Flat Roofs

Flat Roofs

In contrast to the sloped form of a roof, a flat roof is horizontal or nearly horizontal. Flat roof systems are roofs that are built so that they are nearly flat. They are not completely flat; otherwise water would build up on the roof (called ponding) and would eventually start to get inside the building. Materials that cover flat roofs typically allow the water to run off freely from a very slight inclination. In general, a flat roof lasts longer if it is properly maintained. The life expectancy of a flat roof can be proportional to the maintenance done on it. Some assessors use 10 years as an average life cycle, although this is dependent on the type of flat roof system in place. Some old tar and gravel roofers quietly acknowledge that unless a roof has been neglected for too long and there are many problems in many areas, a BUR (a built up roof of tar, paper and gravel) will last 20 – 30 years. There are BUR systems in place dating to the early 1900s.

All flat roofs should be inspected semi-annually and after major storms. During the roof inspection particular attention should be paid to the flashings around all of the roof top penetrations. The sharp bends at such places can open up and need to be sealed with plastic cement, mesh and a small mason’s trowel. Additionally, repairs to lap seams in base flashings should be made. 90% of all roof leaks and failure occur at the flashings. Another important maintenance item, often neglected, is to simply keep the roof drains free of debris. A clogged roof drain will cause water to pond, leading to increased “dead load” weight on building that may not be engineered to accommodate that weight. Additionally, ponding water on a roof can freeze. Often, water finds its way into a flashing seam and freezes, weakening the seam. Roof coatings can be used to fix leaks and extend the life of all types of flat roofs by preventing degradation by the sun (ultra-violet radiation).

Type of flat roofs:

SINGLE PLY: Composed of a single level of a plastic or rubber type compound.

EPDM: Ethylene Propylene Diene Monomer is a synthetic rubber most commonly used in single-ply roofing because it is readily available and relatively simple to apply.

TPO: Thermoplastic Polyolefin single-ply roofing. This roofing material can be fully adhered, mechanically fastened, or balasted. TPO seam strengths are reported to be three to four times higher than EPDM roofing systems. This is a popular choice for “Green” building. It is available in white, grey, and black.

PVC: Polyvinyl Chloride (PVC) membrane roofing is also known as vinyl roofing. Vinyl is derived from two simple ingredients: fossil fuel and salt. Thermoplastic PVC roofing is extremely strong, as its heat-welded seams form a permanent, watertight bond that is stronger than the membrane itself. Vinyl roofs are inherently fire resistant due to their chemical composition and have a broader range of fire ratings over common substrates. Vinyl roofs provide an energy-efficient roofing option due to their inherently light coloring. While the surface of a black roof can experience a temperature increase of as much as 90 degrees under the heat of the full sun, a white reflective roof typically increases only 10-25 degrees Fahrenheit.

MULTIPLE-PLY: These are made by overlapping rolls of felt or mats that are alternated with layers of asphalt. This is covered by a granular mineral surface, ballast or tile to protect the layers.

Built-up Roofs (Asphalt BUR): Is made up of multiple layers of reinforcing plies and asphalt forming a redundancy of water proofing layers. The reflectivity of built up roofs depend on the surfacing material used. Gravel is the most common and they are referred to as asphalt and gravel roofs.

Modified Bitumen: Polymer-modified roof membranes are composed of reinforcing fabrics that serve as carriers for the hot polymer-modified bitumen as it is manufactured into a roll material. Modified bitumen roof system membranes are composed of multiple layers, much like built-up roofs.

Metal: Flat Seamed metal roofs made out of small sheets of metal that are soldered together.

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