Welded, Taped and Torched Seams
The repair has to suit the sheet. On PVC and TPO, we clean the surface, hot-air weld a new cover strip or reweld the lap, and probe it afterward to prove it closed. On rubber sheets joined with splice tape, we remove the failed tape and prepare the surface properly before a new splice goes down, since adhesion to a contaminated sheet is the usual cause of a second failure. Reseating the edge with a bead of sealant only after the seam is mechanically sound is the finishing step, never the repair itself.
On modified bitumen, the trouble sits at laps where the cap sheet has fishmouthed or where granules were left in the seam. We cut the fishmouth, heat or adhere a new patch with generous laps, and set it in the right direction for water. On metal roofs, we check lap fasteners and sealant tape, because the seam may be open at the fastener rather than at the panel itself, and the repair follows the cause.
Preparation Before the Weld
Most seams that fail twice were repaired once without being cleaned. A weld needs a clean, dry surface of fresh material, and a roof that has sat under dust and traffic for years has neither. We remove the dirt, scrub or wipe the weld area according to the manufacturer's method for that membrane, and wait for the surface to be dry. If the sheet is chalky or aged beyond the point where it will fuse, we say so rather than force a weld that will not last.
A repair strip over a failed seam also needs rounded corners, because a square cut becomes a stress point where it starts to peel. We cut each patch with radiused corners, lap it generously, and probe the edge afterward. The finished work is documented with photographs and a marked roof plan, so a property manager knows exactly which seams were touched and can find them again after the next heat wave.
Dry Heat, Thermal Movement and Large Flat Roofs
A roof sees its seams stressed every day in this climate. The membrane expands in afternoon heat and shrinks overnight, and Pasadena's recorded June high of 113 F shows how hot the sheet can get. In a dry climate where rain is only about 20.4 inches a year, a hidden seam opening can wait months for the first heavy storm to prove itself, and that long delay is why the problem is so often found only after water arrives.
The effect is largest on wide roofs with long uninterrupted runs of sheet. In Lamanda Park, where East Foothill Boulevard holds the city's most concentrated industrial space, any flex or warehouse unit with a single-ply roof has the same exposure, and a seam that opens near a wall can run in either direction with the movement of the field. We therefore check the seams nearest to curbs, walls and low areas first, since those are the places a roof moves the most.
Surveying Seams Before the Rainy Season
The cheapest seam repair is the one made before water enters. We recommend a seam survey in the autumn, after summer heat has done its worst and before the rain starts, probing the T-joints, corners and any lap that sits near a low spot. A survey that finds three small openings costs far less than a wet ceiling and a section of saturated insulation. We write up the findings with photographs, so a facilities manager at a campus like Caltech or near Huntington Hospital in the HeArt District can see which areas are sound and which need work.
Where seams keep opening in the same place, the cause usually sits beside them. A ponding area that keeps a seam wet, foot traffic that rubs through the sheet, or a puncture near the lap will all undo a good repair. Ordinary commercial roof repair on a seam is only half the job until that second cause is corrected.
Commercial Roofing Questions
How can I find an open seam before it leaks?
Look along the laps for a thin line of dirt at one edge, a lifted corner or a lap that a probe slides under. A seam with a gap you can see is already open. Walking the roof after rain also helps, since a seam that wicks water will show a damp line at the edge for a while after the surface dries.
Can an open seam be fixed with caulk or mastic?
On a few membranes, a small, clean and dry opening can take a sealant as a short-term closure. On most, the sealant covers a bond that is already gone, and the opening returns with the next cycle of heat. We reweld, retape or replace the seam, and use sealant only as a finish over a sound edge.
Is a failed seam a sign the whole roof is finished?
Seams that open in one area often trace to local movement, a pond or a poor original weld, and the rest of the roof can be perfectly sound. We probe the surrounding laps and sample the sheet. If the membrane still fuses cleanly and the seams elsewhere are tight, repair is worth doing.
Does the membrane type change how a seam is repaired?
Entirely. Thermoplastic sheets are welded with hot air, rubber is joined with tape or adhesive, bitumen is torched or mopped, and metal laps rely on sealant tape and fasteners. A repair that suits one will damage another, so we begin by identifying exactly what is on the roof.
Our seams have been repaired twice already. At what point do we stop?
The condition of the sheet decides it. When the same lap line keeps opening and the membrane no longer takes a clean weld, further repairs are money spent on a roof past recovery. We show you the test welds and the sheet condition, and price a recover or replacement beside the next repair.
