Flat roofs

Once you’ve got the building regulations under your belt, building a flat roof is quite simple.

Despite the name, flat roofs are never truly flat. They have to have an ever-so-slight angle to ensure water will drain off properly, instead of letting water sit and soak through. Despite the slightly confusing name, flat roofs are constructed quite simply. Joists span the gap between two walls, and the joists are usually covered by timber and a waterproof layer, though the joists can sometimes be covered by a single, more waterproof layer.

The size of the joists is determined by the weight of the timber and the waterproof layer, as well as the distance that needs to be covered by the joists. Other factors can influence the size of the joists needed, including the weight of snow (in certain areas), and this is all covered in the Approved Document A (Structure) of the Building Regulations.

Level of Difficulty

Level 2: Advanced
Felt is the traditional choice, however many roofs today are lined with fibreglass or even rubber. You may also find that in the course of construction, the timbers of your own roof are inserted at different points. Nonetheless, many things remain consistent. Timber noggins are always inserted in between joists at the 600mm centre to prevent the joists from twisting.

The materials used in the construction of flat roofs depends on their purpose. The timbers usually consist of timber sheets, often 18 or 22mm ply or OSB Stirling board. The arris rail helps drain water away from the wood and helps prevent rot. It usually stands about two inches tall and can be found at your local timber merchant’s shop. Felt roofing must be made to British standards and it is laid out all over the roof in a designated number of layers. Strips of the felt are then attached to facia boards. The felt and boards are turned over the arris rail and sealed to the top of the roof covering, giving the roof a nicely finished edge.

The building regulations

Before building a flat roof, you must get approval from a certified specialist, who can determine if your plans meet Approved Document A (Structure). There are eight tables in the regulations that discuss the maximum spans of roofs. These tables also include information on the maximum loads flat roofs can bear, as well as regulations on areas of flat roofs that, in order to aid drainage, are pitched at over 10 degrees.

How to construct a flat roof

The building regulations will determine how large the joists need to be, how far apart they should be and more, so have a certified builder check your plans before you start.

Note: Be sure also to sweep off or clean off the roof in between each step to make sure no debris gets caught in the roof.

  1. Once you have the expert’s approval, you will want to construct the frame of the roof according to your plans
    a) First fit the wall plates onto the walls. These are usually 1-inch by 4-inch rough cuts of timber, secured to the wall with cement and other fixings to make sure they are level
    b) The joists can then be attached to the wall plates
    c) Install the noggins every two metres, across the span of the roof
    d) You may also want to install firings on each of the joists. These are wedges of wood that run the width of the roof, giving it some of the slight angle it needs to drain properly
  2. Attach the roofing felt to the facia board
  3. Lay down the felt boards on top of the frame
  4. Place the timber sheets on top. Keep a gap at all the joints, so that the wood can expand and contract
  5. Attach the arris rail, and wrap the felt boards around the rail

Why you may just want to get the professionals in

In theory, flat roofs shouldn’t be too hard to build, but there are several reasons why you probably should leave it to the professionals.

First of all, the building regulations and associated calculations can be very complicated, and mistakes in calculations can lead to some dangerous situations. Having joists that are the wrong size can mean that, if the roof moved for any reason, the cover could weaken, split and – best case scenario – form a leak.

Similarly, though certainly less worryingly, installing a felt or fibreglass roof without any real experience can lead to leaks. If you then attempt to find and patch up the leak yourself, you will probably just waste your time. After all, by the time you find the leak, water will probably have already penetrated the felt and started causing soft spots in the covering.

Please note that all our DIY guides and ‘Expert answers’ advice have been written strictly for reference only. Rated People do not accept any liability for any damage caused to an individual, property or anything else as a result of following our DIY guides and using our ‘Expert answers’ advice.

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