Dormers are a classic part of the British building vernacular and typically associated with traditional-style properties. Specified on a practical level to introduce more light and head height than a simple rooflight, and aesthetically to break up a roofline’s mass, they have developed an unfortunate reputation in recent decades as a result of badly designed loft conversions.

Luckily, architects and designers are beginning to reassess them for the modern home and at the same time are doing them better than ever on traditional properties. So how do you make sure your dormer is a good one?

Planning Permission

The addition of a dormer window will often require planning permission. (If your home is listed or lies within a Conservation Area it will always require planning permission.) For example, if the dormer is to be placed on a roof slope which forms the principal elevation of the house and fronts a highway, or the dormer will extend above the ridgeline of the property, planning will be required. Planning consent will also be required when the dormer features a balcony or veranda, or would increase the area of the original roof space by more than 40m3.

However, if the dormer window is going to be on a roof that does not front a highway then you should be able to proceed without planning permission.

Construction

A dormer window is basically a vertical window with its own roof. They are at least partially positioned within the slope of the roof.

Installing a dormer window is a fairly complicated process, more disruptive than a rooflight installation and, in all honesty, best left to the professionals. A section of the roof is cut out, a frame for the dormer constructed and supports for the new window put into place. There is also the matter of insulation — get this wrong and the thermal efficiency of the whole roof can be compromised.

A dormer window is typically made up of a timber frame projecting from a pitched roof. They usually comprise of a roof, two sides or ‘cheeks’, and a window at the front. Most dormers will be ‘projecting’ – positioned entirely above the line of the pitched roof – although others are recessed or semi-recessed.

Although their construction varies, dormer windows are often cut into the eaves of a building and installed on one-and-a-half-storey buildings to allow light into the rooms within the roof space.

Insulation

If you are insulating the dormer window at the same time as the main roof, this will be a far easier task than retrofitting insulation into an existing dormer.

If you are, however, adding insulation to an existing dormer, your main aims should be to:

  • upgrade the thermal performance of the dormer window as much as possible;
  • add enough insulation to the dormer to prevent heat loss;
  • prevent ‘cold bridging’ to ensure that airtightness is maintained around the dormer, but that ventilation paths to any ventilated roof spaces above the dormers are not disturbed;
  • finally, you should minimise the risk of corrosion to the underside of any lead used to clad the dormer cheeks or roof.

The effectiveness of insulation to the main roof can be significantly compromised if dormer windows are left uninsulated. In most cases, the only space available to insulate the cheeks will be within the spaces between the elements of the frame. This may mean insulation thicknesses of 75mm or less, which is not normally enough to meet Building Regulation targets. To make the insulation work effectively it should be packed consistently into all corners of the spaces, otherwise cold bridging will be likely.

It is also a good idea to add a thin layer of insulating board either outside or inside the frame — even 20mm can have a big impact on the overall insulation value. Kingspan offer good solutions.

Positioning and Style

The key to getting dormer windows right is all down to style, positioning and proportions. Dormers should be carefully positioned to respect the symmetry of the building, regularly spaced – not too close to the edge of the roof – and balanced in their size.

Dormers can look better if they are set well in from the eaves and ridge. In terms of positioning, dormers tend to be at their best when placed well down from the main ridge of the house — slightly lower than halfway down the roof is ideal.

Aim for the pitch of the dormer’s roof to mirror that of the house or surrounding houses — note that keeping the pitch of the dormer the same as that on the roof means you can have tiled rather than lead valleys. Dormer roofs can be pitched with a gable end, hipped or half-hipped, flat or curved, or be mono-pitch.

The most commonly seen dormer roof is pitched with a central ridge, which creates a neat triangular gable above the window. Others are hipped at the front or have roofs that slope to the front (sometimes called a catslide roof). Thatched roofs and cottage roofs suit dormers with matching roofing materials that curve at the sides, creating a smooth ‘eyebrow-shaped’ dormer (see lead image).

The other common form of dormer roof has a flat roof. These are actually slightly pitched to allow rainwater to run off and are commonly covered with lead, zinc, copper, asphalt or felt.

Constructing the dormer roof in the same style as the main roof – i.e. a hipped roof with a hipped roof – looks best and on roofs with a shallow pitch (usually covered with slate), a flat-roofed dormer generally works better than one with a steeply pitched roof. In terms of shape, study the shapes within your home’s existing design and aim to repeat these shapes within the dormer window in order to help it blend in as much as possible.

Although the material used for the roofs of dormers is usually the same as that found on the main roof, a dormer window is actually the ideal way to introduce a new material. Cedar shake shingles, tile hanging and weatherboarding used for the cheeks, for instance, all look great. Just make sure to create some visual cohesion with the rest of the house in some way, by matching the paint colour on the windows, for example. Of course dormer cheeks can match the roof covering, but more often they are of lead or wooden boarding. Masonry cheeks are unusual because of the weight they add to the roof structure.

When it comes to proportions, beware of going too big. Two or more small dormer windows tend to look better than one large, overpowering dormer — although this is a slightly more expensive option. And finally, aim not to exceed 1.2m in width.

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