Is a rooflight or dormer the right choice for my period home?
The installation of rooflighting or a dormer can be an effective way to bring daylight into an old building, especially one that’s undergoing major changes, such as a loft conversion or extension. Rooflighting or dormers can also provide additional natural ventilation, as well as easy access onto roofs for maintenance or escape in the event of fire.
Yet the provision of new glazing at roof level is one of the most sensitive issues involving work to old buildings. This is because of the intrusive impact that such insertions can have on the character of a building, not least where a steep roof is the dominant element. In addition to aesthetic factors, other considerations when contemplating the installation rooflighting or dormers on an old building include the need to minimise disturbance of existing roof timbers, desired performance characteristics, such as good thermal insulation, and cost.
How much will a rooflight or dormer cost?
Typical prices to supply and fit a medium-sized conservation rooflight range from £1,600 to £1,900. If your home is not in a conservation area, you will not be required to choose a low profile option and can expect to pay much less (around £700 including fitting).
The cost of installing a dormer varies greatly with the range of designs, but a general guide would be £6,000 to £12,000 plus VAT. The figures given here relate to prices for the south-east of England outside London, and exclude any costs necessary for scaffolding and skip hire.
Are there different styles of rooflights available?
- Rooflights, sometimes known as skylights, are available for pitched or flat roofs. Their excessive use on a roof, though, can create visual clutter and many standard designs are incongruous with older properties.
- On an older building, conservation rooflights will generally be preferable. Of slender section steel with a vertical emphasis, these lie flush and emulate the look of traditional cast-iron rooflights.
- Bespoke sizes can also be made where rooflights of standard dimensions are unsuitable. The use of excessive flashings between the rooflight and roof should be avoided to maintain the character of the building.
What are the latest technologies?
Modern rooflighting has low-emissivity (low-e) double-glazing and thermally efficient frames. Some installations have motorised operation linked to rain, temperature or smoke sensors for automatic activation.
How many rooflights do I need?
Many roofs have larger and more numerous rooflights than necessary, particularly given that they admit more light than a normal vertical window of the same dimensions. Two smaller rooflights will frequently be preferable to one larger one. In some cases, a number of conservation rooflights banked together to form an elegant feature window will create a better solution than peppering units individually elsewhere across a roof slope.
The positioning of rooflights is also important. While a random pattern with a variation in size can work well for a barn conversion, a number of similarly sized rooflights at the same height on the roof may well be in better keeping with, say, a more formally designed house.
What are the benefits of a dormer?
Dormer windows – upright windows, usually with their own roofs, built into pitched roofs – have been used on domestic buildings since the 16th century. The diversity of dormers found reflects the pitch and construction of roofs along with local traditions. Designs vary from the gabled or hipped types with rendered, lead, slated or tiled cheeks (haffits) to wedge-shaped (‘pent’) flat-roofed dormers.
- A dormer has the advantage of creating additional space outside the main roofline, which can be beneficial for providing staircase headroom, for example. New dormers of an appropriate scale, design and position may be successfully integrated where there are already precedents, either original or more recent, and do not result in the roof looking overcrowded.
- There are many situations, however, where the use of a dormer is not appropriate. This might be, for example, where they unbalance a building, or disrupt the continuity of a terrace or block of buildings of uniform design. Dormers also tend to look out of place on barn or stable conversions, where they undermine the simple form so vital to the character of many former agricultural buildings.
What size dormer is best for my home?
The shape, size and materials of new dormers should reflect any already on the building or in the neighbourhood. Dormers usually create better visual harmony if they are a little smaller than the windows of the floor below. Well-considered modern touches should not be ruled out, however. For example, additional glazing could perhaps be installed in place of traditional render for the small triangular gable on the front of a pitched-roof dormer.
Great care is needed when insulating dormers to avoid increasing the thickness of their cheeks excessively, which adversely affects their proportions. Aerogel, a thin, high-performance, breathable form of insulation has been employed successfully to insulate dormers.
Do I need planning permission?
