EXTENSION MASTERCLASS
Paying close attention to the structure, insulation and coverings of flat roof extensions is key to building a low-maintenance, leak-free and durable roof
Flat roof extensions can provide a simple, costeffective solution that ticks all the right boxes, especially for many smaller extensions. The comparative ease of constructing flat roof extensions, as well as their discreet visual impact, can prevent you having to shell out for a more expensive tiled roof in many cases.
Flat roofs can also come in handy where using a bulky pitched roof would overshadow or block existing windows. There may even be an added bonus where the space can be utilised as a balcony, subject, of course, to planning permission and Building Regulations.
The main concern with flat roofs is their unenviable reputation for developing problems. Unless carefully designed and constructed they are notoriously prone to leakage and heat loss, along with defects like sagging decks, ponding water and prolific plant growth. All of which may explain why mortgage lenders and insurers have a tendency to be wary of properties endowed with large areas of flat roofing.
Thankfully, such concerns can be dispelled by taking the trouble to consider the three main aspects of flat roof extensions: the structure, insulation and coverings.
GETTING THE STRUCTURE RIGHT
Extension flat roofs are normally constructed like timber floors, in essence comprising a series of joists with a timber deck plonked on top. Obviously, it’s essential to get the size, type and spacing of joists right. (These can be calculated with reference to published span tables — see the Right Survey website.)
Building control will normally require a structural engineer’s calculations to demonstrate that the completed structure will be able to support the various loadings and resist wind uplift. The design will also need to accommodate any roof windows – where the joists need to be cut, trimmed and strengthened – as well as factoring in additional loadings, such as where foot traffic is anticipated.
Flat roof joists are conventionally cut from standard C16 or C24 treated softwood or, less commonly, can comprise manufactured I-joists or Posi-joists, or even concrete beam and block construction. They’re normally spaced at 400mm or 600mm centres and in most cases will span the shortest distance between the walls.
Where spans exceed 2.5m, additional strutting or noggins are required between the joists to prevent twisting. Joists are normally supported using steel joist hangers, or on timber wall plates, or on beams or lintels above openings.
