Flat & Low-Slope Roof Repairs

Warning Signs a Low Slope Roof Needs Attention

A low slope roof sheds water slowly, so small defects often show up indoors weeks after the damage started. In Ontario, freeze-thaw cycles open seams and flashing each spring, and summer storms test every drain at once.

Ponding water is the sign owners ask about most, and the roofing industry has a clear definition for it:

• Ponding: water still on the roof 48 hours after rain in drying weather, the threshold set in the OIRCA technical bulletin on ponding water.
• Minimum slope: 2%, or ¼ inch per foot, per the same bulletin.
• Rule of thumb: a puddle gone by the next afternoon is normal; standing water on day three is a drainage problem.

Left alone, a roof leak or ponding water can lead to these problems:

• Wet insulation loses its insulating value and adds weight to the roof structure.
• Mould and mildew growth starts in ceilings, drywall and stored goods.
• Long-term moisture causes structural damage, rotting wood decks and rusting steel.
• Water from one split seam travels metres under the membrane and raises the bill.
• Leaks over stock or equipment can close part of a building.

Roof Repairs for Homes, Businesses and Industrial Sites

We handle commercial and residential roofing repairs, and the priorities change with each property type.

Residential Flat Roof and Overhang Repairs

Homeowners usually call about a flat roof over an addition, a garage, a porch overhang or a rooftop deck. The usual causes are failed seams, flashing pulled away from a wall, and an ageing roofing membrane that has blistered.

On residential flat roofs, we check where the flat section meets the wall or the shingles of the main roof, because that joint leaks more often than the open surface. One homeowner’s overhang had been poorly built by another roofer 10 years earlier and rot had set in, so our crew rebuilt it.

Commercial Building Roof Repairs

On a commercial building, leaks start at drains, seams, curbs and pipe penetrations rather than across the whole surface. Water travels under the membrane before it shows inside, so we trace each leak to its source instead of patching above the stain.

When a tenant’s unit leaked during a heavy rain, our crew patched the roof the same day, and the owner’s roof replacement was finished in four days, before the next rainfall. For plazas, offices or schools, we work around tenants and opening hours.

Industrial Flat Roof Repairs

Industrial roofs carry HVAC units, exhaust fans and foot traffic from service crews, and some are exposed to chemicals from production. Punctures around equipment and worn paths between units are common.

We repair the damaged areas, install walkway pads where technicians travel, and set up maintenance visits for large roofs. Many older plants have a metal roof; when panels rust or seams leak, we price both a repair and an insulated PVC retrofit.

Matching Roofing Materials on PVC, EPDM and Gravel Roofs

A repair only holds if it bonds to the roof underneath, so the first question is what the existing roof is made of.

• PVC and TPO: seams are heat-welded, so a well-made patch becomes part of the membrane instead of a glued layer on top, unlike EPDM.
• Tar and gravel: the gravel is pulled back before a repair and put back afterwards, because it shields the asphalt from UV rays.
• Never mix systems: asphalt cement on a PVC roof does not bond, and it can damage the PVC.

What Flat Roof Repair Costs in Ontario

Minor repairs, such as sealing a leak or patching a crack, cost $500 to $2,000, and repairs for extensive damage can exceed $5,000. Three factors set the price.

• 1. Roof size and access. Larger roof areas need more material and more labour hours. Setup takes longer on roofs with no interior hatch, or no ground space for a lift.
• 2. Membrane type. Some membranes cost more to patch than others; material costs per square foot are in the table below.
• 3. Labour rates and roofing industry safety rules. Roofing labour in Ontario averages $25 to $100 per hour, higher in urban centres such as Toronto. Ontario also requires workers on construction projects who use fall protection to have completed Working at Heights training, valid for three years. Ask any roofing contractors you compare for proof of current training.

Book a Roof Inspection Before the Next Storm

Call 1-800-263-8452 or request a free quote online, Monday to Friday, 8 a.m. to 4 p.m. To make the first visit faster, tell us:

• Where the water shows inside, and whether it appears during rain or days later
• What the roof is made of, if you know
• The roof’s approximate age and size

After the inspection you get a written report on the roof condition and a price.

When a Patch Is Not Enough: The Overlay Option

Some roofs have been patched so often that the next repair only buys a season. A full roof replacement means tearing off and disposing of the old membrane, gravel or shingles, which adds several dollars per square foot.

The middle ground is an overlay: a new PVC membrane installed over the existing tar and gravel, modified bitumen or built up roof, or over a metal roof. Why we recommend it:

• Fewer seams: Duro-Last membranes are custom-made for each roof, and the manufacturer reports up to 85% of seams completed in the factory.
• No open flame: the remaining seams are hot-air welded, and PVC has good fire resistance.
• Lower cost: no tear-off, no disposal, and a new roof with a long life expectancy.

We recommend an overlay only when the deck and insulation are dry and sound, because wet insulation has to come out first. One industrial owner chose this route for a 35-year-old corrugated metal roof.

Flat Roof Repair FAQs