves |
|---|---|---|
| One layer of asphalt shingles | Included in the installed rate (about $0.90 to $1.30 per sq ft of it) | Shingle forks, a dumpster, a tarp over the garden, a magnet over the lawn |
| Two layers | +$1.00 per sq ft | Twice the weight, twice the nails, a second dumpster on a big roof |
| Three layers, or shingles over cedar shakes | +$2.00 per sq ft | The cedar comes off in pieces and the deck underneath is usually boards with gaps |
| Metal panels | +$0.50 to $1.00 per sq ft | Screws out, panels down, usually strapping to remove too |
| Flat roof, built-up tar and gravel | +$1.50 to $3.00 per sq ft | Heavy, slow, and disposal is charged by weight |
| Overlay (new shingles over the old) | Saves the tear-off | Allowed over one layer in most Ontario municipalities; not a good roof, see below |
The dumpster is $400 to $700 for a bungalow’s worth of shingles and is inside the rate. Shingles are heavy: a 20-square roof is about 5,000 lb of them, more if there are two layers, and the transfer station charges by the tonne. Roofers who quote “disposal extra” are quoting the per-square number without it; ask what the extra is, in dollars, before signing.
What a roofer looks for on the bare deck
- Soft spots. Walked on, every sheet. Plywood that gives underfoot or sounds hollow has delaminated from years of damp. Around chimneys, at the eaves where ice dams sat, under old skylights, and along valleys are the usual places.
- Gaps. Board decks (1×6 or 1×8 planks) from before the 1960s shrink and leave gaps that shingles cannot bridge and nails miss. Over about 1/4 in. and the roof needs sheeting over the boards.
- Thickness. 3/8 in. plywood on 24 in. rafter spacing, which some 1970s builders used, flexes under a crew and under snow. Code today wants 1/2 in. at 24 in. centres (7/16 OSB with clips). Thin deck on wide spacing is a re-sheet.
- Rot at the edges. The first 2 ft above the eaves is where ice dams held water. If the fascia is soft, the deck edge is too.
- Previous repairs. A patch of new plywood from the last roof, screwed down in a hurry, tells you where it leaked before and probably still does.
Decking costs
| Decking work | Cost | Notes |
|---|---|---|
| Replace individual sheets, 1/2 in. plywood or 7/16 OSB | $100 to $140 per 4×8 sheet installed | Cut out the bad, fit the new, nail to the rafters. Two to eight sheets is typical on a 25-year-old roof. |
| Re-sheet an entire board deck | $2.75 to $3.75 per sq ft | New plywood over the old boards; a 20-square roof is $6,000 to $7,500 on its own |
| Re-sheet after full plywood failure | $3.25 to $4.50 per sq ft | Old sheets off, new on; the extra is the removal |
| Sister or replace a rotten rafter end | $150 to $400 each | Carpentry, not roofing; a good roofer does it or stops and calls one |
| Fascia board replacement | $8 to $14 per lin ft | Done while the eave is open, before the drip edge |
The clause that matters: “Decking replacement beyond [two] sheets at $[120] per sheet, installed, approved by the owner before installation; photographs of the condition provided.” Get that on the quote. A roofer who will not put a unit price on decking before the roof comes off is a roofer who will name the price when you have no roof.
The overlay question
Putting new shingles on top of the old saves about $1 a square foot and a day, and most Ontario municipalities allow a second layer. The case against it: the new shingles follow every curl and ridge of the old ones and look like it; the roof now weighs twice what it did; the deck was never inspected, so the rot stays; nails have to go through two layers to find wood and often do not; the manufacturer’s warranty is shorter or void over an existing layer; and the next roofer tears off both layers at $2 a square foot. On a garage or a shed, fine. On a house, the tear-off is the cheap part of a good roof.
What the bare deck means for the rest of the job
The two hours with the deck open are also when the roofer confirms the ventilation (are there soffit vents, are they blocked by insulation, is the bathroom fan dumping into the attic), checks the chimney for loose brick before flashing it, and sees whether the skylights are worth keeping. A good crew photographs everything and shows you before the underlayment goes down; it takes five minutes and it is the record if anything is argued about later. The installation checklist has the list for that morning and the one for the last day.
Questions people ask
How much does it cost to replace roof decking?
$100 to $140 per 4×8 sheet installed for individual sheets; $2.75 to $4.50 per square foot to re-sheet a whole roof. A typical 25-year-old roof needs two to eight sheets; a board deck from before 1965 usually needs full sheeting.
Is tear-off included in a roofing quote?
It should be, for one layer, and the quote should say so. A second layer is about $1 per square foot extra. “Disposal extra” means the number you were given is not the number you will pay.
Can you put new shingles over old shingles in Ontario?
Over one layer, most municipalities allow it. It is a worse roof: heavier, uneven, uninspected deck, shorter warranty, double tear-off next time. Fine on an outbuilding, not recommended on a house.
How do roofers know the decking is bad before they start?
They do not, beyond the attic and a walk on the roof. The honest quote carries an allowance and a unit price; the dishonest one carries nothing and finds a lot.
What is roof decking made of?
1/2 in. plywood or 7/16 in. OSB on houses since about 1975; 1×6 or 1×8 boards before that; 3/8 in. plywood on a few 1970s houses that should not have used it. Code calls for 1/2 in. at 24 in. rafter spacing.
Before the roof comes off
Tear-off layers named, disposal in, decking allowance and a unit price per sheet, photographs promised. Those four lines are the difference between a roofing quote and a roofing estimate.
Sources and further reading
- Ontario’s Building Code: the official page, with the current regulation (O. Reg. 163/24). Roofing is Section 9.26, attic ventilation 9.19.
- O. Reg. 163/24, the Building Code regulation: the full text if you want to read 9.26 and 9.19 yourself.
- Your rights when starting home renovations or repairs: Ontario government. The 10-day cooling-off period, the 10% estimate rule, and what a contract has to say.
