Flat roofs are a popular choice for garages, extensions, garden offices, and outbuildings.  The basic construction is fairly straightforward which is great if you want a self build project.  A  timber frame forms the supporting structure with flat roof joists across the short side of the roof providing the load bearing element of the roof. The joists are attached using either timber wall plates or metal flat roof joist hangers.

Cold Roof vs Warm Roof

One thing to consider is whether you will have a cold roof or warm roof.  A cold roof is created with the insulation between the joists, a warm roof is above the deck.  There are advantages and disadvantages to both.  You can read more about those here - Cold roof vs Warm roof.

What Is The Fall On A Flat Roof?

The fall is the slope you create to allow rain water to run off easily.  A smooth surface like plastic or corrugated iron will require a fall of just 1:80 as rain water will be more free flowing on a smooth surface, rougher surfaces like felt will require a fall of 1:60.  There are a few ways to do this; the first way is to install your joists at a slight angle.  A second option is to install a tapered decking (known as furrings) on top of your joists.  Finally you can buy insulation with the fall built in (called ‘cut to falls’), this will only work if you are doing a warm roof.


If you don’t build in a fall, the roof may create pools of rain water which can eventually weaken the roof leading to leaks.  This is a very common cause of leaks, and is much better avoided at the build stage!

Roof Decking Construction

To recap, once your frame and joists are in place, you will lay your roof decking, considering your fall in the process.  Once this is in place you need a flat surface on top of this which you will attach your waterproof roof covering to.  This is most likely going to be a sheet of Smartply or Plywood - a minimum thickness of 18mm is required.  A waterproof layer is then required to be laid on top of this.  This will usually be a bitumen based felt which you apply in sheets, the easiest to apply are the self-adhesive versions.  Professional builds may use mastic asphalt at this stage.

Preventing Flat Roof Leaks

Leaks are one of the most common issues with flat roofs and can be prevented by ensuring the fall is sufficient, that the waterproof layer is properly applied and sealed, and adding a little skirting where the roof meets the walls.  Prevention is better than cure as they say!

Regular Maintenance Of Your Flat Roof

Check your flat roof annually and tend to any minor tears in the felt (these can often be solved by applying a patch of waterproof felting with adhesive), this can prevent more major repairs like having to relay the felt, which will take time and will be quite costly.

Flat Roof Blistering

If there is no leakage, blisters are best left alone.  If you think blisters pose a leak risk you can soften them using a blow torch and then flattening with something heavy like a block of wood.  This will solve the issue if the blister does not include water.  If you have blisters filled with water, it’s time to call the professionals in.

Major Repairs

If your flat roof is in poor condition it may eventually need a major repair, for example a whole surface treatment.  This involves covering the whole roof in a waterproofing agent which could extend the life for a few years before the need to replace and relay the felt.


We hope this has been useful article on the basics of building a flat roof, considerations, and how to maintain and repair your flat roof.