for ventilation. Most proprietary roof windows such as Velux have built in ventilators.
as a means of escape[for the purposes of part B of the Approved Documents] - Structural means whereby [in the event of fire] a safe route or routes is or are provided for persons to travel from any point in a building to a place of safety. if there is a fire more +/-»
problems with planning permissionthe legal basis for being allowed to do some form of development such as building a house. (not to be confused with Building Regulations which is all about whether the building is properly constructed). see more on Planning Permissionmore +/-»
rain running in if a roof light is opened too far more +/-»
Roof lights tend to be very efficient in terms of allowing light into a room compared with the same size of window in a wall. They not only have an unimpeded view of the sky but they are more centrally placed over the area they are lighting.
Using natural light whenever possible saves a considerable amount of energy. The amount of light entering a room through a window is partly to do with the window size but more importantly is to do with how much sky is visible from the window and the size and shape of the room. The light entering directly from the sky may be hundreds of times that which gets reflected from the ground or trees (although sometimes water can be used to reflect light up into windows). The maximum amount of light is provided by roof lights positioned over the middle of a room. One of the ways of measuring the natural daylight falling on a particular area in a room is by calculating the daylight factor (DF). This is the ratio, as a percent, of the daylight that reaches a point inside a room compared with what light would be there if there was no roof or building, in other words just an open sky. (It is measured under an overcast sky and is measured at table top height, 850mm, and is averaged over the room).
The measurement takes into account how much direct sky is visible from within the room, how bright the external surfaces are outside the room and how reflective they are within the room. The diagram shows how the light coming in through a window tends to be much more restricted by surrounding shading than is the case with a roof window.
It works out that a DF of lower than 2% looks gloomy and artificial lighting is required. Between 2% and 5% is usually suitable without artificial lighting provided it is not for a working area (such as reading or in a kitchen) and anything above 5% looks quite brightly lit. There are various formulas for calculating this but it usually comes down to the experience of the architect.
If you are very concerned about a particular lighting situation you can ask your architect for a calculation to be done or you can use the BRSBuilding Research Station Daylight protractor and the Simplified Daylight Tables described here. The brightness of the sky is assumed to be 5000 lux (lumens/sq.m.) and so a DF of 6% requires a minimum level of lighting of about 300 lux for a horizontal working area. Offices are often lit at 500 lux whereas domestic situations may go down to around 50 lux where only a little background lighting is needed.
This of course depends on how low down in the roof the roof window is situated. Quite often with older houses a low roof light will interfere with an existing timber purlin. Cutting a purlin is usually a serious structural matter although a structural engineer may be able to work out a way round this.Powered by Hackadelic Sliding Notes 1.6.5
The Building regulationsThese are the legal regulations which govern how a house is constructed. (not to be confused with Planning Permission which is about whether you are allowed to build the house at all or what it might look like) see Building Regulations) B1 page 17 and para. 2.8 on page 19 describe how roof windows may be needed for escapePowered by Hackadelic Sliding Notes 1.6.5
Occasionally, when new stairs are provided to serve an attic conversion there will be limited headroom if the landingA platform or part of floor structure at the end of a flight of stairs or ramp. is near the eaves. A carefully situated roof window may give slightly more headroom along with a brighter space. The Building Regulations cover the headroom required over stairs in K1 paragraph 1.10 . It can be reduced slightly for loft conversions.Powered by Hackadelic Sliding Notes 1.6.5
opening roof lights can usually be turned right round to enable cleaning. Fixed ones can be glazed with self cleaning glass. Dome roof lights are usually constructed of formed polycarbonate or acrylic, in two or three skins. Dirt does tend to adhere to plastic surfaces, especially as they start to age and roughen slightly. Usually they cannot be cleaned from the inside. As they are normally employed on flat or nearly flat roofs it may be possible to easily access them from the outside for cleaning. They often work well with flat(ish) living roofsA roof with a covering of soil or growing medium and plants. They tend to be divided into turf roofs with a 150mm layer of soil and sedum roofs with a thinner layer (about 40mm).
see Living RoofsA roof with a covering of soil or growing medium and plants. They tend to be divided into turf roofs with a 150mm layer of soil and sedum roofs with a thinner layer (about 40mm).
see Living Roofs which tend to deposit bits of plant material on them when birds pick around for food. Of course you don’t get a clear view through a plastic roof light but that might not matter on a flat roof.Powered by Hackadelic Sliding Notes 1.6.5
In certain sensitive situations such as conservation areas planners don’t like the look of large areas of glazing on roofs. It may be acceptable to use the smaller ‘conservation style’ roof windows which have a vertical dividing bar to visually split up the glazing in the manner of the old cast iron roof lights. It may also be necessary to let the roof light into the roof slightly so that its surface is level with or very slightly above that of the surrounding roof finish. Powered by Hackadelic Sliding Notes 1.6.5
Especially with loft conversions, it may be necessary to cut into a purlin to be able to insert a roof light at a level low enough to provide a decent view. As the purlin is one of the main structural members holding up the roof joists it will be necessary to insert some other means of support. This can become quite expensive.Powered by Hackadelic Sliding Notes 1.6.5
nothing worse than trying to sleep off a hangover with the sun belting in through a roof light without a blind. Providing the top of the window is reachable then a normal blind is fine. If it is too high up in the roof then motorised blinds are available.Powered by Hackadelic Sliding Notes 1.6.5
it’s surprising how annoying it can be to leave a tilting roof light (just above your bed) slightly too far open during a thunder storm, so that all the rain runs back and into the bed. And you didn’t realise and you come back home and climb into bed……..urgh! A restricting chain can be fitted to limit the tilt of the window.Powered by Hackadelic Sliding Notes 1.6.5
