Renovation & Design

ASK THE INSPECTOR: Don't cheap out on flat-roof replacement

QUESTION: I would greatly appreciate your advice on replacing our 20-year-old tar and gravel roof. It has been inspected and maintained for the last 20 years, but with a couple of leaks. Because of the difficulty in finding future leaks on this roof, it now has to be replaced.

We have dealt with a large roofing contractor for 30 years and do trust them. However, their replacement quote was $22,700, which is extremely expensive. I have done some investigation on the different options for a residential flat roof and am thoroughly confused.

I read your column and you did have a short article on flat roofs. You reported that tar and gravel was no longer being used for residential. You mentioned a membrane system which you did not explain. There are many options of membrane.

From my roofing contract quote, it seems they are quoting top-of-the-line material. The quote is for overlay with half-inch recovery board and one layer of granular torch-applied cap sheet. It also included four-inch-wide by half-inch-thick plywood strips to all perimeters, pre-painted flashing, replacing soil stack extensions, base stripping to perimeters and projections. They reported that the decking was in good condition but, should planks need replacing, extra cost would apply. This quote will eliminate the gravel, which will be appreciated as it gradually disappears into the eavestroughs over the years.

In your opinion, can I replace my roof at less cost without losing quality?

Regards, M. Renaud

ANSWER: Replacing older roofing on a flat or very low-slope roof is a very labour-intensive project. While it may seem like the quote is quite high, scrimping on this type of specialized roof system can only lead to trouble in the long run.

When we look at a typical pitched residential roof system -- normally a simple hip or gable design with asphalt shingles -- replacement can be a straightforward process. A good portion of the work involves removing the old shingles, cleaning the existing sheathing of nails and debris and hauling away the old materials. Installing the new roofing may only involve minimal flashing and other repairs.

But replacing a flat-slope, built-up roofing system with rock cover involves many additional items to be inspected and repaired prior to installation of the new membrane. The initial layers of rock have to removed and discarded, and that can be a very difficult job, as much of the rock may have been embedded in the top layer of bitumen for many years. It will have to be manually shovelled, scraped, or chipped out of the tar before removal.

Once all that is gone, the uneven surface will have to scraped relatively smooth before the new layer of sheathing is put down. Alternatively, the sheathing under the layered tar and gravel roofing could be completely removed, but that's even more work.

To accommodate the new membrane, smooth sheathing is required to give a clean, even surface for proper adherence. The membrane can then be rolled out, cut to fit and installed by heat-bonding the bitumen to the sheathing with a large propane torch. There are other similar membranes that do not require heating the adhesive for installation, but the torch-on varieties are considered the highest quality.

The extreme fire hazard during this type of installation is why only trained professionals can purchase and install this product. That may be one reason why the overall installation costs are quite high. Use of a self-adhesive membrane may save some labour costs, but it may not be that significant a portion of the entire bill.

Your quote also appears to include major repairs to the curb and flashing that surrounds the roof area. It's always a good idea to repair this area when the roofing is changed to ensure the new roof doesn't leak through deteriorated curbing.

I assume that the plywood and pre-painted flashing quoted allows for a completely new cap to the perimeter curb, ensuring a leak-proof installation of the new membrane around the entire roof. Lesser-quality roofers may try and cut corners by patching any deteriorated curb flashing with asphalt cement or small pieces of metal, but complete removal and replacement of the old material will ensure the highest-quality job. It should also ensure that the flashing and curb last as long or longer than the new roofing, which should have a life expectancy greater than two decades.

In short, you can try to minimize the costs of some home repairs and renovations without much peril, but flat roofing replacement does not fall into that category. It's one job where a less-costly replacement can only be achieved by cutting corners and reducing the quality of the job. Many residential roofing contractors do not even do repairs on flat roofing systems, so the few that will tackle this difficult task may charge a premium for their services.

If you can't afford the high-quality work that your contractor has quoted, you may be able to extend the life of your current built-up roofing system for a few more seasons with yearly inspection and patching while budgeting for the proper replacement.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property Inspectors - Manitoba ( Questions can be e-mailed to the address below. Ari can be reached at (204) 291-5358 or check out his website at


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