A Beginner’s Guide to Roof Windows, Rooflights and Skylights
Want to draw more light down into your room? Read this planning checklist before investing in roof glazing
Before you install a window in your roof, take a look at our expert guide. Find out whether you need Planning Permission, how to choose window treatments, and what you can expect to spend to transform your particular loft space or extension.
Professional advice from: Angus Eitel of Fiftypointeight Architecture + Interiors; Marienne Pachonick of Marienne Pachonick Architects; Mark Warburton of Renka
Professional advice from: Angus Eitel of Fiftypointeight Architecture + Interiors; Marienne Pachonick of Marienne Pachonick Architects; Mark Warburton of Renka
What do I need to consider when choosing roof glazing?
Marienne Pachonick has a checklist. “Orientation of the house and where the sun path is; privacy; the view; the height of the rooflight and access for opening, cleaning and maintenance; the amount of glazing in relation to the space; security, and whether the rooflight needs to be able to carry any loading, such people walking across it.”
“Consider what type of roof it’s being fitted on or in,” Mark advises. “Flat roof or pitched? If pitched, at what angle? Make sure you and your supplier have a shared understanding of size; by that I mean whether the dimensions quoted are ‘internal’ [the size of the hole in the roof] or ‘external’ [to the external face of the kerb or upstand].”
Whether the window will be fixed or openable is another important consideration, he says. “There are often limits to the size of a section of window that can be electrically openable.”
Marienne Pachonick has a checklist. “Orientation of the house and where the sun path is; privacy; the view; the height of the rooflight and access for opening, cleaning and maintenance; the amount of glazing in relation to the space; security, and whether the rooflight needs to be able to carry any loading, such people walking across it.”
“Consider what type of roof it’s being fitted on or in,” Mark advises. “Flat roof or pitched? If pitched, at what angle? Make sure you and your supplier have a shared understanding of size; by that I mean whether the dimensions quoted are ‘internal’ [the size of the hole in the roof] or ‘external’ [to the external face of the kerb or upstand].”
Whether the window will be fixed or openable is another important consideration, he says. “There are often limits to the size of a section of window that can be electrically openable.”
What size should I choose?
Products will be significantly cheaper if you go for an off-the-shelf item rather than bespoke, Angus advises. “The constraint is that you then have less flexibility in the size – both width- and lengthways.
Widths tend to be based on the standard spacing of timber roof rafters. “Velux roof windows, for example, are available in sizes from 470 x 980mm to 1340 x 1600mm, and 940 x 550mm to 780 x 1800mm,” Angus says. “So you can get small ones or wide ones or long ones, but you can’t get long and wide ones. You would need to go to a bespoke manufacturer for those and that would be noticeably more expensive.”
Be aware, too, he says, that it’s possible to have too much glazing. “It can cause overheating and glare. Roof windows and rooflights can also be noisy in loft conversions when it rains.”
Planning a loft conversion? Search for professionals in your area.
Products will be significantly cheaper if you go for an off-the-shelf item rather than bespoke, Angus advises. “The constraint is that you then have less flexibility in the size – both width- and lengthways.
Widths tend to be based on the standard spacing of timber roof rafters. “Velux roof windows, for example, are available in sizes from 470 x 980mm to 1340 x 1600mm, and 940 x 550mm to 780 x 1800mm,” Angus says. “So you can get small ones or wide ones or long ones, but you can’t get long and wide ones. You would need to go to a bespoke manufacturer for those and that would be noticeably more expensive.”
Be aware, too, he says, that it’s possible to have too much glazing. “It can cause overheating and glare. Roof windows and rooflights can also be noisy in loft conversions when it rains.”
Planning a loft conversion? Search for professionals in your area.
Are there any Planning Permission or Permitted Development issues that might affect my choice?
You don’t normally need to apply for Planning Permission to re-roof your house or to insert rooflights or skylights, as the Permitted Development rules allow for roof alterations.
“But this is within certain limits and conditions,” Marienne says. “With houses, for example, no alteration should project more than 150mm from the existing roof plane or be higher than the highest part of the roof. Any side-facing windows must have obscured glazing and any opening has to be 1.7m above the floor.”
When it comes to planning constraints that affect Listed Buildings and Conservation Areas, Angus suggests looking for heritage-style windows. “Companies such as Velux offer a traditional, conservation-style roof window that blends well into an old building by utilising a vertical centre bar and black finish exterior,” he says. “It can be recessed flat in line with a slate or tile roof finish.”
Mark adds, “In terms of Planning Permission, local authorities have different rules on what’s possible. Always consult your architect or chosen consultant regarding this.”
You don’t normally need to apply for Planning Permission to re-roof your house or to insert rooflights or skylights, as the Permitted Development rules allow for roof alterations.
“But this is within certain limits and conditions,” Marienne says. “With houses, for example, no alteration should project more than 150mm from the existing roof plane or be higher than the highest part of the roof. Any side-facing windows must have obscured glazing and any opening has to be 1.7m above the floor.”
When it comes to planning constraints that affect Listed Buildings and Conservation Areas, Angus suggests looking for heritage-style windows. “Companies such as Velux offer a traditional, conservation-style roof window that blends well into an old building by utilising a vertical centre bar and black finish exterior,” he says. “It can be recessed flat in line with a slate or tile roof finish.”
Mark adds, “In terms of Planning Permission, local authorities have different rules on what’s possible. Always consult your architect or chosen consultant regarding this.”
What about Building Regulations?
