Assembly
Self-adhered underlayment: the layer that decides how a tropical system ends
Every other layer of a roof assumes water is running downhill at a reasonable speed. Self-adhered membrane exists for the situations where it is not: driven sideways and uphill by sustained wind, concentrated in a valley that cannot clear fast enough, or standing on a patio cover that should never have had shingles on it.
You will hear it called ice and water shield, which is what the category was named for and what it is still labeled as. On the Gulf Coast the name is a historical artifact. The product is identical and the reason for specifying it is completely different.
What it actually is
A sheet of asphalt modified with rubber polymers, with an adhesive back and a release film. It is rolled onto the bare deck and sticks permanently. Two properties make it different from anything else on the roof: it bonds continuously to the deck rather than being nailed over it, and it self-seals around a nail shank driven through it. So even where the shingles above it are fastened straight through the membrane, the penetrations do not leak.
Synthetic underlayment, by contrast, is a shedding layer. It is mechanically fastened, it sheds water that gets past the shingles, and every fastener through it is a hole. That is entirely adequate across the field of the roof and inadequate wherever water concentrates or arrives from the wrong direction. See synthetic underlayment.
Why wind-driven rain is the mechanism here
A shingle roof is a set of overlapping shedding laps. It works because water runs downhill fast enough that it never has time or pressure to travel back up under a lap. A tropical system removes both of those assumptions at once: rain arrives close to horizontal, for hours rather than minutes, with a pressure difference across the roof surface actively pushing water upslope. Wind that lifts a tab also opens the lap it was sealing.
That is the condition the membrane is for, and it is the reason the coverage decision matters more on this coast than a northern estimate template assumes. A roof detailed to shed vertical rain is not detailed for a hurricane, and the difference is under the shingles where nobody will ever see it.
Where it goes on a roof in this climate
- Valleys
- The single most important run. A valley concentrates the water from two planes into one line and it carries far more than any other part of the roof, and rainfall intensity in this region is high. Membrane the full length, wide, under whatever valley treatment is used. See valley.
- Penetrations
- Around every pipe, vent, turbine, skylight and chimney. These are the places where a shingle course has to be interrupted, and interruptions leak. Boots are also the shortest-lived component on a roof here, so the layer beneath them is what buys you time when one perishes. See pipe boot.
- Wall lines and headwalls
- Where a roof runs into a wall, behind the step flashing. Wind-driven rain moves horizontally at a wall junction and no shingle detail on its own handles that indefinitely. On the inner-Loop houses with a low-slope rear addition against a taller original wall, this is the leak. See step flashing.
- Low-slope sections
- Below about 4:12 the water moves slowly enough that shingles alone are marginal, and in this region wind-driven rain makes marginal into wet. Below about 2:12 shingles do not belong at all — see roof pitch.
- The eaves
- Worth including, and for a different reason than in a cold market. There is no ice to back water up behind, but the eave is where uplift is highest, where wind-driven rain is pushed hardest against the first course, and where the drip edge and starter detail have to work together. A run at the eave is cheap insurance on a coastal roof rather than a code requirement.
- Hips and ridges
- Sometimes specified, particularly on the coast, because these are the lines where uplift is highest and where a lifted cap exposes the deck directly.
On a Houston roof the valley and penetration coverage is the thing to ask about specifically, because it is the thing an estimate leaves out. A quote that says "install underlayment" and nothing else may or may not include it, and after the roof is on nobody can tell.
What to ask for, in the words that get it
- Whether valleys get it full length and full width, under the valley treatment.
- Whether penetrations and wall lines get it, or only the valleys.
- Whether the eaves and rakes get a run, and how wide.
- What happens at the low-slope sections — the patio cover, the carport, the rear addition. If the answer is "shingles", ask what the pitch is.
- What goes over the rest of the deck, because a synthetic underlayment across the field is normal and appropriate. The membrane is not meant to cover the whole roof.
- Whether the deck is being cleaned and dried first. It is an adhesive product. It does not stick to sawdust or a damp deck, and on this coast a deck left open overnight is a damp deck by morning.
Full-deck membrane coverage is often sold as the premium option. On a ventilated attic assembly in this climate it is usually the wrong answer: the membrane is a vapor barrier, and a vapor barrier on the underside of the deck in a hot humid climate can trap moisture in the sheathing that has nowhere to dry to. The place for full coverage is a low-slope section, an unvented assembly designed for it, or a specific problem — not the whole roof by default.
How it fails
| Problem | Why it happens | What it looks like later |
|---|---|---|
| It was never installed | Left off an older roof, or omitted from a cheap scope | A valley or a wall line that leaks in every sustained blow, and is dry in ordinary rain |
| Valleys only, nothing at the penetrations | A template scope written for a different climate | A perished boot becomes a ceiling stain instead of a maintenance item |
| It was laid on a dirty or damp deck | Rushed sequencing, or a deck left open overnight in humidity | It releases from the deck in sheets and stops sealing around fasteners |
| Laps run the wrong way | Upper course lapped under the lower one | Water runs into the lap rather than over it, which is the one thing the layer exists to prevent |
| Used instead of flashing | Membrane wrapped around a chimney in place of metal | It works for a few years, degrades fast in this ultraviolet load wherever it is exposed, and then leaks |
| Left exposed too long before the roof went on | Weather delay, or a job that stalled | Most of these products have a stated exposure limit. Past it the adhesive and the surface are compromised |
Want to know what is actually under the shingles at your valleys?
Request a free inspectionIs self-adhered membrane required by code here?
Requirements come from the code your jurisdiction has adopted and amended, and this metro is dozens of cities plus large unincorporated areas, so there is no single answer. In the designated coastal catastrophe area the applicable windstorm building code governs and the work has to be inspected against it. The practical approach is the same either way: ask your roofer what the requirement is for your specific address, and have the answer written into the scope.
My roof does not have it. Do I need to replace the roof to add it?
Effectively, yes, for the valleys and the field penetrations — it goes on the bare deck under everything else. That is why it is a question to settle before a replacement rather than after one. Targeted improvements are possible where a section is being reworked anyway, which is a good reason to deal with a problem valley properly rather than patching it.
Does it make the attic sweat?
It can, if it is used over the whole deck on a ventilated assembly, because it is a vapor barrier. That is exactly why it belongs at valleys, penetrations, wall lines and low-slope sections rather than everywhere. Adequate intake and exhaust ventilation is what keeps the deck dry in this climate — see attic ventilation.
Is one product better than another?
The meaningful differences are between granular-surfaced and film-surfaced products, in high-temperature stability, and in the stated exposure limit before the roof goes on — all three of which matter more on a roof deck in a Houston July than they would further north. This site does not name manufacturers. What matters more than the product is the coverage, the direction of the laps and the state of the deck it went onto. On the coast, add that the product needs a published evaluation for the designated area.
All work is performed by insured local roofing professionals. Texas does not license roofing contractors at state level, so insurance, references and a written scope are what there is to check.