Flat Roofs
Flat roofs are amongst the most convenient and straightforward style of roofs to install. They are very common in modern UK residential construction, featuring prominently in extension and renovation works. They feature a number of favourable benefits over pitched roofs that make them an excellent choice for a practical roofing solution, as well as a clean aesthetic.
What is a flat roof?
At its simplest, a flat roof is a very shallow pitch, engineered timber deck that effectively protects the space underneath from the elements. By definition, a flat roof features no rafters in its construction, with timber joist acting as the primary structural components. These joists are installed level between wall plates in accordance with the standard procedure for the installation of joists. To provide the correct fall (pitch) for water towards the desired run off point of a flat[1] roof, shallow pitch furring strips are installed on top of the joists before the first layer of structural sheathing is installed. The furrings are lengths of timber that taper steadily between two set thickness and can be purchased pre-cut in bundles from the merchants.
It's important to take a moment here to make a distinction between flat roofs and shallow mono pitch roofs. The underside of the joists in a flat roof are always level from plate to plate, with a pitched component being applied above this in the makeup of the roof. A mono pitch roof is a pitched roof that features one set of rafters, and only one pitch in its assembly. The plates of the mono pitch roof are at different heights in the building, and the structural members are rafters, featuring plumb and seat cuts. The reason this is worth mentioning, is that some very shallow mono pitch roofs can be the same pitch as some of the more aggressive pitched flat roofs.
Typically, the pitch or fall of a flat roof is somewhere between 1 and 10 degrees, in order for water to flow correctly. Some very shallow mono pitch can come down as low as 10 degrees, depending on the specific requirement for the roof covering system. Whilst this isn't particularly common, it's not impossible to come across a setup such as this.
This classification differentiation becomes more apparent when a flat roof or mono pitch meets a building. A flat roof that is supported by a plate or by hangers from the external skin of a pre-existing construct is exactly that, a flat roof. This roof may cover an extension, or could be the covering for an outside patio area. A mono pitch that receives into the external skin of a building however, is known as a lean to. The specific terminology here denotes two separate ideas.
Flat roof assemblies installed in exposed applications such as car ports or patio coverings require no insulation, as there is no climate to be contained. The same is also true for flat roofs installed over garages. However, flat roofs that cover habitable spaces require some form of insulation to meet current regulations. Here we encounter some more flat roof terminology - warm and cold decks. A cold deck is a flat roof that features insulation, typically fibreglass quilt insulation in between the joists, and under the plywood layers of the flat roof assembly. These decks are cheaper to construct, requiring fewer materials and less labour, though they are less energy efficient than warm decks in their insulative capacities. In addition to this, modern regulations rarely allow for cold decks as part of extension construction and new builds, opting for the more energy efficient warm roofs. The term warm roof refers to a flat deck that features thick rigid foam insulation sandwiched between layers of structural ply, all above the structural joists of the construction. We shall look closely at how each type of deck is assembled later on.
What are the criteria of a flat roof?
Flat roofs require a solid starting point in order to be assembled, meaning quality wall plates that are level to each other and fixed in place. Appropriately sized joists must be used to cover the span of the roof, in accordance with the engineers’ calculations. The joists and tails should be installed at appropriate centres for the sheathing, typically 16” centres. The overhang of the joists needs to accommodate the desired soffit width. If the deck build up is large, resulting in a deep surface to be capped with fascia, the joist tails must be reduced before installation to reduce the overall width of the fascia. An appropriate amount of fall is required within the finished flat deck assembly to allow for correct run off of water. An understanding of the direction of fall is imperative, with suitable provisions such as plastic guttering, boxed guttering, hoppers, or run offs being allowed for. Insulation must be installed correctly in relation to the desired finish of the roof, with all required vapour barriers being present. Where flat roofs meet other constructions such as vertical surfaces, appropriate weather proofing provisions should be installed.