Garden room roof design & the 2.5m rule

From the conversations we have had with many of you, we know that you are hoping to make use of the Permitted Development rule that allows a garden room no higher than 2.5 meters to be positioned within 2 meters of the boundary, without the need for Planning Permission.

It is quite tricky to design a garden room that has structural integrity, is well insulated, has good internal height and has a suitable fall for water runoff, and get it to come in under 2.5 meters. You are going to have to put a lot of thought into your build up, and you’ll probably have to make some compromises along the way.

In our first set of sample plans which look at the construction of a 3 meter x 3-meter cube that is 2.5 meters high, and we demonstrate a typical build-up. Reading these plans you will see that we have had to be canny in our choices of the floor and roof build-up.

One of our first tasks when designing a garden room that is 2.5m high is to create a cross section drawing. We find its helpful for seeing where we can save a few valuable millimetres.

One of our first tasks when designing a garden room that is 2.5m high is to create a cross section drawing. We find its helpful for seeing where we can save a few valuable millimetres.

When designing your own garden room that comes in at 2.5m give careful consideration to the following:

  • Foundation – think carefully about your choice of foundation. You want something that will end up flush with your ground level, as every millimetre, it protrudes above ground needs to come out of your 2.5m height allowance. In addition, your choice of a foundation will have an impact on the floor build-up. A traditional concrete slab will, for instance, allow you to use narrower joists than a pad foundation system. You could, of course, use a pad foundation combined with steels and say 100mm joists, but you need to give this all a good deal of thought.
  • Insulation – garden rooms are designed to be used all year round, so you don’t want to compromise on your insulation levels. When designing your floor and roof build-up’s you want to ensure that they are as well insulated as possible while keeping the profiles of your insulation thin – rigid insulations come into their own in a build-up like this.
  • Ceiling height – if you plan to use standard off the shelf doors, these will dictate your ceiling height and in turn how much height allowance you have left for your roof build-up. If you were to order non-standard doors of say 2 meters high you could reduce your ceiling height by 50mm, which can then be incorporated into your roof structure = the opportunity for thicker rafters and/or more insulation.
  • Roof build-up – warm roofs are the build-up of choice for many garden rooms as you don’t have to worry about ventilating the roof space, they also are considered better performing. With a warm roof build-up, the insulation sits on top of the rafters rather than between them. This obviously has an effect on your height allowance. You will have to really consider your rafter thickness and insulation depths.
  • Fall – even though you are building what is considered a flat roof building you need to consider a fall for run-off of water. The recommended fall for a roof like this is 1:40. Depending on the depth of your roof this is quite a taper, and your 2.5m height goes to the highest point of the roof, not the lowest. You are going to have to weigh up how you incorporate this fall without compromising the structural integrity of the structure. One option is to place your rafters closer together and make use of structural boards in the deck build-up. On larger roofs, you may need to consider a supporting beam or incorporating some steel.

So as you can see, there is quite a lot to consider here. When we are working on our designs, we start off by creating a cross section drawing, to scale and find this is an ideal way to highlight where we can make millimetre savings in our design, try it!