Why does a flat roof hold water when it should drain?
A flat roof is built with a slight fall toward its drains, but that fall is often too small to move water quickly. Where the deck has sagged or the drain sits higher than the lowest point, water collects and stays.
Standing water is a load, and a membrane holding a pool for days stretches and wears faster at the seams. The water also carries dust and debris that settle into the surface, and those deposits feed the growth that blocks the drain.
A pool that dries and reforms on the same spot shows a level fault. Correcting it is a larger job than a seam repair, and it is often the reason a repaired roof fails again in the same place.
What are the common membrane types on a flat roof?
The common types are single-ply sheets, such as EPDM or TPO, and modified bitumen, which is a layered asphalt product. Each has its own seam method, its own weakness and its own repair.
Single-ply membranes are joined with heat-welded or adhesive seams, and a patch on them has to be bonded to a clean surface. Modified bitumen is torched or cold-applied at its laps and is repaired by cutting out the blister and laying a new cap sheet.
The technician identifies the type before any repair is chosen. A patch from the wrong family will not bond, and a bond that fails in a season is a waste of the owner's money.
The technician also looks at the membrane age and its exposure to the sun. A sheet that has been in full sun for many years loses its plasticiser, and it becomes stiff and prone to splitting at the seams. A patch placed on a stiff sheet can crack at its edge within a season, so the repair is matched to the age of the material as well as to the size of the split. Where the membrane is well past its useful life, the technician says so and recommends a full covering rather than a row of patches.
How is the flashing at the parapet checked?
The flashing is the strip that carries the membrane up the wall, and it is the place most flat roofs leak first. It is checked at every parapet, curb and penetration, with the top edge sealed and the lap facing the way water flows.
A flashing that has been nailed through the top edge without a cover can let water run behind the membrane. The fix is a new cap, a bonded lap and a reseal of the top edge, which keeps the water outside the wall.
Curbs around air handlers and roof hatches are checked the same way. A curb that has lost its sealant is one of the more common sources of an interior stain on a commercial building.
What should a building owner keep on file?
A short record of each repair: the date, the location, the membrane type, the patch size and the test result. It makes the next inspection quicker and helps any later decision about replacement.
Photographs of the ponding areas before and after the work show whether the drainage has changed. A dated set of photographs is the simplest evidence of maintenance the owner can keep.
Drain cleaning is recorded on the same sheet. A roof with a clear drain and a sound seam usually lasts longer than one with neither, and the record shows which of the two has been managed.
When a patch is placed on a building that has been patched before, the earlier patches are mapped on the same sheet. A new patch next to an old one needs its own overlap, and a patch laid over an earlier patch can trap a film of moisture between the two layers. Mapping the patches also shows whether the failures are clustered around a drain, a parapet or a single seam, which tells the owner where the next spending should go.
