Flat Roof Repair For Homes: Why They Leak and What Actually Fixes Them

A "flat" roof is never truly flat. It has a slight pitch so rain drains toward a gutter, scupper or drain. When that pitch is too shallow, or the roof sags, water stays put. One home inspector's long-standing observation is that some flat roofs are adequately sloped toward drains but many are not, and water simply ponds and will only be dispersed by evaporation.

Most leaks on these roofs come from a short list of causes:

• Ponding water. Carlisle, a major membrane maker, says ponded water should evaporate within 48 to 72 hours after a rainfall, and that debris can inhibit drainage. Polyglass, another manufacturer, notes that 1 inch of ponding water can weigh more than 5.2 pounds per square foot.
• Seams. InterNACHI says that on low-slope roofs seams which get soaked by ponding may fail.
• Flashing at walls and penetrations. Roof penetrations and intersecting planes are sealed with flashings, and those joints are a common leak point. IKO notes that ponding deeper than the flashing is tall can let water into the structure.
• Membrane movement and age. On EPDM, shrinkage may pull flashing loose or cause craze-cracking, and it can stress the seams too.
• Freeze and thaw. In New England, standing water that freezes and thaws works on the surface. IKO says freeze/thaw cycling can erode the mineral surface on some roofing materials, such as modified bitumen.

The spot where a stain shows up inside is often not the spot where water gets in. GAF's manual warns that leak water may move some lateral distance from the roof leak, which makes the entry point harder to find.

Manufacturers publish repair methods, and they are written for trained installers. They involve torches, heat welders, primers and solvents on a roof surface. This section describes what a proper repair involves so you can ask better questions. It is not a how-to, and none of it is a job for a homeowner.

EPDM. Carlisle's restoration instructions call for a repaired area where the new membrane extends a minimum of 6" past the repair area for proper splicing, with surfaces cleaned and primed first. The same document says that cuts, holes, tears, and punctures that will lead to moisture infiltration in the future must be repaired. Carlisle's owner's manual, written for owners of its roofs, says that if minor emergency temporary repairs are made to a suspected leak area, its lap sealant or a good-grade rubber caulk should be used, and not asphaltic roof cement. Reaching the spot means going onto the roof, which this guide does not recommend.

TPO. GAF's installation manual says remedial work must be performed with like materials, consistent with the rest of the roof, so that the number of patches stays low. TPO patches are bonded with hot-air welding equipment, not with roof cement from a tube.

Modified bitumen. Roofing Contractor magazine says open or unadhered seams are often the result of improper torch application and that proper repairs require torch or heat welding of the seams. Before any patch goes on, the article says the area has to be cleared of debris and contaminants, and the prepared area should extend beyond the perimeter of the patch so the patch sits on clean material.

The pattern is the same across all 3. The surface is cleaned, the patch is the same family of material as the roof, it overlaps the damage, and the edges and seams are sealed. A bead of the wrong sealant over a crack rarely does that.

Homeowners often ask whether a flat section can be re-covered with ordinary shingles. The model residential code says no below a set slope. In the Rhode Island edition of the 2021 IRC, section R905.2.2 states that asphalt shingles shall be used only on roof slopes of 2 units vertical in 12 units horizontal (17-percent slope) or greater. Between 2:12 and 4:12, the same section requires double underlayment.

Low-slope coverings have their own minimum. The same chapter sets a design slope of not less than 1/4 unit vertical in 12 units horizontal for modified bitumen, thermoset single-ply (EPDM) and thermoplastic single-ply (TPO) roofs. A roof pitched under 2:12 needs a membrane system that is built to hold water out, not shingles that rely on gravity to shed it.

This guide cites the Rhode Island text only. For a Massachusetts home, a roofer or the local building department can confirm the rule that applies in your town.