What Makes a Rooflight a Conservation Rooflight?

Older roofs often carry small cast-iron rooflights, set into slate or tile so that they sit almost level with the covering. They have a thin metal frame, a single pane or two panes divided by a bar, and no protruding box. A conservation rooflight reproduces that shape in a modern, insulated unit.

The aim is a rooflight that disappears into the roof from the street. A bulky plastic dome or a deep frame announces itself. A low, slim one reads as part of the original roof.

The older units were small, and many were no wider than the gap between two rafters. Modern versions keep those proportions, which is part of why they suit the roofs they copy.

The conservation rooflight rules guide covers the planning side of fitting one on an older or protected house.

Looking at the existing roof is the best test of whether a conservation unit suits it. If the house already has an original rooflight, the new one can be matched to its size and bar pattern.

There is no agreed standard that a unit must pass to carry the name, so the label is a description of design. The features below are what makers and conservation officers look for.

The centre bar is optional on some units and fixed on others. The centre bar answer explains when it is worth having. The finish is covered in the colours answer.

Depth is the feature that differs most between products. Some units rise a few centimetres above the covering, while the lowest-profile ones sit almost level with it. When the unit is viewed from the street, a few centimetres of upstand is the difference between a rooflight that disappears and one that shows.

Old rooflights were single glazed and let heat out. A modern conservation unit keeps the old outline but adds a sealed double-glazed unit inside a narrow frame. The glazing is made thin enough to fit that frame, which is part of what separates a good conservation unit from a basic one.

Part L names its figure by how a unit sits. In Approved Document L a rooflight is a glazed unit raised on a kerb or upstand, out of line with the roof surface, and a roof window lies flush in the same plane as the roof around it. Laid in the line of a pitched slope, a conservation unit fits the second description, so the figure to check is the roof window limit for an existing home: a U-value no higher than 1.4 W/m²K, though a unit rated Band B for window energy passes instead. A kerb-mounted flat version is held to the rooflight limit of 2.2 W/m²K. The maker's data sheet gives the figure for the exact model. The double glazing answer looks at the glass in more detail.

The frame is usually steel or aluminium, finished to look like painted iron. The metal is narrow because the glazing is bonded in or held by a thin bead, depending on the maker.

Condensation is worth a word, since slim frames carry less insulation than a deep one. Thermally broken frames and warm-edge spacers in the glazing lower the risk of cold surfaces, and opening versions help ventilate the room.

Conservation rooflights suit several situations.

• Slate and tile pitched roofs. The unit sits low against the covering, with flashing matched to the roof material.
• Older houses and outbuildings. Barns, coach houses and converted stables where a modern box would look wrong.
• Roof slopes visible from the street. The low profile limits how much of the unit can be seen.
• Listed or protected buildings. The style suits them, though the consent question stays separate.

The style does not remove any consent requirement. A listed building will almost always need listed building consent for a new rooflight, and where an Article 4 direction covers a conservation area, planning permission may be needed. The planning authority (BCP Council in Bournemouth) makes the decision, and we can check the position at the survey.

Inside the room, the unit shows as a narrow glazed opening with a splayed lining, a detail that older roofs also have. The lining is formed to suit the thickness of the roof build-up, which the survey measures.

A conservation rooflight is fitted into the roof covering rather than on top of it, so the opening is cut to suit the frame and the flashing is dressed into the slates or tiles around it. The sizes are small, often a single bay between rafters, and the fit is tight.

At the survey we measure the rafter centres, the pitch, the covering type and the gauge of the slates or tiles. Those readings set the unit's size and the flashing, and they go on the measured survey sheet. Our conservation rooflight installation page describes the full service.

Colour and bar pattern are also noted. The result is recorded on the survey sheet with the unit's maker, size and finish, so you can compare what is proposed with the existing roof before anything is ordered.