Is a Flat Roof really flat?

September 27, 2008


Is a Flat Roof really flat?

by Brad Grieve


It’s 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, an 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 to check the 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. 


Finally, the roof should be reviewed 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.

Sid’s Note: J. Brad Grieve, P.Eng. MBA formerly associate Broker with RE/MAX Fenix.   As a registered professional civil engineer with over 22 years of service in Canada and Mexico, he brings a multi-faceted perspective of house construction, renovation, restoration and maintenance of homes here at Lake Chapala.  

Brad, also recently completed his Masters in Business Administration (MBA) at the prestigious IPADE business school in Guadalajara ( ranked as one of the top five schools in the world (outside USA) by the Wall Street Journal (Sept. 2005).   You can contact Brad at 766 – 2836 or by cell (045) 333 904 9573 or by  E – Mail:




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