Flat Roofs

All roofs are designed to be either watershedding (steep roofs) or waterproof (flat roofs) Flat roofs use membrane roofing systems. They can be a s simple as tar and gravel or as advanced as the “torch on” modified bitumen or single ply systems . All roofs slope to a drain somewhere. They can be in the middle of the roof or at the edges in the form of gutters or scuppers. This guide offers information on the variety of flat roofing systems available and how to  recognize the pros and cons of each.

A good roofing system depends on the following; a good roof design, quality materials for the design and installation process, and professional installation practices. When one or more of these  factors  is left out or modified, the result is a less than adequate roof.

Flat roofing systems can be installed on buildings  with roof slopes from dead flat up to 3 in 12, but  the norm is usually in the 1/8″ per foot to 1/4″ per  foot  range.

Roll Roofing: Sometimes called selvage roofing, roll roofing consists of the same material as asphalt shingles (asphalt impregnated felts covered with granules). The surface can be completely covered or only 50% covered with granules. It is installed as a single ply and has a limited lifetime expectancy of between  5 and 10 years. Roll roofing is installed in long strips with little overlap, and because it expands and contracts with temperature changes, it buckles and wrinkles. Though in some cases the entire system is cemented in place, often only the seams and  edges  are secured.

Single Ply Roofing:  This   type of roofing system is  fairly new to North America. Some are a modified bitumen (asphalt) base while others are  plastic  (primarily PVC) or synthetic rubber. Some are torched or tarred down, and some  are glued or mechanically fastened with strips or buttons.  They  can  also be laid loose and held in place with  gravel.  The  life expectancy of these roofs are unknown, though some manufacturers offer a 10 year warranty and claim a life expectancy of 30 years.

Metal Roofs: These can be made of copper, galvanized steel, pre-painted or coated steel, or tin. They are the most expensive type of roofing system (especially copper). Metal roofs are very difficult to repair. Once they have begun to rust and  leak, so replacement is usually required. Metal roofs should never be covered with tar as moisture trapped below the tar causes accelerated rusting.

Corrugated Plastic Roofs: These are single ply, translucent roofing systems which are generally placed over patios and light structures. They should not be used over living spaces, and are not considered water tight. Corrugated roofs are low quality, easily damaged, discolor with sunlight, and leak at the joints. They are extremely weak and  should never be walked on.

Built-up Roofs: These are normally called tar and gravel roofs. They consist of two to five layers of roofing felt with a layer of asphalt in between. A flood coat of asphalt is then applied over the top and covered with a layer of gravel. This protects the roof from ultra-violet light and mechanical damage. Two ply roofs have a life expectancy of about  five years, while a four ply roof may last up to 20 years. If moisture gets trapped between the layers, the membranes will crack and buckle and reduce the life expectancy of about five years, while a four ply roof may last up to 20 years.  If moisture gets trapped between the layers, the membranes will crack and buckle and reduce the life expectancy. A lack of gravel can cause a condition called alligatoring, which occurs as the surface breaks down and deteriorates due to exposure to sunlight.

New Products: There are always new roofing systems being developed. Most of these  products carry a 20 year plus service life rating and are easy to install and repair. Because they are made of a strong EPRM (ethylene propylene roofing membrane) rubber. They are almost twice as puncture resistant than standard PVC (Polyvinyl Chloride) roll roofing. They resist ultraviolet degradation and chemicals typically found on flat roofs. They require very low maintenance. These systems come in large rolls, and are fastened to the deck with buttons or spikes and then hot welded at the seams to offer a complete seal. Walkway pads are puncture, UV, chemical and weather resistant.

Cost:  Typical roll roofing costs approximately $1.00- $2.00 per square foot, depending on the number of layers installed and the choice of material. Newer rubber roofing systems cost between $3.00 and $7.00 installed, and range according to various factors such as preparation work, slope, etc. Every roof is unique, and it is important to have a professional roofer assess the needs and requirements of both the roof and the client.

Common Problems with Roll Roofing:

Ponding: Proper drainage is vital for any roof. Drains can be gutters, central drains or scuppers. Good installation includes a secondary drain on flat roofs. Water ponding on a flat roof  can shorten the life expectancy by as much as fifty percent, and because of the construction of built up roofs, leaks are difficult to isolate and repair. Water can travel a significant distance through the plys of a roof before emerging on the interior. When ponding occurs, the roof may sag and can be in danger of collapse. The weight of the water accelerates the sag which, in turn, holds more water.  Often  the roof bends to the point where the ends  of the structural members are pulled off their bearing surfaces. When this happens, the roof can collapse. Roofs that pond will have no valid material warranty and generally last only half as long as a roof that sheds water properly. While it is common practice  to  install  new  built -up roofs over existing built-up roofing systems, the practice is not advisable. Moisture trapped in the old roofing system  will  cause  premature deterioration of the new membrane.

Blueberry Blistering: When low melting point bitumens are exposed to too much warmth from the sun, they expand and create blisters that resemble blueberries. These usually appear on surfaces that have a very thick coating and poor gravel coverage. They can be repaired by re-coating and covering  with  an extra  layer of  gravel.

Blisters Between Felts: Water or air trapped between layers, that expand during sunny weather can cause buckling in areas of poor adhesion. Most blisters can be cut out and repaired.

Alligator Cracking: When alligator cracking of surface bitumen occurs on bare spots or on smooth  surfaced roofs without a protective covering. This type of problem can be rectified either by removing the original bitumen and replacing  it, or  by  re-coating  and  covering with gravel.

With the large variety of roofing systems available, it is important to keep some basic principles in mind. The roof surface should perform its intended function, which is to keep water out. The life expectancy and guarantee should be reasonable.