Dormers and roof windows

1. Dormer

1.1. Advantages

One of the key advantages of a dormer is added space, and therefore, more functionality. A dormer will also make your home stand out.

1.2. Disadvantages

It seems that the biggest downside of a dormer is that it allows much less daylight in. This is the result of poor refraction of light. Moreover, sometimes the upper edge of the roof window is not flush with the roof. Any shading solution or fly screen can only be installed on the outside. Multiple joints and corners increase the chances of water ingress, especially if not installed properly. Its also important to enable air circulation around thermal insulation, which increases costs.

2. Construction of a dormer

Construction of a dormer requires a deeper dive into the roof structure, which is why it involves construction of that part of the roof structure, the dormer, and window installation. Since a dormer is a house within a house, it has to include all relevant parts, including quality thermal insulation, which allows no thermal bridging. Its because of all these reasons that we recommend you leave this construction to professionals.

2.1. Do you need planning permission for a dormer?

The answer is yes. You need to get planning permission to construct a dormer, because of the alterations to the roof structure and therefore to the appearance of the entire house. If you live in an apartment block, youll also need permission from other owners.

2.2. Dormer insulation

Tuck 20 cm thick external wall thermal insulation around the dormer. The roof of the dormer, on the other hand, should include 40 cm thick insulation like the rest of the main roof. Be sure to install a vapour barrier as efficient protection from moisture, a secondary roof structure as protection against stormwater, and to allow free air circulation above the thermal insulation layer.

2.3. Dormer windows

Like with roof windows, a wide choice of materials is available in this case, too - wood, PVC, aluminium. The recommended type of installation is RAL. Opening actions also vary and some of the possible options include tilt and turn, hinged, and sliding.

3. Dormer price

Dormers are always costlier than roof windows.

4. Roof window

4.1. Advantages

The most obvious advantage of having a roof window is lots of daylight and heat (up to 30% more), and if you include more than one, youll get more light. A roof window facilitates ventilation, gives the illusion of greater space, isnt as conspicuous as a dormer, and boosts the aesthetics.

4.2. Disadvantages

Possible downsides include the risk of water ingress in case of improper installation and a somewhat limited view (except in case of panoramic windows).

5. Installation of a roof window

Although theres no doubt that its much easier to install a roof window than construct a dormer, you should definitely hire a professional in either case. For full protection against water ingress, the window should be sealed as allowed by the roof covering in place. Another important thing is to include two barriers - an air barrier at the bottom and a vapour barrier at the top. Also, the thermal insulation underneath battens has to be as thick as that on the rest of the roof. This, however, is not possible when a roof window is installed between rafters. In this case, additional elements allowing free air circulation are installed.

5.1. Planning permission

Unlike a dormer, a roof window doesnt require planning permission.

5.2. Insulation and glazing

Laminated glass windows are a highly recommended option. This type of glass doesnt shatter, so when broken, it holds glass shards in place. The insulation layer is installed over the internal lining of a roof window. Use of a special installation kit is always recommended. The LSG kit includes an insulation collar, waterproof flashings, and a vapour barrier.

5.3. Opening action

Roof windows can open in several ways. First, they can either open manually or automatically. Centre-pivot windows, with the handle at the top and hinges in the middle, are the most popular choice among homeowners. There is also a panoramic opening action with the handle at the bottom. A panoramic window ensures a better view and is more appropriate for low slope roofs.

5.4. Timber, PVC, or aluminium roof windows?

Youre free to choose among PVC, timber, and aluminium, or a combination of these. PVC is the cheapest option, followed by timber, while aluminium is the most expensive option. External aluminium, copper, or zinc protection makes a roof window more resilient.

6. Price of roof windows

How much do roof windows cost? Windows are always cheaper than dormers. A standard PVC roof window, composed of wooden internal parts and tempered glass can cost around NGN50,000 - NGN100,000 excluding installation. Needless to say, there are always costlier options that can double the price.