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The Sky Is Falling

J. Brad Grieve

Once the rainwater drains off the roof, where is the water going?

After two strong rainfalls in Guadalajara, I was up on my father-in-law's roof last weekend to help a contractor from Chapala clean and reseal his roof. Typical of many older roofs, it showed peeling sealant, a missing base coat, mold- and mildew-stained tiles, and even a series of vines climbing up the wall and over the flat roof area. The two storey house has a flat roof area that covers the stairway, hallway, four bedrooms and two bathrooms on the upper level: a total of about 150 m2 (1,600 ft2).

After moving the stationary gas tank, it took almost six hours of preparation… yes, preparation. After clearing, sweeping, scraping, removing layers of previous sealing and waterproofing attempts, washing, performing minor repairs, raising wires/cables and, finally, acid washing, the roof surface was ready for the base coat. The final acid wash actually removed a lot of the mildew and deep seated dirt. It also helped etch the roof surface, creating a clean surface that was rough enough for the final coat to grab onto the surface.

No, we did not just seal it with waterproofing paint; we actually made up a mixture of white Portland Cement, marmolina (powdered marble), caolin (hydrated aluminum silicate) and water. After finally achieving a homogeneous creamy mixture, the final ingredient was added, which is a water-base paint sealer. The sealer is added last to prevent the formation of lumps in the mixture. Once blended, the mixture will only last one hour. All the buckets had to be applied carefully and uniformly onto the wetted roof surface.

To apply the coating to the roof surface was part art and part skill. It was applied using a common broom; however care has to be taken to not apply the mixture too thick or thin. If it is too thick, the mixture may crack as it dries and, if too thin, the mixture could not provide enough protection or might lift off almost immediately.

The finished product was brilliantly white and will tolerate solar exposure and not heat up much compared to darker colors (i.e. red oxide, black, green, etc.), and it was noticeably cooler inside the house since the roof/ceiling was not radiating much heat into the bedrooms.

This leads me into a couple other notes about roofs that need to be addressed during the winter dry season, before the rains of summer. First, go up yourself or send the gardener, and check the roof drains and gutters to assure that the rainwater is able to flow freely and quickly off the roof.

So often, homeowners discover leaks in their roof just after the first rainfalls. However, the minor leak becomes a torrential stream of water entering the house, because the roof drain is blocked and the minor leak is located under the puddle that has formed on the roof. Even if you miss seeing and repairing the minor crack on the roof, the damage by the torrent of water could have been prevented if the drains had simply been reviewed in a timely manner.

Another issue has come up that is not unique to older homes, but has been seen in some newly built houses as well. Once the rainwater drains off the roof, where is the water going? Is it draining against the exterior wall, another section of the roof, the foundation, sidewalk or terrace? Ideally, we want the water to come down off the roof and then drain away from the house. So many times, I have found water draining against an exterior wall or foundation and this water seeps into the masonry structure causing salitre (masonry efflorescence) on the interior walls. In some cases, the excessive amount of water against the house has caused the fill material around and under the foundations to consolidate, which have caused the foundation to settle faster than another part of the house. This uneven settlement ends up causing cracks in the floors, walls and ceilings, which leads into other bigger problems.

Make sure the rainwater gets off the roof and away from the house. After time, water will always find some way to cause problems in, on or under your home.

Published or Updated on: March 1, 2008 by J. Brad Grieve © 2008
Contact J. Brad Grieve

J. Brad Grieve is a professional civil engineer who has lived and worked in the Lake Chapala area since 1994. He is the owner of Ajijic Home Inspections and you can be reach him by phone: (376) 766-2836 or e-mail.

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