Flat Roof Leak Repair Preparation
Steps to prepare the roof surface before sealant application

Prepare to Repair: steps before applying roof sealants & coatings to stop roof leaks

There are several steps to an effective diagnosis and repair of a leaky roof. It's not just a matter of slapping on some sealant and calling it a day.

The leak repair process will include cleaning, scraping, sweeping, inspection, and THEN the actual repair.

What to bring onto the roof for surface preparation

The tools you'll want to bring onto the roof for surface preparation include at least a broom, dustpan, and one or more scrapers (shown above).

I didn't bother to bring up the roof sealant nor any roof crack repair materials as the roof being repaired here is so small I wanted minimum clutter on the surface during cleaning and inspection.

When I had finished prepping the roof I lugged up our impermeable roof coating: a Comex product called TOP.

Watch out: While this is a pretty nice material to brush out onto a roof the solvent vapors from this or any paint can be unsafe not only as a health hazard but it might make you lightheaded enough to fall off of the roof - one more cause of deaths by roof falls.

Wear a respirator whose canisters are rated for organic and inorganic vapors. The warnings above include:

Advertencia: No fume y no inginera alimentos durante su aplicacion.

Warning: Do not smoke and do not eat food during application.

A mere dust mask won't do a thing about solvent vapors.

Thorough roof surface preparation is key to a successful roof coating repair job

If you do a thorough job of preparing the concrete roof surface for coating with an impermeable sealant such as Comex® Top (illustrated below) you should find and remove all of the old, poorly-adhered roof coating to expose clean dry (let it dry if needed) concrete surface.

In our article series on painting buildings we emphasize the three key steps to a successful paint job: Prepare, Prepare, Prepare. If you do not adequately clean and prepare the paint surface the paint job will fail quickly.

See PAINTING MISTAKES.

Tips & tricks of the trade when preparing & scraping a previously coated concrete roof surface

As when scraping painted siding for re-painting the question is always how much scraping, sanding, and preparing is really necessary. For the new coating (primer + one or two top coats) to adhere to the roof surface you must

• Remove all loose debris
• Scrape and remove existing roof coating that is not soundly adhered to the roof surface. Below you can see that I'm pretty aggressive about testing the edges of old roof coating.
  
  If I can peel it up with my scraper it needs to be removed. I know it's depressing to think you're up on a boiling hot roof and you need to scrape and peel still more, but I figure it's better to sweat just once than to have to do the job all over again next year because I did a poor surface preparation job the first time.

Take a look at my photo at above right: that green algae showed up where I removed some otherwise good-looking but old roof sealant coating.

The fact that we see algae growing under the roof sealant coating confirms that water had been leaking under this area of sealant. The sealant had to be removed and this area dried and scraped again.

• Find and seal any large cracks
• Feathering: Scrape of if necessary sand the raised edges of old but sound roof coating that is to remain in place. Below you can see those very white areas where I've scraped and feathered the edges of old roof coating that is well-adhered to the roof. A sharp scraping tool is essential if you're to have success with feathering the edges of any sound paint or roof sealant coating.

[Click to enlarge any image]

Feathering or tapering the edges of thick but sound roof coating.

This will help the new coating seal where it overlays the old roof coating edges and will reduce the chances of leaks at those locations. Without a perfect seal at the edges of thick roof coating water may enter under the thick old coating edge, lifting it and ultimately causing coating failure and roof leaks.

• Sweep all dirt, dust and debris from the roof very thoroughly. There must be no fine powder or debris on the roof surface or your new coating may fail to bond well to the roof. Below you can see that even in a small roof area there were both large old slabs of roof coating and plenty of small bits and chips of debris to loosen and remove.

• Prime any areas of bare exposed concrete.
  
  The Comex® Top roof coating instructions suggest thinning their product with water to prime the roof surface. My opinion is that such priming is important on new concrete (be sure it's cured) and on bare concrete where all old roof coating has been removed, but it is less critical if there is a sound existing roof coating to be painted over.