Flat Roof Repair – How to Repair a Leaky Rubber Roof – DIY Guide

This guide covers the basics of flat roof repair, also known as low-slope roof repair, including:

  • Locating the leak
  • Repairing flat roofing
  • Knowing when replacing the roof is a better option
  • The cost of flat roof repair and replacement
EPDM Rubber installed on a flat roof by GemTile

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If your home or building has a flat roof, then you know that water is its major nemesis and leaks must be aggressively addressed with top-quality flat roof repair materials and methods. We explore whether DIY flat roof repair is something you want to attempt or if it makes more sense to hire a professional flat roofing specialist.

These flat roof repair techniques can be used on the most common flat roof materials:

  • EPDM, aka Rubber
  • Modified bitumen
  • BUR (Built up roofing), aka tar and gravel

PVC roof repair
Note: If you happen to have a PVC or TPO roof leak, such as a leaky seam or a hole/puncture caused by a fallen tree branch or an accidental cut, then you will need to find a qualified specialist equipped with all the necessary tools and supplies; a roll of appropriate membrane and a heat air gun. Look for a company specializing in PVC or TPO.

Locating a Flat Roof Leak

While finding a leak in a flat roof is easier than on a steep-sloped, shingled roof, leaks can still be deceiving. Where water enters your interior space might not be exactly where the leak is, because any water that penetrates a roofing membrane can travel for some distance until it finds a seam in the sub-roof or sheathing and seeps inside.

These steps will allow you to isolate the source of the leak if it isn’t readily evident when you inspect the roof.

  • From inside, measure the distance from where the leak is visible to the two nearest walls.
  • Use those measurements on the rooftop to create a starting point for your search.
  • Most flat roofs are very slightly sloped to facilitate runoff, so explore from the point of the water’s entrance up the slope, since water runs downhill.
  • Look for punctures, popping screws, rips, worn spots, seam separations, blisters and other imperfections in the membrane.
  • If no damage is found in the membrane, examine nearby vent pipe boots, flashing, caulk around vents and similar structures for damage.
  • When you still cannot locate the leak, call a professional flat roof contractor for assistance and repair.

Caution: It’s always smart to respect ladders and heights, so proceed with extreme care! If you can’t access the roof from inside, use a quality ladder rated for more than your weight, and have a strong adult hold the ladder steady for you. If the roof is icy, even greater precaution is required, or a pro should be called in.

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Repairing a Minor Flat Roof Issue

Warranties for flat roofs can range from 5 to 20 years. If your roof is under warranty, call the contractor that installed it to make the repair. Even if the warranty is prorated for the age of the roof, you might still save money with pro repair. You’ll have the confidence the job was done correctly and have recourse if it wasn’t.

If the warranty is up, then you can attempt the repair yourself. If it holds, you’ve saved money; If not, you won’t make it worse with the methods we suggest, and a flat roof contractor can repair it.

Rubber Roof Repair Tools:

  • Roofing Seams Probing tool
  • Roofing Hand Roller tool
  • Roofing Membrane Cleaner
  • EPDM rubber glue
  • EPDM peel and stick roofing tape
  • Appropriate Ladder with ladder stabilizer
  • Paint brush used to spread the glue
  • Bucket of warm water and clean towels

Other tools you may need will depend on the roofing system in question and a specific problem you’re repairing. The common tools used for repairs include a utility knife ($3-$6), tube of roofing sealant or cement ($4-$6), a caulk gun ($5-$20), a 2” putty knife or brush ($1-$4) and peel & stick flashing ($35-$125/size of the roll).

EPDM repairs require a few specialty items including EPDM glue ($12/tube or $40/gallon), weathered membrane cleaner for older roofs ($12/quart or $25/gallon) and EPDM/TPO primer ($15/quart or $40/gallon).

