What are the layers of a roof, and how do I know mine was installed properly?
A modern asphalt roof is four things: the OSB decking, a synthetic underlayment over it, an ice and water membrane in the valleys where water concentrates, and the shingles on top. Every shortcut in this trade is one of those being skipped, reused or laid over instead of replaced. The tell is usually visible from the street a few years later, as waves in the shingle courses or a vertical line of shingles that has started to slip.
The four layers, from the attic up
The decking is the wood, usually OSB, that everything else is fastened to. It is structural, and it is the layer nobody can assess until the shingles are off.
Over that goes the underlayment, the felt. Synthetic felt is the better product and it is what we use. It is the layer that keeps water out during the install and the backstop after it.
In the valleys, where two slopes meet and all the water on that roof concentrates into one channel, an ice and water membrane goes down. It is self-adhering, it sticks to the deck, and it stops water getting underneath the valley metal. Valleys are where roofs leak, so this is not a place to economise.
Then the shingles. Three layers of protection with the shingle as the outermost, which is why a roof that looks fine can be failing underneath it.
Felt over felt is the shortcut that defines the trade
The shortcut is leaving the old underlayment on and laying new felt over it rather than stripping to bare decking. It saves hours per roof, and it fails in a specific, recognisable way.
When the old shingles come off but the old felt stays, the nails holding that felt come up slightly. You then lay a new roof over a surface with hundreds of small high points in it. The shingles cannot lie flat, so they never fully seal to each other.
What follows shows up over the next few years. Waves across the courses where the surface underneath is uneven. Then zippering, which is a vertical run of shingles working loose and starting to walk down the slope, and it is as obvious from the street as it sounds.
The other cost is the one nobody mentions at the time: an install like that voids the manufacturer warranty. You paid for a thirty-year shingle and no longer have a thirty-year shingle behind it. Multiple layers also add weight the structure was never designed to carry.
The other shortcuts, in the order we find them
Old nails left in the deck, which produces the same unevenness as old felt for the same reason.
Soft metals reused rather than replaced. Vents, flashing and pipe boots are cheap and they are exactly what fails, so putting the old ones back under a new roof is putting the failure back.
Ventilation holes cut wrong, or cut to fit the old vent rather than the new one. A roof that cannot breathe cooks the shingles from underneath and shortens the life of everything above it.
And a small one that is a personal irritation: ladders going up against a gutter run that is already lifted, with nothing to protect it. It is thirty seconds of care and skipping it causes problems for years.
How a bad roof actually fails, which is slowly
People imagine a bad roof announces itself with a dripping ceiling. Occasionally. The common version is worse, because it is quiet.
A slow leak does not run. It seeps, and it goes into the attic, where nobody looks. By the time it comes through a ceiling and you finally go up there, the water has been in the insulation and the framing for a long time, and what you find is black mold across an area far larger than the stain downstairs.
The other slow failure is structural. Water in the decking over years softens it. From the street the roofline starts to dip, rise and dip again. From inside the attic you can see the deck has gone concave between the rafters. Then someone walks it and it cracks underfoot. At that point the shingles above it are most of what is holding the deck together.
None of that is dramatic and all of it is expensive, which is the argument for stripping to bare deck and looking at what is there. It is also why we say what the per-sheet rate for decking replacement is up front, rather than pretending the question will not come up.
The good version exists too
It is worth saying that we tear off plenty of roofs and find the work underneath was done properly: stripped clean, decking sound, valleys done right. It is a pleasant surprise and it says something about whoever was there before us.
That is the standard. Not a heroic one, just a roof somebody put on the way it is supposed to go on.
Related questions
Can a new roof be installed over the old one?
It is sometimes permitted and it is very rarely a good idea. You inherit whatever is underneath, you cannot see the decking, the extra weight is real, and it commonly voids the manufacturer warranty on the new shingle.
What is zippering?
A vertical run of shingles working loose and slipping down the slope, usually because they never sealed properly to one another. It is a common consequence of laying a roof over old felt or old nails, and it is visible from the ground.
What is ice and water shield, and do I need it in Texas?
A self-adhering membrane that bonds to the decking in the valleys so water cannot get under the valley metal. The name is about cold climates, but the reason it belongs on a Texas roof is water volume rather than ice: valleys concentrate everything the roof catches.
How would I know if my last roof was done badly?
Look along the courses from the street for waves, and for any vertical line of shingles that has started to slip. Inside, look at the attic decking for sagging between the rafters and for staining. Neither is conclusive, and both are worth an inspection.
Why does bad decking matter if the shingles are new?
Because the shingles are fastened to it. Soft or cracked decking does not hold nails properly, so the roof above it is only as attached as the wood underneath, and the failure appears in the next storm rather than in year twenty.
