Flat roofs are usually quite straight forward, flat roof construction usually consists of timber struts, or joists as they are more commonly known. But sometimes the use of steels are needed and are laid across two parapet uprights or walls.
Steel joists or what are referred to as RSJ’s (RSJ = Rolled Steel Joist. If you were wondering) are usually fitted into one or both of the supporting walls to give a much greater stability to the roof structure itself.
Once the RSJ’s are fitted, the timber joists are then laid on top. Sometimes they are cut into the steels, but typically just laid. You need to lay the joists at specified distances apart for structual integrity, usually around 400mm is what I do but this all depends on the width or span of the roof itself. Also depending on the surface that is being laid and whether the roof will be used as a terrace or walkway for example.
Most of the time when creating a flat roof I will be using a plywood or OSB3 timber board, which is more commonly called Norboard or Stirling board.
Because we use timber in the construction of a flat roof, they need to be waterproofed or weathered. So once we have laid the OSB across the joists we then use whatever chosen waterproofing to seal the roof. We use GRP, Felt, EPDM and Liquid Coatings. They all do the same job when laid correctly, but all have their own perks. Something else we should mention is that where two OSB boards meet and the joint is not on top of a joist, tongue and grooved OSB make the joint really strong, so this is a must and this should be taken into account when planning the roof and purchasing materials.
Sometimes architects or other designers will not allow the height required for a pitched roof, most of the time this is because it obstructs a neighbours view or puts their property into shade. This is why pitched roofs may not be appropriate for an extension, but this is where Flat Roof Extensions come in!
Pitched roofs can be really expensive too as they require a lot more material to construct! Roof valleys or box gutters for water drainage adds to cost. Where as a flat roof allows water to run easily away from the building into new gutters which can be connected to existing drains.