A flat roof garden room should not have a flat roof

In our work for The Garden Room Guide & Garden Office Guide we use the term ‘flat roof garden room’ a lot. So called flat roof garden rooms dominate the industry as they are the most flexible when it comes to positioning under the Permitted Development rules.

The Permitted Development rules say that garden rooms no higher than 2.5 meters can be situated close to the boundaries of your garden. Garden rooms higher than 2.5m have to be positioned at least 2 meters from each boundary. This rule makes a 2.5 meter building a desirable choice particularly if your garden is small.

So, that is why flat roof garden rooms are popular. The reality is that no garden room should have an absolutely flat roof overwise it will be destined for problems. Flat roofs need to have a fall so that moisture ie rain, snow etc will not just sit on the roof but run away into the gutters.

This fall doesn’t need to be huge. A fall of 1:60 is commonly used – this means for every 60 units the roof drops down by one unit. As a working example a 3 meter wide garden room would have a fall of 50mm ( 3000mm / 60 = 50mm).

How do the professional garden room designers approach this?

There are three ways that professional garden room designers approach the fall on a flat roof design – using firring strips, cutting birdsmouth joints or cutting the frames at an angle. We’ll look at each one:

Firring strips

This is maybe the easiest way of creating the fall you need. With this method you build all your wall frames to the same height and fit the rafters levelly on top of them. You then fix a tapered firring strip on top of the rafters.

You can buy firring strips from builders merchants or roofing suppliers. They are treated timbers that have been cut down to form a taper which is normally 50mm down to 0mm. You can also create these strips yourself by ripping down lengths of timber using a circular saw.

Birdsmouth Joints

A traditional method of fitting rafters is to cut a birdsmouth joint which allows a sloping rafter to sit snugly on the header of the wall. This is most commonly used on roofs with more than a pitch than a flat roof offers, but it is still a method. With a flat roof your birdsmouth joint will be quite shallow.

You would create a taller front wall than the rear wall. You then need to cut your birdsmouth, as we say this will be very shallow in this instance. You’ll need a carpenter’s square to mark this up.

Cut frames at an angle

The last method is to cut the walls at an angle. You’ll need to work out what this angle is – cast your mind back to school mathematics!  Once you have the angle you need you’ll want to cut the top of your studs on the front and rear wall. Once you attach your header plates you will have a frame with a sloped top which your rafters then sit on.

You will need to to slope your side walls too.

So, if you are designing a flat roof garden room remember to ‘design in’ the fall.