Flat and low-slope roof problems: what goes wrong and how it gets fixed

Why flat roofs need different treatment

A pitched roof sheds water fast. Shingles and tile work by overlapping, so gravity pulls rain down and off before it has a chance to get underneath. A flat or low-slope roof does not have that advantage. Water moves slowly, and sometimes it does not move at all.

That is why flat roofs use continuous, sealed materials instead of overlapping pieces. It is also why shingles and tile are not allowed on very low slopes. Under California code, asphalt shingles generally need a slope of at least 2 in 12, and tile needs a bit more, so anything flatter needs a low-slope system made for the job.

In the High Desert and the Inland Empire you see flat and low-slope roofs on additions, garage conversions, porches, patio covers, and on some whole homes. Each one comes with the same basic weak points.

Ponding water: the number one problem

Ponding is water that stays on the roof long after the rain stops. A common rule of thumb in the industry is that water still sitting there about 48 hours later is a drainage problem, not just a puddle.

Ponding causes trouble in several ways:

• It keeps constant pressure on seams and any small defect in the membrane.
• It speeds up UV and heat wear around the edges of the puddle, where wet and dry areas meet.
• It collects dirt and debris, which holds more water and can grow algae.
• Its weight adds up, and a low spot tends to get lower over time.

Ponding usually points to a sagging deck, clogged drains or scuppers, or a roof that was never built with enough slope. A proper repair addresses the cause. That can mean clearing and resizing drains, adding tapered insulation or crickets to push water toward the drain, or repairing the deck under the low spot.

Seams, edges and flashing

Most flat roof membranes come in rolls or sheets, so every roof has seams. Those seams are sealed with heat, adhesive or torch depending on the system. When a seam loses its bond, water gets in, often far from where it shows up on the ceiling.

Other common leak points include:

• Edges and drip edge, where the membrane turns down over the fascia and wind can lift it.
• Wall flashing, where a low-slope roof meets a taller wall of the house. This is one of the most common leak spots on additions and patio covers.
• Penetrations like vent pipes, skylights, HVAC stands and swamp cooler lines.
• Blisters and cracks in the membrane surface, often from trapped moisture or years of sun.

High Desert heat and cold nights make materials expand and contract every day, which puts a lot of stress on seams and flashing. A roofer checking a flat roof should look closely at all of these, not just the spot above the stain.

Patio covers and additions

Patio cover roofs are often the lowest slope on the property, and many were added after the house was built. That creates a few typical problems:

• The connection to the house was flashed poorly, so water runs behind the patio roof and into the wall.
• The slope is too shallow, so water sits instead of draining to the edge.
• Rolled roofing or another short-life material was used and has simply worn out.
• Solar panels, lights or fans were mounted through the roof without proper sealing.

Additions can have the same tie-in problem where the new roof meets the old one. Fixing the flashing at that joint, and making sure water has somewhere to go, solves a lot of these leaks. If the house roof is shingle or tile and the tie-in is leaking, the repair may need to open up part of the main roof as well. Our tile roof leaks post covers how that works on tile.

Repair or replace a flat roof?

A flat roof repair makes sense when the problem is local: one failed seam, a damaged flashing, a single penetration, or a small blistered area, and the rest of the membrane is in good shape.

Replacement usually makes more sense when:

• The membrane is cracked, brittle or blistered across much of the roof.
• Leaks keep coming back in different places.
• There is wet insulation or soft, damaged decking under the membrane.
• The roof ponds badly and the slope needs to be rebuilt anyway.

Coatings can extend the life of some flat roofs, but they are not a fix for a wet or failing roof underneath. Ask what is being coated over and whether the existing roof is dry. For more on this decision in general, see repair or replace? How to decide in the High Desert.

Code and energy rules for low-slope roofs

Replacing a flat roof is usually a permitted job, the same as a pitched roof. Check with the City of Hesperia Building and Safety or San Bernardino County Land Use Services, depending on where you live, or let your roofer pull the permit.

California's energy code also has cool roof rules for low-slope re-roofing, and the reflectance targets are higher than for pitched roofs. We explain them in California cool roof rules for re-roofing. If your home is in a Fire Hazard Severity Zone, the fire rating of the roof assembly matters too, so ask your roofer whether the system being quoted is Class A.

Keeping a flat roof healthy

Flat roofs reward a little attention. A few simple habits help:

1. Keep drains, scuppers and gutters clear, especially before monsoon season and winter storms.
2. Keep foot traffic to a minimum and stay off the membrane on hot afternoons when it is softer.
3. Have the roof looked at after strong wind, since lifted edges are easy to miss from the ground.
4. Get small problems repaired early, before water soaks the insulation or deck.

A regular inspection catches most of this. Here is what happens during a roof inspection.

How Ocelot Roofing can help

We repair and replace flat and low-slope roofs on homes, additions and patio covers, and we pay close attention to where the water goes. We will find the real source of the leak, show you what we found, and give you a free, itemized estimate for the repair, and for replacement if that is the better long-term answer.

The owner, Manny Torres, oversees every project, so you have one accountable person from the estimate to the finished roof. See our flat roofing services, request a free estimate, or call (866) 837-3311.

Common questions

About the author

Manny Torres, Owner, Ocelot Roofing

Manny Torres owns Ocelot Roofing and personally oversees every project from estimate to final cleanup. Ocelot Roofing is part of Verde Home Design Inc., a third-generation family business based in Hesperia, CA.