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Flat Roofs: Designer's Dream / Contractor's Nightmare!

Posted on 20 July, 2013 at 15:50
An award winning French architect, designer and urban planner named Le Corbusier is said to be the pioneer of of flat roofs in modern architecture. But even one of his most famous building designs- the Villa Savoye - began leaking almost immediately after its completion. Mr. Corbusier only avoided a lawsuit when the German Army invaded France and he fled the country (He then brought his ideas to the U.S.).
Here in the Southwest, flat roofs have been a common sight since the Hohokams and Incas roamed the land. I sometimes theorize, they made so many clay pots to partly catch rainwater from their leaky roofs.
Flat and Santa Fe style roofs remain a popular feature in Arizona's design/architecture community. They are versatile, efficient, space saving and look great within the desert landscape. But as with most good ideas, the devil is in the details.
The two most important aspects in keeping a flat roof problem free are "Slope" and "Drainage".
Slope is a common term used to describe how level or in this case, how out-of-level a surface is. This number is obtained by measuring the surface's horizontal inclination vs. its vertical drop. For example, most shingle and tile roofs will drop 3 or 4 inches per every 12" of horizontal space. This is referred to as a 3:12 or 4:12 pitch or slope.
Flat roof is actually a mis-nomer because flat on a roof is never a good thing. The flatter it is, the more likely it is to retain water and leak. Most flat roof slopes range between 1/4:12 to 1.5:12. Obviously, the more pitch the better to keep the surface as clear as possible of standing rainwater. Many designers however, are reluctant to give up their flat design by incorporating the higher pitch ratios. So in order to achieve the better slope while maintaining their flat appearance, the Santa Fe concept became popularized. Santa Fe style roofs are designed to hide the sloped roof behind a "Parapet" wall. All this is, is a level, false wall built around the entire perimeter of the building at a height slightly higher than the highest point of the roof itself. In order to keep the roof drained, "scuppers" are cut into the walls where it meets the lowest points of the roof. Scuppers are pre-formed round or square metal inserts installed in wall cut-outs prior to the roof surface installation. This is where the issue of drainage is most important. Statistics compiled by the Liquid Roofing & Waterproofing Assn. (http://www.lrwa.org.uk/) reveal that up to 90% of all flat roof leaks occur at scuppers and other flashing locations. Maintaining good water drainage is most critical, as ponding water almost always has a tendency of eventually finding its way beneath the surface.
One of the most common repair errors I've found in my many years of investigating these types of leaks are repairs made to cover the problem without cutting away original cracks, buckled material or previous repairs. Although this may remedy the situation in the short term, it usually worsens the problem in the long term. That's because each repair done over another creates a Damming effect, where water ponding is actually increased. A true professional will know that the best way to correct scupper leaks is to actually cut out the buckles and previous repairs, down to the metal flange. This is the only way to identify the scupper and substrate condition and determine whether those need replacement as well.
On a lesser scale, flat roof leaks may also occur in open field areas, around pipe penetrations and along seams- lines where the original roof materials overlap. While many of these problems are fairly visible, many others can remain hidden to the untrained eye. A few lessons I've learned in seminars I've attended over the years;
A nail hole, too small for most people to see, can create a leak of up to 3 gals per hr;
Its possible for a flat roof leak to travel up to 20' before penetrating a building's interior;
On very smooth surfaces, its possible for water to creep uphill, creating leaks in unsuspecting locations.
For these reasons and more, most roofing contractors prefer not to even deal with flat roof problems or refuse to guarantee repairs made by their employees. That's also why most will try their darnedest to sell you a new roof instead.
I realize I'm not the only expert when it comes to flat roof repairs. But I will not sell you a repair unless I can personally guaranty its integrity for a minimum of 2 years!

Categories: Roof Repair Tips

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