Don't Roof Over Old Shingles Until You Check These Four Things

What California Code Actually Allows

A roof over has a few names. Roofers call it an overlay, a nail over or a layover. The code calls it a roof recover. It means new roofing goes on top of the old roofing with no tear off.

California's Residential Code covers this in Section R908.¹ It says a roof recover is allowed when it is installed per the maker's approved instructions. It also allows a few special cases, like a standing seam metal system that carries its own load, or a new coating over an old coating or a low slope membrane.

Then it lists three times a recover is not allowed at all. First, when the old roof is water soaked or worn out enough that it cannot hold the new one. Second, when the old roof is slate, clay tile, cement tile or asbestos cement tile. Third, when the roof already has two or more layers of any kind.¹

So the most a house can carry is two layers. If your roof was already covered over once, the next roof is a full tear off. No exceptions are listed in that section.

The same part of the code says the roof frame has to be able to hold the roof covering system and the load of the crew and materials during the job.¹ A second layer is extra weight that stays there for the life of the roof.

Why Most Ontario Roofs Can't Be Covered Over

Drive through Ontario Ranch, Eastvale or most tract streets built in the last few decades and you see concrete tile. Under the code, that roof cannot be covered over.¹ Period.

That surprises people who read national advice, which is often written for towns where nearly every house has asphalt shingles. In the Inland Empire, the tile rule rules out a big share of homes before anything else is checked.

The good news for tile owners is that the tile itself often outlives the paper under it. The code lets sound slate, clay or cement tile be put back after new underlayment goes down, as long as broken or cracked pieces are not reused.^1] Our [tile roofing page explains that re-lay job.

The houses that can be candidates are the older shingle ranches, some newer homes with shingle roofs, garages and patio covers. Even then, the roof has to pass the checks below.

Four Checks Before You Say Yes to a Roof Over

A roofer who offers a roof over should be able to answer all four of these on the spot, with photos.

Count the layers. Look at the edge of the roof along the side of the house. A second layer shows as a thick double edge. If there are already two layers, a third is against code.¹

Find the soft spots. Walk the roof, look for sagging lines, and check the attic for old water stains on the wood. The code bans a recover over a roof that is water soaked or too worn to hold the new layer.¹ A hidden soft spot is the reason a roof over can fail early, because nobody sees the deck again until the next tear off.

Look at the metal. The code says flashing, edging, vents and outlets must be replaced where they are rusted, damaged or worn out, and that flashing has to be rebuilt per the maker's instructions.¹ On an older roof, that list gets long.

Get the maker's instructions in writing. The code's first condition for a recover is that it follows the maker's approved instructions.¹ Ask which shingle is going on, and ask the roofer to show you what that maker says about going over an old roof. If they cannot, that is your answer.

The Weight, the Nails and the Ridge

Shingles are heavier than they look. The data sheet for PABCO Premier, a common architectural shingle, lists 255 pounds per square, and a square is 100 square feet of roof.⁴ Other products weigh more or less, so check the sheet for the one on your bid.

As a worked example, a roof with 20 squares of that shingle carries about 5,100 pounds of new shingles, plus whatever the old layer still weighs. The code puts the job of checking that the frame can hold it on the project.¹

Nails have to go through two layers now. The Asphalt Roofing Manufacturers Association says roofing nails should go at least three quarters of an inch into the wood deck, or fully through a thinner deck, and that the number of layers is one of the things to count when picking nail length.² Short nails that only bite into the old shingles are how a roof over loses shingles in a Santa Ana wind.

The ridge needs attention too. Owens Corning's hip and ridge instructions say that when a new roof goes over an old one, the old hip and ridge shingles come off first so the new ones lie level.³ A crew that skips that step leaves a hump along every ridge.

The Cool Roof Rule Still Applies

Skipping the tear off does not skip the Energy Code. All three Ontario ZIP codes, 91761, 91762 and 91764, are in state climate zone 10, per the Energy Commission's ZIP list.⁷

The Energy Commission says its roof rules apply when more than half of a roof is replaced.⁵ An Energy Code Ace fact sheet, written for the 2019 code, spells out that recovering or recoating an existing roof counts the same as replacing it once more than half the roof is done.⁶ A whole house roof over is all of the roof.

For pitched roofs in zone 10, that means a product with a Cool Roof Rating Council label, unless the house meets an exception. The Commission lists exceptions like R-38 ceiling insulation, a radiant barrier in the attic, or R-2 insulation at the roof deck.⁵ Cool roof shingles come in dark colors too, so this is about picking the right product, not about a white roof.

Check this before you sign. It belongs in the permit, and the City of Ontario takes detached single family reroof permits through its online Symbium portal.⁸

When We Would Still Tear It Off

A roof over saves the tear off labor and the dump runs. That is real money on a simple one layer shingle roof that is flat, dry and sound.

But stack up what the code and the makers still require. New flashing where the old is rusted. Old ridge shingles removed. Longer nails. A cool roof product in zone 10. A frame that can hold two roofs. By the time those are done right, the gap between a roof over and a full roof replacement gets much smaller.

And the biggest cost of a roof over never shows up on the bid. Nobody sees the deck. A tear off shows every soft board, and they get replaced before the new roof goes on. With a roof over, a rotted spot stays under two layers until it leaks.

Our take. On a shingle roof with one layer, no soft spots and good metal, a roof over can be a fair choice, especially on a garage or patio cover. On anything older, anything that has leaked, or any roof where you plan to stay 15 more years, we would tear it off and see the wood. A roof inspection with photos of the edge, the attic and the flashing settles which roof you have.

If you want a second set of eyes on a roof over bid, call Ontario Roofing at (909) 404-9464 for a free written estimate.