Gravel/asphalt roof to shingles

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My gravel/asphalt (built up roof) needs to be replaced with shingles. The roof deck is 3/8" plywood 60 years old. Do I need to remove the old 3/8" plywood or is it okey to lay a new deck (1/4" plywood) on top of the old deck?

my first question is who told you it needed to be replaced with shingles and why? also i would say you might consider using thicker ply if it is in your budget. i just finished a $110,000.00 20 yr warranty A.P.P. Manville where the general installed 1/2 over 1/2, the architect considered the weight savings from the removal of the 600 lb per sq. rock roof and the total installed weight of the added plywood and the A.P.P. system and concluded there would be a weight savings. what is the pitch of the roof and why shingles?

The built up roof(BUR) was damaged in a hail storm. I am having difficulty finding a roofer willing to install another BUR. I understand that 3/8" deck is the minimum for installing shingles. Until the old BUR is removed I won’t know what condition the deck is in. I am being quoted $75.00 a sheet to replace any bad decking. I want to be prepared if the deck needs to be replaced. The roof is 4/12 pitch.

It is the same here…most wiil convert to torchdown for lower pitch and shingles for 4/12 and above. It is difficult to find anyone that does bur anymore.

I have had bur roofs where the tar is present on every piece of plywood (remaining tar will not chip off anymore and you can’t shingle over it). In this case, you are looking at a full resheet.

If this happens to you, then i would upgrade to a thicker plywood (1/2")

If your roof is flared upward, with cant-stripping and perimeter flashing, then your probably looking at having your rakes and eave cut down to the level of plywood (for shingle). If your doing torchdown, it could probably stay.

Do I need to remove the old 3/8" plywood or is it okey to lay a new deck (1/4" plywood) on top of the old deck?

Best practice here is to re-sheet. Call your local building dept and ask them what’s code in your area, 1/2" ply may not
be allowed. Going over existing ply may not be allowed. These things differ from municipality to municipality.

use 1/2 structural 1 plywood minimum for a good result. be sure your nails are long enough and 4 inch on ends and 6 inch field. put a pitch gauge on the roof to double check.

Many thanks to all! From your advice my best choice is to remove the old deck and use 1/2" plywood which is accepable in our town.