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Is a flat roof really flat?

J. Brad Grieve

It is there above us, protecting us from the sun, the wind and the rain however; generally we are not worried about our roofs until we see water leaking through it. Typically, roofs here at Lake Chapala are constructed using the boveda construction technique where red bricks are arched between steel beams and the top or exterior is capped with a mixture of mortar and concrete. The basic roof structure is then typically covered with a flat tile and sometimes if the roof is inclined, overlaid with a traditional rounded red tiles which overlap each other to cover the roof surface. The rounded tiles are generally decorative since alone the porous tiles will allow water to filter through however, does help the roof pitch the rainwater away from the roof surface below, and of course provide solar protection to this waterproofed roof surface.

The key element of waterproofing a roof surface is the final layer at the top of the roof structure. Typically, this surface is painted with an impermeable paint or roof coating to prevent water from entering the roof structure. These paints are typically elastic and resistant to the small movements of the roof structure and will fill in any cracks in the roof surface. Some roof sealing products work with a fibrous cloth, which acts as a reinforcing medium for the sealant to help the roof sealant to resist the future formation of small cracks on the roof surface. Other roof sealant products have different recipes of ingredients, which will help the roof sealant resist deterioration caused by the ultraviolet and infrared rays of the sun. Also the various sealants will have different guarantees as to the durability of the product claiming it will last anywhere from one to five years.

Some homes have flat roofs, which is a euphemism since they are typically not perfectly flat. If they were, the rainwater would not easily flow off the roof. Typically the roof surface is pitched towards a central roof drain or a drain along the edge of the roof. These types of roofs will also have a parapet of varying height around the perimeter of the roof. Usually, a interior corner filler where the interior of the parapet and roof meet, is installed to help water pitch from the parapet towards the centre of the roof rather than settle into this same corner. A primary concern on these flat roofs is to prevent any “puddling” of water on the roof, which would allow the water to have time to find a small crack and enter the roof structure. Care is taken during construction to assure that roofs pitch towards any given drain on the roof. The quality of these details will determine the long-term ability of the roof to shed rainwater quickly and avoid roof leaks.

The best prevention of leaks is for the homeowner, is to check his roof before during and after the rainy season. Before the rainy season, thoroughly examine the surface of the flat roof area looking for loose or bubbled up roof sealant and small cracks. On the inclined roof areas with round roof tiles, check to see all roof tiles are sitting correctly and have not be damaged or moved. The loose material on the flat roofs should be removed and the individual spots and small cracks resealed with an appropriate roof-sealing product. Also remove any debris that is on the roof and in particular, accumulating at the openings of the roof drains. Similarly, the roof should be reviewed during the rainy season to assure that the roof drains are free of debris the drains are free flowing. Any puddling can be detected at this time. Puddling does not guarantee a leak however; it certainly is giving more time for the hydraulic pressure of the water to find a leak.

And finally, the roof should be revised after the rainy season. The rainy season is the best time to prove a roof system and at the end of the season, minor repairs can be done where necessary. It is always a surprise leak when a rare winter storm comes along and dumps a heavy rainfall to test the roof system.

Published or Updated on: February 14, 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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