Flat Roof Repair and Replacement for Garages, Porches, Additions and Two-Flats

Signs you may need flat roofing

• Shingles or roll roofing on a porch or addition that barely pitches
• Puddles still sitting on the roof two days after a storm
• Blisters, alligator cracking or bare patches on an asphalt or coated surface
• Membrane or flashing pulling away where the low roof meets the house wall
• Ceiling stains under a porch roof or in the garage after snowmelt
• Soft, spongy spots underfoot

Why shingles leak on a flat or low-slope roof

Shingles do not seal a roof; they shed water. Each course overlaps the one below and relies on gravity to carry water across the laps faster than it can work underneath. On a steep roof that takes seconds. On a shallow one, water lingers, wind pushes it uphill under the laps, and in winter an ice ridge at the edge holds meltwater against them for days.

That is why codes based on the International Residential Code do not allow asphalt shingles below a 2:12 pitch, meaning 2 inches of rise for every foot of run, and require a double layer of underlayment from 2:12 up to 4:12. Below 2:12 the roof needs a continuous waterproof membrane. Porch roofs and additions built by a previous owner are common places to find shingles on a pitch that was never going to work.

To check your own roof from a ladder at its edge, rest one end of a level on the roof surface, hold it level, and measure straight down from the 12-inch mark to the roof. Less than 2 inches means shingles are the wrong material.

TPO, EPDM, modified bitumen and coatings compared

TPO

Thermoplastic polyolefin is a single-ply sheet, usually white, with seams fused by hot air into one continuous membrane. The white surface reflects summer sun, which helps a bedroom or office over a garage stay cooler. It suits roofs with few penetrations and light foot traffic.

EPDM

EPDM is a synthetic rubber sheet, typically black. On a small garage it can often go down as one piece with no field seams; where seams are needed, they are bonded with primer and seam tape. It stays flexible in deep cold and is simple to patch, though the dark surface runs hot in July.

Modified bitumen

Asphalt blended with rubber-like SBS or plastic-like APP polymers, installed in two plies with a granule-surfaced cap sheet. It stands up to foot traffic better than most single-ply sheets, which makes it a common choice under a walkable area. Self-adhered and cold-applied versions keep open flame off a wood-framed house.

Coatings

Silicone and acrylic coatings are rolled or sprayed over an existing roof that is still sound and dry underneath. They can add years to a weathered membrane but cannot fix wet insulation or rotted decking. Acrylics do not tolerate standing water; silicone handles ponding better but is slick when wet, and generally only more silicone will bond to it later.

Product details and expected lifespans are in our flat roof membrane guide.

Drainage first: tapered insulation, crickets and scuppers

Membrane roofs are designed for a minimum slope of about a quarter inch per foot. Many older flat roofs have less, or had it once and lost it as joists sagged. Before we price a membrane, we work out where the water should go:

• Tapered insulation. Rigid foam boards made with a built-in slope, commonly 1/8 or 1/4 inch per foot, are laid over the deck to steer water toward an edge or drain. They also add insulation, which matters when a living space sits below.
• Crickets. Small built-up diamonds or triangles split water around chimneys, skylights and the high side of a wall so it cannot collect there.
• Scuppers and gutters. A roof ringed by a low parapet drains through scuppers, openings through the wall, into a collector box and downspout. An open-edge roof drains into a gutter that must be sized for the flow; our seamless gutter page explains sizing.
• Reframing. On a porch roof with almost no pitch, it is sometimes simpler to fasten tapered sleepers or new joists over the old ones and build real slope.

Drains on the north side of a building can freeze shut in January while the rest of the roof melts. Placing outlets where the sun reaches them, and clearing them every fall, prevents a lot of winter ponding.

Flashing where a flat roof meets the house

On houses, the leak is rarely in the middle of a flat roof. It is where that roof meets something taller: the wall of the house, a door onto a deck, or the bottom of a shingled slope that drains onto it.

• Siding walls. The membrane should turn up the wall well above the roof surface and be covered by the siding or a counterflashing, not caulked to its face.
• Brick walls. Counterflashing is set into a cut mortar joint so water running down the brick lands on top of the flashing, not behind it.
• Doors. A door onto a flat roof needs a threshold high enough for the membrane to run underneath, with a pan flashing below the sill.
• Shingle-to-flat transitions. Where a pitched slope empties onto a flat section, the membrane continues up under the lowest shingle courses so snowmelt crossing the joint stays on top of it.

Flat roof repair or replacement on Chicago-area homes?

A punctured membrane, an open seam or a failed flashing at the wall is a repair, as long as the surrounding material is still flexible and the insulation is dry. We press on the roof as we walk it; soft areas mean wet insulation or a deck that is starting to rot, and small core samples confirm how far it reaches.

Replacement makes more sense when several layers of old roll roofing or built-up asphalt are already stacked on the deck, when wet areas are spread across the roof, or when the drainage itself has to be rebuilt. The old layers come off, damaged decking is replaced, and the new system goes down over insulation and slope that work. For leaks on the pitched parts of the house, see roof repair.

What it costs

Flat roof pricing depends on the area, how many old layers come off, whether insulation or decking is wet, the membrane chosen, and how many walls, doors and penetrations need flashing. Small porch and garage roofs cost more per square foot than large ones because setup, edge metal and wall details make up a bigger share of the job. Your written estimate after inspection is the firm number.

How the work goes

1. Inspect and probe

   We walk the roof, check every wall tie-in and test soft spots for wet insulation.

2. Plan the drainage

   Slope, crickets and outlets are laid out before a membrane is chosen.

3. Tear-off and deck repair

   Old layers come off and damaged decking is replaced.

4. Membrane and flashing

   The new system goes down with wall, edge and penetration details finished before the crew leaves.

5. Water check

   We watch how the roof drains and photograph the finished details for your records.

Flat Roofing questions

Can I put a deck or patio furniture on my flat roof?

Only if the roof and framing were designed for it. Chair legs and grill feet press into a membrane and eventually cut through, and a deck adds weight the joists may not have been sized for. A proper rooftop deck sits on pedestals or sleepers over a protected membrane, with a walking surface that lifts out for repairs, and it may need a permit and a structural review.

Is a white roof a good idea on a house in Illinois?

Often, when a living space sits below. A reflective membrane absorbs less summer sun, so the room under it heats up less. The trade-off is less winter heat gain, so snow may linger a little longer. On a detached garage the choice is mostly about appearance and glare from upstairs windows.

Why does my flat garage roof leak only in winter?

Usually for one of three reasons. Snow and ice dam the roof edge or freeze the drain, so meltwater backs up into seams and flashing that stay dry in a summer rain. Daytime melt runs into a small crack, freezes overnight and widens it. Or it is not a roof leak at all: in a heated or poorly ventilated garage, warm, damp air condenses on the cold underside of the deck and drips. Checking the edge and drain during a thaw, and the underside of the deck on a cold morning, tells a leak from condensation.

Can a flat roof be converted into a pitched roof?

Yes. New framing can be built on top of the existing roof to create a shallow-pitched or gabled roof that takes shingles. It changes the look of the house, adds load, and needs a permit and usually drawings. If the only goal is drainage, tapered insulation under a new membrane solves the problem for much less.