Seam Failure Repair in Daly City, CA

A seam is where two sheets become one roof, so when it opens, the roof is no longer a single skin. On commercial roofs in Daly City, CA, we find open seams with a probe, not just with the eye: a thin tool run along the edge of each lap tells us whether the weld or bond is tight. Some seams fail as a visible gap, some as a slow lift, and some only show when a probe slides under the edge. We mark every failure on the plan, photograph it and note the seam type, because a hot-air weld, an adhesive splice, a tape and a torched lap each call for a different repair. Then we look for the reason the seam let go, since a repair that ignores the cause will open again.

Access matters as well. A seam under a rooftop unit, behind a parapet or inside a crowded penetration cluster is harder to reach and to weld, so we note any obstruction and plan the sequence. Sometimes a unit has to be lifted or a conduit moved to expose the lap, and we coordinate that with the building's mechanical or electrical contact before starting.

The next decision is whether to repair individual laps or treat a long run.

Thermoplastic sheets such as TPO and PVC are joined by hot-air welding, so a failed seam is repaired by cleaning the surfaces and welding a cover strip across the lap or re-welding the open section. EPDM is joined with adhesive or tape, and repair means cleaning, priming and applying new tape or a patch. Modified bitumen uses torched or cold-applied laps that can be reheated or re-adhered. Built-up roofs have felts that can delaminate at laps. We confirm the system before bringing materials up, because mismatched adhesives and welding temperatures can damage the sheet. On a roof with more than one membrane, such as an older section tied into a newer one, we repair each seam by its own system and make a clear note at the tie-in. The lap between two different materials is a flashing detail and not a simple seam, and we treat it that way.

Seams open for a handful of reasons. Dirt or moisture on the surfaces at the time of welding keeps the sheets from fusing. Movement across a joint in the deck stresses the lap. Ponding water sits on a seam and softens the bond. A shrinking membrane pulls on the lap. Thermal cycling and foot traffic add wear. We look at the failure pattern: a single open seam suggests a local fault; a series of openings along one line points to deck movement or a systemic problem. Seams near drains and parapets deserve special attention, since those are the places where water and movement are strongest.

An open seam is rarely alone. We probe the entire length of the seam and the seams on either side, and we mark soft or weak spots even if they have not opened yet. We also pull a test sample on welded seams when the system allows, to measure whether a new weld will bond to the old sheet. Aged membrane can become contaminated with dirt or plasticizer loss, which makes welding difficult, so we clean it and test again. If the sheet will not accept a weld, the repair may call for a mechanically fastened strip or a liquid-applied seam sealer instead.

Quality control is simple but strict. After every weld or tape application, we probe the finished repair, check for voids and pinholes and look at the edges of the cover strip. Each strip is rounded at the corners so water does not catch an edge, and strips that cross a T-joint get an extra patch because that is where a seam is most likely to open. We photograph the completed repair with a ruler or a marked tape in view so the length and width can be checked later.

Per the San Francisco International Airport station, the year totals 19.64 inches, with 16.76 of those in November through March and only 0.18 in June through August. Welding and taping need dry, clean surfaces, so seam repairs are best done in the dry months. In the wet season, a temporary seam closure holds until conditions allow a permanent weld, and emergency calls are taken day and night if water is entering the building. On any day, we check the seam for condensation and wipe it dry before working, because moisture trapped under a weld or tape defeats the bond. The same routine applies to a building in Pacifica, where commercial areas sit at district centers and along Highway 1.

A seam repair is priced by length, by type and by access. A few feet of re-welded lap near a drain is a short visit. A run of failed seams across a large field takes crew time and may be cheaper to treat with a restoration coating or a recover than to patch one by one. In Brisbane, the Crocker Park Trade Commercial designation covers light fabrication, printing, warehousing, retail and services, and South San Francisco lists Oyster Point among its specific plan areas. Roofs across such mixed uses differ, so we price each roof on what the probe finds. A leak repair that depends on a seam is often part of a larger repair plan.

Closeout: Document every lap that was opened and closed

At completion, we photograph each repaired seam, show the cover strip or tape, and mark the plan with the length repaired. We also list seams that were probed and found sound. The record shows the owner which sections are repaired and which are on watch. If ponding contributed to the failure, we pair the seam work with a drainage correction for the ponding water, and if punctures are present, a puncture repair goes on the same plan.

Commercial Roofing Questions

How can I tell whether a seam is failing?

Look for gaps, lifted edges, dark lines of dirt under a lap and fish-mouth wrinkles. A probe shows what the eye misses, so a walk with someone who carries one is worth more than a look from the roof hatch. If water is coming into the building, the nearest seams are among the first places we check.

Is a seam failure a sign the roof needs replacing?

One failed seam is usually a repair. A roof with seam failures across the field, wet insulation and a brittle membrane may be near the end of its life, and then the better comparison is a recover or replacement priced against the cost of continuing to chase individual laps.

What if the membrane will not weld?

Aged or contaminated membrane may not accept a weld. We clean the surface, test a small weld and, if it fails, use a mechanical or liquid-applied method instead.

Can seams be sealed with a coating?

A liquid-applied coating can seal and protect seams on a sound roof. It does not repair a roof with wet insulation or a deteriorated sheet, so we check those first.

Does the city require a permit for seam repairs?

That depends on the scope, and the City Building Division can confirm it. Its Title 24 plan review and field inspection cover construction work, and the 2025 California code edition, with local amendments, has applied since January 1, 2026.