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Flat Roof Repair

Learn about flat roof leak repair, how to repair different types of flat roof and how much it costs to do it yourself vs using professional flat roof repair contractors.

All roofs need care and maintenance but flat roofs are especially prone to leaks and benefit from more consistent monitoring. A well installed and maintained flat roof should cause you no problems, but if you do find a problem then quick and effective flat roof repair is essential.

In this RenoCompare flat roof repair guide we’ll walk you through the typical problems and flat roof leak repair methods for the most common materials. We’ll also help you to detect where exactly a flat roof leak is coming from and of course outline flat roof repair costs.

Rather than a step-by-step guide on how to fix a flat roof, we’ll outline the repairs, so you’ll have an idea of their scope and cost. In most cases we don’t recommend DIY flat roof repair unless you are very confident in what your doing. Why not get some free, no-obligation quotes from trusted local contractors to see if going the do it yourself route is even financially advantageous.

Of course, if the roof is still under warranty, the first step is to call the roofing contractor that installed it. Even if the warranty recently expired, they’ll often repair a roof at their cost to maintain their reputation.

Let’s start by taking a look at the different flat roof materials and the typical repairs required.

Built-up Roof Repair

This material is also called BUR and tar & gravel roofing. You might spend more time finding the leak than repairing it, since the roof is covered in a layer of gravel. Hint: The leak is likely in a low spot where pooling water is common.

Use a flat shovel to remove the gravel, being careful not to gouge the roof. You’re looking for a crack or for a blister in the top layer, which is evidence that water is getting beneath it.

If it’s a blister, cut it with a sharp knife from the side, so the blade won’t penetrate deeper, causing further damage. Pull back the edges of the blister, to let the area dry thoroughly.

Once the repair location is dry, spread tar or roofing cement over the damage with a trowel. Cover it with roof repair fabric patch followed by another coat of tar. That’s called a three-course BUR patch. Some contractors suggest adding another layer of fabric plus more tar for a 5-course repair if the damage is deeper than one layer or longer than a few inches.

BUR flat roof repair cost:

$50-$85 for materials. If you hire pros, expect to pay $50-$100 per hour for them to locate the damage. Most pros can do it in an hour or less. These flat roof repairs costs are $250-$400 each based on the extent of the damage.

If you do have low areas, get estimates for building them up, so water moves to drains rather than pools. Cost will range from $400 to $1,200 depending on the size of the depression that must be raised.

Repair or replace: Leaks in a built-up roof mean it is breaking down. More leaks will follow, so it is probably time to call roofing contractors for estimates on a new roof.

Modified Bitumen Roof Repair

Mod bit roofs leak when seams come apart or when a heavy object falls on them. You have two flat roof repair options here: Hot and cold.

Cold modified bitumen patch is a short-term fix for seams that are lying flat. The patch can be applied with a trowel, fully covering the loose seam several inches on either side. Expect the repair to keep out water for up to a year.

The long-term repair, and one required if the seam is cupping, is a hot patch. A mod bit sheet patch is heated with a torch to soften it and make it sticky for superior adhesion. This is a pro job, not a DIY flat roof repair for most homeowners.

Modified bitumen repair cost:

$40-$55 per gallon of patch. Hiring a pro for hot-patch repair costs $300-$600 based on the size of the repair. Once multiple seams begin coming apart, money spent on repairs is wasted. The roof needs replacing.

Single Layer Membrane Roof Repair (PVC, EPDM, TPO)

Flat roof repairs to these materials using a patch are affordable. The process is similar for all three and for Hypalon/CSPE roofing too.

The first step is to clean the roof with a universal cleaner like EternaClean or one recommended by the manufacturer of your roof. Once clean and dry, the roof is gently scored with a dish scrubber to create a rough surface for better adhesion of the patch. EPDM and TPO roof repair areas should be primed with a product like Weatherbond multi-purpose primer formulated for those materials.

The final step in TPO and PVC roof repairs is to apply a sealing tape like Eternabond RoofSeal. Rubber roof repair (EPDM) is better completed with a peel-and-stick patch made from EPDM.

Single layer membrane roof repair cost:

DIY flat roof leak repair cost for these materials range from $60 to $90 for cleaner, primer if needed and the patch material. Pro flat roof repair averages $275 to $750 per location for these materials. EPDM is the least expensive followed by TPO and PVC.

How to Find a Leak in a Flat Roof

Some leak locations are obvious – find the pooling water, badly cupped seam or tree branch poking a hole in the roof.

However, small leaks that are not in a low spot like those that result from material degradation are hard to find.

Remember that even flat roofs have slope – a minimum of ¼” per foot, so water will run downhill. To find the leak, begin searching at the approximate spot where the roof leak is appearing on the ceiling below. If it’s not found, then search your way up the slope and side to side at least five feet.

If you can’t find it, check nearby flashing. Flashing is the material used to create a water-tight seal where the roof meets any curb, wall, vent stack, skylight or other protrusion. The flashing might be metal or made from the same material the roof is constructed with.

If flashing is damaged, blistered or has pulled away, that could be your problem. It needs to be repaired regardless. Flashing repairs cost 33% to 100% more than roof repair cost because they are time consuming.

Here’s a tip that might save you hassle and money: Call a pro for flashing issues. Flashing is one of the most important safeguards against leaking. Repairs must be properly made or its likely the problem will grow worse from the attempt.

Of course, depending on the severity of the damage, repair might be out of the question. In which case you will need to consider flat roof replacement.