Do Sun Tunnels Work on a Flat Roof?

Flat roofs on garage conversions, kitchen extensions and rear additions on newer estates often cover a deep, dark space: a kitchen reaching back from the house, or a utility room tucked against the party wall. They are exactly the rooms where a sun tunnel helps.

The collector looks straight at the sky, so it catches light from the whole day without being tilted. On a pitched roof the dome sits on a slope and favours one direction. Here it is level, and the daylight it collects stays steadier from morning to evening.

On a pitched roof a tunnel is flashed into the tiles. On a flat roof it stands on a small upstand, often called a kerb or curb, which lifts the dome above the waterproof covering. The kerb stops standing water reaching the joint, and the roof membrane is dressed up its sides and finished under the collector's skirt.

The kerb is low, much lower than a lantern, so the 150mm projection limit for permitted development is easy to stay within on a house. The Class C planning rules for rooflights set out that limit and the homes where it does not apply, such as flats.

This is the real question on a flat roof. A pitched loft has open space for a tube. A flat roof build-up is often a thin sandwich: the covering, insulation, the deck, joists and the plasterboard. In many extensions the joists run right to the ceiling, so the tube has only the depth of the joists to cross.

Where a joist sits in the way, we do not cut it. Moving the tunnel a little to land between two joists is simpler than trimming timbers, and keeps the structure as the builder intended. A rigid tube suits the short, straight runs best; the choice is covered in rigid vs flexible sun tunnels.

On a flat roof the insulation layer is vital to the warmth of the room below. A tunnel passes through that layer, so the tube is sealed and surrounded by insulation, and the dome is double-skinned. This is the same principle explained on our page about whether sun tunnels lose heat.

Where the work touches the roof structure or its thermal performance it can be notifiable, and when it is, we send the notice to Building Control for you. Each evening the roof is left weathertight, and dust sheets protect the floors below until the job is finished.

A flat roof tunnel takes less time on site than you might expect, because the roof stays whole and the tube is a ready-made part. The order is the same each time. First the covering is opened over a small area, then the kerb is formed or fitted, and the membrane is dressed up to it. After that the collector goes on and the tube is dropped through from above.

Inside, the tube is joined to the diffuser ring fixed to the ceiling, and the gap between tube and plasterboard is sealed. If you have chosen a light kit or a vent, those are wired or connected at the same visit. Check that the room below is clear of furniture, and we will lay protection over the floor and any fittings we cannot move.

The point that catches people out is the finish on the roof. A felt or single-ply covering must be compatible with the kerb material, and an old covering near the end of its life is better renewed around the tunnel than worked around. We raise this at survey, because a short run of new membrane now costs far less than reopening the area later. We look at the covering for its age and its ability to take the new upstand, and we tell you straight if it should be renewed in the same visit.

If you want to see the sky, or open something for air, a flat rooflight gives more. A tunnel brings daylight only, unless it is fitted with a vent kit, which you can read about in sun tunnel ventilation kits. Flat rooflights with an opening sash are described under flat roof rooflights.

For a narrow passage or a small cloakroom, though, the tunnel is often right, because it asks little of the roof. The full service is on our sun tunnel installation page, and we carry out a free survey with a written, itemised quote before you commit to anything.