Bear in mind that planning permission could be required before inserting new rooflighting or dormers, for example in listed buildings or if your property is located in a conservation area. Regardless of this, it will be necessary to obtain building control approval.
Buildings that are listed or of traditional, breathable construction, however, are exempt from compliance with the energy efficiency requirements of the building regulations where harm to them would otherwise result.
Are there any other considerations?
- The positioning of new rooflighting or dormers must carefully balance the visual impact internally and externally while also optimising daylight.
- Roof-level glazing is often best located in areas least visible from public vantage points, such as at the rear of buildings or on the inner slopes of double-pitched roofs.
- Rooflights or dormers also generally look more comfortable when not inserted too high on a roof slope (the main exception being ridge rooflights).
- New rooflighting or dormers should be designed to fit in between, or have the least physical impact on, historic timber roof structures, ceilings and plasterwork.
- It is important to minimise the removal of historic fabric and ensure that the structure of the building is not seriously overloaded or weakened.
- You’ll also need to consider maintenance and security. For example, there might be an opportunity to improve roof access or reduce maintenance requirements by using self-cleaning glass.
- Future maintenance will also be minimised by ensuring that the rooflight is designed for its intended environment, particularly in coastal areas.
- It may be prudent to fit security bars below rooflights in single-storey flat roofs.
Is restoring original roof glazing a suitable option?
Apart from dormers, many buildings rarely had any form of glazing in the roof historically. Its use became increasingly practical with advances in glass technology from the Industrial Revolution onwards. Early fixed rooflighting comprised simply a pane of glass inserted in place of tiles or slates. The first opening rooflights, installed in Georgian and Regency times, were timber-framed and often weathered with lead.
These early rooflights were superseded in the mid-19th century by mass-produced cast iron versions that were top-hinged. The Victorians also employed large areas of overhead glazing, for example, in railway stations and the famous Crystal Palace.
Any surviving old rooflighting or dormers could well be of historic significance so worthy of consideration for retention, rather than replacement. Original dormers should be repaired where possible, although some modifications may be justified, particularly where weatherproofing has been known to be problematic.
Is there an alternative to rooflights or dormers?
- A sun pipe can duct natural light through a roof space or storey, and although they do not provide ventilation, they can be used where conventional rooflights are not practical. Easy to install, the light enters a rigid or flexible reflective tube via a translucent collector on the roof and is spread into the room or space below by means of a diffuser that looks like a normal electrical light fitting.
- Conservation sun pipes are available with a flat top rather than a standard dome, so resembles Victorian cast-iron rooflights that sit flush with the plane of the roof. Fittings are made to suit different roof coverings, such as slate or pantiles.
- A variation to a sun pipe is a sun tile, which is a clear tile connected to a mirror duct.
- The wider the sun pipe tube, the further light can travel.
- The flexible tube versions lend themselves well to channelling light into awkward spaces. They transmit less light, though, than rigid tubes due to their crinkled interiors and are designed for use where there are shorter distances between the roof and ceiling. The amount of light reflected decreases at every bend and with the pipe length.
- Motorised shut-off dampers can be fitted to sun pipes for remote control from a wall switch.
- It is also possible to wire the diffusers so that they work like conventional electric lights when light levels fall.
Additional glazing solutions
- Patent glazing comprises glass in a lightweight, non-loading framing system that can be used for roofs and walls. The use of patent glazing works well in some situations, for instance linking a new extension to an existing building. The framing is typically of aluminium although, as with other forms of glazing at roof level, heritage patent glazing is available. This comprises lead-clad steel glazing bars designed to accommodate double-glazing.
- A frameless alternative to patent glazing is structural glass, which creates a seamless, continuous glazed look.
- The insertion of a simple pane of glass in place of tiles or slates may be an option for outbuildings and garages. Glass pantiles are produced for pantiled roofs, that is roofs covered with large tiles with an S-shaped profile.
Douglas Kent is a chartered building surveyor who oversees the technical activities of the Society for the Protection of Ancient Buildings (SPAB)