As with many industries, there’s a whole world of confusing jargon built up around flat roof construction. The reality is that flat roofs are relatively straightforward, so we have written this simple guide to help you get to grips with flat roof lingo.
This is the basic material and construction that forms the roof. The stronger the better, so you can avoid sagging. Concrete is best but rarely found on domestic homes. So if you’re having a new roof built go for plywood, timber board or oriented strand board. Avoid chipboard or particle board. It’s cheap but can cause problems as it likes to absorb moisture. This layer sits above the normal ceiling joists, which are the long thick planks that hold everything up.
Reinforced Bitumen Membranes (RBMs)
This catchy title is simply two or three layers of waterproof and/or flexible materials (e.g. felt) rolled out and then bonded together with hot bitumen. This forms a waterproof membrane.
Most RBM roofs need a protective layer on top of the membrane. Gravel or shingle is most common. This helps hold the membrane down in high winds, and provide protection from the sun and also when anybody walks on it. It’s essential that materials are bonded to the membrane so they don’t move, thus exposing the membrane. Other protective materials include reflective paint and mineral finishes where small salt flakes are pre-bonded to the felt.
One of the oldest types of flat roof. Felt is bonded with asphalt in the factory. It is then rolled out and can be held down with nails or Asphalt cement. There will be multiple layers of which the top one is gravel coated, so similar to RBM. It’s the least expensive but also needs replacing most regularly.
EDPM (Ethylene Propylene Diene Monomer)
Whilst felt has been the traditional material for RBM roofs, new rubber based materials have come to the market which are harder wearing and last longer. They still come in a roll and provide a much plainer smoother finish.
Some flat roofs are designed to be regularly walked upon, such as a roof terrace or balcony, so a more substantial material is required. These tiles come in a variety of materials including porous concrete, rubber, bitumen, plastic and fibre cement. As with any load-bearing surface it’s important that the roof is strong enough to take the increased weight of the materials and people on top of them.
Single ply membranes are an extremely strong and flexible polymer material made from the likes of PVC. It is 1 to 2mm thick and usually comes in 20 metre rolls. The membrane is held in place by mechanical fasteners, which are long plastic tubes with short screws that go through the membrane to the structural deck.
Glass Reinforced Plastic (GRP)
A very strong material that is also on the hulls of boats. It is used more on commercial buildings and is very strong and durable, and more expensive. It has a life expectancy of around 40 years if maintained well and can be walked upon. Grey is the most common colour due to its likeness with a lead roof. It’s fairly inflexible so can only be used for a straight surface where there is no movement.
Green roof systems
A lawn in the sky with hardy grasses planted in a layer of organic material. It looks visually stunning and is very eco-friendly but needs careful construction. The organic material can carry a weight of water so needs to be on strong roofing foundations. The grass will also need ongoing maintenance, much like a garden.
A flexible metal most common on churches. It is hardy and has a very distinctive look. Copper and Zinc are other metals that are coming into common usage, particularly on more design focused new builds.
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Why do you put gravel on a flat roof?
Gravel is used on a flat roof to weigh down the covering surface and to protect the roof from sun damage. Gravel must be properly bonded to the surface material to prevent it being blown off in high winds. It must also be evenly distributed across the surface to prevent the roof becoming dry and brittle from the sun.
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