Underlayment
Also called: felt, roofing felt, synthetic underlayment, tar paper
Underlayment is the water-resistant layer laid over the decking and under the shingles. It is the roof's secondary barrier, catching anything that gets past the covering, and it is one of the most frequently disputed line items on a roof estimate.
What it means
Underlayment is the layer between the decking and the roof covering. It is a water-resistant sheet, historically asphalt-saturated felt and now increasingly a synthetic woven material, rolled out over the deck and fastened before the shingles go on.
Its job is to be the second line of defence. The covering is the primary barrier, and it is not perfect: wind-driven rain gets under shingles, ice forms at eaves, and individual shingles fail. The underlayment catches what gets through and keeps it off the structural deck.
Why it matters more than its cost suggests
It is a modest line item on an estimate and it protects the most expensive component of the assembly.
Water reaching bare decking causes rot, delamination, and eventually structural replacement, along with everything that follows underneath: insulation, ceilings, finishes. The underlayment is what stands between an ordinary shingle failure and that sequence.
It also does real work during construction, keeping the deck dry between tear-off and covering, which matters on any job that runs across weather.
Why it appears in scope disputes
Because whether it is included depends on whether the scope is a repair or a replacement, and that is exactly the thing carriers and contractors argue about.
On a full tear-off the underlayment comes off with the covering. It is fastened underneath, it is disturbed by removal, and reusing it would produce a barrier full of holes and tears. So a replacement scope includes it, and a full-replacement estimate without it is missing something.
On a repair confined to a small area, the existing underlayment may reasonably remain. So a carrier scoping a repair will not include it, and that is not an omission but a consequence of the scope.
Which means an argument about underlayment is usually an argument about repair versus replacement wearing different clothes. Resolving the underlying scope question resolves this one automatically.
The code dimension
Requirements for underlayment type, coverage, and fastening are set by local building code and vary. Where current code requires more than what is on the roof today, a replacement may have to meet the current standard rather than reproduce the existing build-up.
Whether the additional cost of that is covered is an ordinance or law question and depends on the policy. The requirement itself is a matter for the local authority rather than for the carrier or the contractor to decide.
Common mistakes
Leaving it off a full replacement estimate is a straightforward error that costs the homeowner.
The second is arguing it in isolation when the real disagreement is about repair versus replacement.
The third is reusing disturbed underlayment to save a small sum on a job whose whole purpose is to restore the roof properly.
On the call
Homeowners rarely raise underlayment themselves, but it is a common omission when a carrier scopes a repair rather than a replacement. If a caller is comparing two estimates and cannot see why they differ, this is one of the first lines worth checking.
This is how our agents handle it on storm appointment calls and day-to-day roofing answering service intake.
Questions people ask about underlayment
- Does underlayment have to be replaced with the shingles?
- On a full replacement it normally is, because it is fastened under the covering and comes off with it, and reusing disturbed underlayment produces a compromised barrier. On a partial repair confined to a few shingles, the question is narrower.
- What is the difference between felt and synthetic underlayment?
- Traditional felt is an asphalt-saturated paper product; synthetic underlayments are woven polymer sheets. Synthetics are generally lighter, stronger, more tear-resistant, and less prone to wrinkling, and they have become the common choice on new work.
- Why would a carrier leave underlayment off the estimate?
- Usually because the scope is a repair rather than a replacement, so the existing underlayment is assumed to remain in place. Where the scope is a full tear-off and the underlayment is nonetheless missing from the estimate, that is a straightforward omission worth raising.
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