EPDM Rubber Roof Repair Kit

Did you know? Rather than trying to get all of the above items individually, you can save a lot of time by getting an all-inclusive Rubber Roof Repair Kit here

  • Small holes and punctures: Minor damage can be repaired by applying rubber sealant from a tube or can directly to the problem and tooled with the knife. Or the newer peel and stick flashing patch can be applied after the area where the patch will adhere is cleaner per the patch instruction. A tar and gravel roof can be repaired by applying tar to the damage using the caulk gun, smoothing it with the putty knife or brush and covering the tar with reinforcing mesh followed by a layer of gravel.
  • Membrane blisters: Cut through the bubble, end to end, with a utility knife. Be careful to only go as deep as the blister. Use towels or a small propane torch (carefully) to dry the area under the blister. When the membrane is completely dry, apply the appropriate roof repair material such as roof cement, tar or peel and stick patch. Some roofers also suggest using 6d roofing nails to secure the blister flaps. If you take that step, be sure to cover the nail heads with tar, roof sealant or peel and stick flashing patch.
  • Large rips and tears: Large areas of damage require the same types of repairs but on a larger scale. Damaged materials should be carefully removed, when possible, so that a clean, dry surface can be prepared for the patch using the appropriate cleaner and primer. Rolls of membrane are available in 3’ to 50’ widths to meet the repairing needs, though rolls wider than 10’ are massively heavy and often require a crane to get them to the roof. The roll cost ranges from $0.55-$0.80 (55-80 cents) per square foot. Seams between the old and new roof materials that aren’t lapped must be covered with specialty seam tape to help prevent leaks. It costs $2-$3 per linear foot depending on the width and the roll length.
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Concerns About Flat Roof Repair vs Replace

Flat/low-slope roof contractors want you to believe that repairing a flat roof is a waste of time. Their argument/sales pitch for replacement is that once you notice a leak, the damage to the roof is likely so bad that a patch will be a short-term (4-12 months) fix at best, and water might still be seeping in and causing mold, rot, and other nightmares during that time. You’ll have to throw more and more money at the repair to protect your home or building beneath, the argument goes, and you would be smarter to put the money toward a new roof.

Are they right? Is replacing a flat roof better than repairing it? Yes and no. Repair is a good idea under these circumstances:

  • The roof is in generally good condition
  • The damage was caused by an incident such as wind-blown debris rather than by age
  • The damaged area is small and can be covered with a single membrane roll width

Under these circumstances and with one huge condition, a repair will likely do the job.

The condition is this: The low-slope roof repair must be done properly, or it might be money wasted. Therefore, if you know what you’re doing and have the right tools, materials and techniques, then making the repair might be worth the cost, either DIY or by a flat roof contractor.

Repairing a flat roof doesn’t make sense when the leaks you’re repairing are a result of age. These types of leaks start showing up after about 12-15 years on quality flat roofs, sometimes earlier when the initial installation wasn’t done properly. You can repair small ones, but when large or multiple leaks appear as the roof deteriorates, replacement is the only lasting option.

The Cost of Flat Roof Repair vs. Replacement

Membrane roofing such as EPDM rubber roofing costs $1.50-$4.50 per square foot for the materials including EPDM roles (45, 60 or 90 mil), adhesive, seam cover tape and other supplies.

The labor cost of installing a new membrane roof averages $3.50-$5.50 per square foot for licensed and insured installers, the only kind you should consider. So, your total average flat roof cost can range from $6.50 – $14.00 per square foot installed, depending on whether it is PVC, TPO, EPDM or other material. Detailed costs are covered in guide to Flat Roof Materials here: https://www.roofingcalc.com/flat-roof-materials/

Roof repair cost is always higher per square foot than roof installation because repair takes longer. First, most roofing contractors charge a minimum service fee of $200-$500. Then, they’ll estimate the job based on the time and materials required to make the repairs. Expect the repair cost to be $12.00-$20.00 per square foot beyond the service fee.

Helpful Resources

There’s more helpful general information in this guide on Typical Roof Repair Costs: https://www.roofingcalc.com/roof-repair-cost/

We hope our DIY guide has been helpful. As you know, if you’ve got a flat roof, the cost of repair and replacement is an ongoing consideration. If you have friends or business associates with a flat roof on their home or commercial property, consider passing it along. They’ll appreciate the information! 😉

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