Flat Roof

Installing a Flat Roof


The first thing that you have to know about flat roofing is that it is not entirely flat. A roof pitch of some sort is added to prevent the pooling of water in the roof that could and would eventually damage your roof, inside and out.

A flat roof is not, to some degree, architecturally sound. A flat roof would not last long when it has to bear the weight of fallen snow or deal with the pool of water after each rain.  A sloped roof is better for dealing with natural elements.  However, flat roof is gaining popularity in terms of aesthetics; linking and reflecting the flatness of the roof to the horizontal and vertical lines of nature. Most homes have part of the roof line flat. In all honesty, a section of flat roofing  in a series of hipped or gabled roofing adds a point of interest.

The introduction of new roofing material specifically for flat roofs has made a tremendous improvement on the strength, durability and life span of flat roofs. Some even have a 20-year warranty. Today opting for a flat roofing is no longer a risk for a leaky house.

Types of Roofing Materials

Here are three types of flat roofing materials that are available in the market.

Rubber Membrane - usually made of EPDM (ethylene propylene diene monomer). The material is true rubber that looks like the inner tubing of a tire.  It has been modified to resist sun damage and the material can anchored with glue, fasteners or weighted down by stone.


Modified Bitumen - most commonly known as torch-down application. This system involves the heating of adhesive as one unrolls the material.  A newer "peel-and-stick" system is  safer and easier to use.


Built-Up Roof (BUR) -this is the traditional three or more layers of waterproof material alternated with hot tar and smooth river stone.

A flat roof basically has a pitch or slopes towards a portion of the whole roofing structure. If this is not done, then you can expect a pool (or pools) of water on your roof when it rains. Another way to build a flat roof is by treating and building a roof as reinforced concrete floor. In order to do this, you have to make sure that your structure's posts and beams can support the weight of poured concrete. If your posts and beams are of wood, forget about the poured concrete flooring.  Here is a step by step description of how to easily build a flat roof.

Building a Flat Roof

Roof Framing. Remember that a flat roof isn't necessarily flat.  An inclination of  minimum of 1/8 inch per foot is ideal. If your locality has more rain than most, then a ¼ inch slope  per foot is better. Let's say the run (total length of the roof) is 5 meters. If the rise is ¼ inch (0.06 M) per foot (0.30 M) then the total pitch for the roof length would be about 1 foot (0.30 meters). Frame the roof by using 2"x 4" or 2"x 6"  joists between the two supporting walls.  Insert noggins between the joists for more support.

Plywood Sheathing.  Cover the framing with 5/8 inch marine plywood with a 1/8 inch gap for construction joints. This should cover the expansion and contraction of the plywood.

Clear the plywood sheathing from any debris. Any dirt or protruding nail heads or wood splinter could puncture the rubber sheathing the you intend to put on the flat roof.

Rubber Sheathing.  A rubber sheathing like ethylene propylene diene monomer comes in 10-foot wide rolls. The rubber sheathing is easy to cut. Measure the finished area to be roofed and allow about 0.25 meters all around. If the roof's width is greater than 10 feet (3 meters) then overlap the rubber sheathing by at least 6 inches (0.15 meters). Lay the rubber sheathing accordingly then fold it back into half.

Glue the Rubber Sheathing.  Use a paint roller to spread glue on the plywood and on the corresponding rubber sheathing. You also have an option to overlay the plywood with iso board - ½ inch thick rigid foam. If you have this option then you need to glue the ISO board to the 5/8 inch plywood first before laying the rubber sheathing on the plywood.

Glue the rubber sheathing starting at the middle of the roof to the edges. The glue bonds instantly. After letting the glue dry a bit, spread and tap the rubber sheathing over the plywood ever so gently without lifting any part to prevent wrinkling of the rubber sheathing. If there is  any vent pipe protruding from the roof, cut the rubber sheathing to make a hole slightly larger than the pipe and slid the rubber over it. Add a rubber collar to  the pipe and glue it over the rubber sheathing on the pipe to prevent leakage.

Finishing the edges. Seal the inside corners by cutting and overlapping the rubber sheathing. Glue the corners. Screw the  vertical rubber sheathing on the walls then apply tri-polymer caulk to the screws to seal them.  You can opt to add a gravel stop (galvanized iron) flashing at the edges of the roof to make the roof more water tight.

"Concern about price is a very sensitive issue but for saving maintenance cost nothing is better than product specs"


"I would just like to mention that there is not only 3 types of flat roofing there is also GRP (glass reinforced plastic)probably the most durable and versatile product on the market normaly gauranteed for as long as 40 years, very hard wearing and can be made in any colour, also looks good and stays looking nice for a very long time, a bit more expensive than felt but you realy get what you pay for with GRP "

simon rolls (ROOFTEC GRP 07547536140

"the author should keep to metric measurements. Also the 1 foot slope for a 5 metre roof has me confused. the rest of the article was helpfull. Thanks"


"Would your recommendations work with reinforced concrete flat roofs? Thank you."