The short answer
Most garden rooms have a flat roof covered in EPDM rubber or fibreglass, because it keeps the room under the permitted development height limits and has few joins to leak. A pitched roof gives more headroom and suits older houses, but it must stay within 4 metres, or 2.5 metres if the room is within 2 metres of a boundary.
The roof does more work than any other part of a garden room. It keeps the rain out, holds a large share of the insulation, decides how tall the building is for planning purposes, and sets much of the look. It is also the part most people think about least when they compare quotes.
This guide covers the main roof shapes and coverings, what they mean for height limits, and the questions worth asking a builder.
Flat or pitched?
Almost every garden room is one of two shapes.
Flat roofs
A “flat” roof is never truly flat. It falls gently, usually towards the back or one side, so water runs off into a gutter. Flat roofs are the most common choice for three reasons:
- Height. A flat roof keeps the whole building low, which matters because a room within 2 metres of a boundary must be no taller than 2.5 metres to stay within permitted development.
- Simplicity. One continuous covering with few joins means fewer places for water to get in.
- Cost. Less timber and less roofing material than a pitched roof of the same footprint.
The drawback is headroom. Inside, a flat-roofed room near a fence often has a ceiling of a little over 2 metres once the floor and roof build-ups are counted. That is fine for an office, less so for a gym where you lift overhead.
Pitched roofs
A pitched roof has two slopes meeting at a ridge, or a single slope (a mono-pitch or lean-to shape). It gives a taller, airier room and often sits better beside a Victorian terrace or a stone cottage.
The planning limits are the catch. Under permitted development, a building with a dual-pitched roof can be up to 4 metres high, and any other roof up to 3 metres, but only if it is more than 2 metres from every boundary. Within 2 metres, the limit is 2.5 metres whatever the shape. In a narrow Chesterfield garden, that often rules out a full pitched roof unless the room sits well away from the fences. Our guide to how big you can build without planning sets out each limit.
Flat roof coverings compared
| Covering | What it is | Points to know |
|---|---|---|
| EPDM rubber | A single sheet of synthetic rubber, glued to the deck | The most common choice. Few or no joins, flexible in cold weather, plain black finish. |
| Fibreglass (GRP) | Resin and glass matting laid on site, then top-coated | Hard, seamless finish that can be coloured. Needs dry weather and a skilled installer to avoid cracking. |
| Torch-on or self-adhesive felt | Layers of modern bitumen membrane | Cheaper and familiar, but more joins. Quality varies a lot between products. |
| Metal sheet | Profiled steel or aluminium, sometimes with a built-in insulated core | Quick to fit and tidy, can be noisy in heavy rain unless the roof is well insulated. |
| Sedum (green roof) | A layer of hardy plants over a waterproof membrane | Looks soft from the house and supports wildlife. Heavier, so the frame must be designed for it, and it needs a little care. |
Whichever covering you choose, it sits on top of a timber deck (usually OSB or plywood), and the insulation either goes between the roof joists or on top of the deck. Ask which, because it affects condensation. Our insulation guide explains why.
Pitched roof coverings
Pitched garden rooms are usually finished in one of:
- Shingles, either bitumen or cedar. Light, suit a timber-clad room.
- Lightweight tiles or tile-effect sheets, which give a house-like look without the weight of real tiles.
- Metal standing seam, crisp and modern.
- Natural slate or stone, which is heavy and costly but sometimes the right answer next to a listed or traditional building.
If you live inside the Peak District National Park and your room needs planning permission, expect the Park Authority to look closely at roof shape and materials. It publishes design guides for building in the park, and they are worth reading before you choose.
Overhangs, gutters and where the water goes
A roof is only as good as its edges. Three details are worth checking on any quote:
- Overhang. A roof that projects past the walls by 100 to 300 mm keeps rain off the cladding and the door. A room built tight to a fence may have little or no overhang on that side, so ask how that wall is protected.
- Gutters and downpipes. Where does the water go? Into a water butt, a soakaway, or onto the lawn? On heavy Chesterfield clay, water dumped beside the base can sit there for days.
- Fall. Ask what fall the flat roof has. Too little and puddles form; puddles shorten the life of most coverings.
Rooflights and roof lanterns
A rooflight floods a deep room with daylight and is a good alternative to more wall glazing if privacy matters. Two things to weigh up:
- Glass in the roof lets heat out on winter nights and in on summer afternoons. Ask for a solar-control glass or a blind if the room faces south.
- Every opening in the roof is a potential leak. Ask the builder how the upstand is sealed to the covering.
What it means for the price
A flat EPDM roof is usually included in the base price builders quote per square metre. Pitched roofs, sedum, standing seam metal and rooflights tend to add to it. If you are weighing up shapes, ask for both prices on the same quote so you can see the real difference, then try the cost calculator for your size.
Questions to ask the builder
- Which covering do you use, and who makes it?
- Is the insulation above or below the deck, and is there a vapour control layer?
- What fall does the flat roof have, and where does the water drain to?
- What is the guarantee on the roof covering, and who gives it?
- How tall will the finished building be, measured from the highest ground next to it?
That last answer decides whether you need planning permission, so get it in writing.
If you would like a builder near Chesterfield to look at your garden and price the options, fill in the form below.