“Loft-level rooflights need to have a minimum U-value of 1.6 W/m²K,” says Marienne. The U-value is a measure of insulation: the lower the U-value, the better insulated the window unit. “Rooflights need to be double-glazed to a certain specification and Window Energy Rating, and be fitted in accordance with the manufacturer’s instructions.”
There are also stipulations for rooflights and skylights in ground-floor extensions. “The total area of glazing – windows, doors and rooflights, old and new – cannot exceed 25% of the new floor area,” Marienne says. “This is to avoid excessive heat loss or gain. If you exceed this allowance, you need to treat the space as a ‘highly glazed extension’.”
“Anything around Building Regulations will generally fall under the remit of your chosen professionals,” Mark says. “That said, if you look for rooflights with outstanding insulation values, most applications will be compliant with building control approval.”
“Loft-level rooflights need to have a minimum U-value of 1.6 W/m²K,” says Marienne. The U-value is a measure of insulation: the lower the U-value, the better insulated the window unit. “Rooflights need to be double-glazed to a certain specification and Window Energy Rating, and be fitted in accordance with the manufacturer’s instructions.”
There are also stipulations for rooflights and skylights in ground-floor extensions. “The total area of glazing – windows, doors and rooflights, old and new – cannot exceed 25% of the new floor area,” Marienne says. “This is to avoid excessive heat loss or gain. If you exceed this allowance, you need to treat the space as a ‘highly glazed extension’.”
“Anything around Building Regulations will generally fall under the remit of your chosen professionals,” Mark says. “That said, if you look for rooflights with outstanding insulation values, most applications will be compliant with building control approval.”
Are there any window design features that will prevent a space becoming too hot or cold?
“U-values [the rate of transfer of heat through a structure] are much improved now, so feeling cold shouldn’t be a problem,” Angus says. “However, triple glazing can lower U-values further. Being able to open them easily will help reduce overheating, improve cross ventilation and encourage air movement, too.”
Mark adds, “Openable sashes can be incorporated in most styles of roof glazing, and these can be operated either manually – dependent upon the location and access to the interior of the roof glazing – or electrically. Electric controls are via an on/off switch or temperature and rainfall sensors.”
More: How to Avoid an Extension That’s Too Hot and Too Bright
“U-values [the rate of transfer of heat through a structure] are much improved now, so feeling cold shouldn’t be a problem,” Angus says. “However, triple glazing can lower U-values further. Being able to open them easily will help reduce overheating, improve cross ventilation and encourage air movement, too.”
Mark adds, “Openable sashes can be incorporated in most styles of roof glazing, and these can be operated either manually – dependent upon the location and access to the interior of the roof glazing – or electrically. Electric controls are via an on/off switch or temperature and rainfall sensors.”
More: How to Avoid an Extension That’s Too Hot and Too Bright
How do you open a window that’s not easily accessible, and when is it acceptable to have a fixed window?
“Most roof windows and rooflights open as standard,” Angus says. “Some can be controlled using solar power. In more hi-tech solutions, the roof windows can be incorporated within a building management system [BMS] that determines when to open and close windows based on current climatic and indoor conditions.”
It’s acceptable to have a fixed window when security is paramount or you simply won’t need to open it. “If the rooflight is part of a space already served by opening windows and doors, then it’s not a requirement to have it opening,” Marienne says.
However, Mark advises, “Be sure background ventilation for these particular areas is dealt with in line with building control requirements.”
“Most roof windows and rooflights open as standard,” Angus says. “Some can be controlled using solar power. In more hi-tech solutions, the roof windows can be incorporated within a building management system [BMS] that determines when to open and close windows based on current climatic and indoor conditions.”
It’s acceptable to have a fixed window when security is paramount or you simply won’t need to open it. “If the rooflight is part of a space already served by opening windows and doors, then it’s not a requirement to have it opening,” Marienne says.
However, Mark advises, “Be sure background ventilation for these particular areas is dealt with in line with building control requirements.”
What window treatments do you recommend?
“First, check that your chosen product is compatible with blinds,” Marienne says. “Then it depends on the space the window sits in and whether you require blinds for modesty, sleeping or shading. I recommend blackout blinds for bedroom rooflights.”
“We provide screening options for almost all of our rooflight combinations,” Mark says. “For flat rooflights, consider an automated blind concealed in the build-up of the insulated kerb.”
“Window treatments are a personal preference,” Angus says, “but I feel it’s always better to use a manufacturer-installed solution. Curtains are impractical. Another option is switchable privacy glass: in ‘normal’ mode, the glass is opaque or milky white, but when you flick a switch and pass a current through it, the glass turns clear.”
More: How to Cut the Cost of Your Extension Project
“First, check that your chosen product is compatible with blinds,” Marienne says. “Then it depends on the space the window sits in and whether you require blinds for modesty, sleeping or shading. I recommend blackout blinds for bedroom rooflights.”
“We provide screening options for almost all of our rooflight combinations,” Mark says. “For flat rooflights, consider an automated blind concealed in the build-up of the insulated kerb.”
“Window treatments are a personal preference,” Angus says, “but I feel it’s always better to use a manufacturer-installed solution. Curtains are impractical. Another option is switchable privacy glass: in ‘normal’ mode, the glass is opaque or milky white, but when you flick a switch and pass a current through it, the glass turns clear.”
More: How to Cut the Cost of Your Extension Project
Roughly how much would standard and bespoke windows cost?
“Velux roof window prices, based on standard sizes and glazing and manual openings, start at around £441 and go up to around £776. Frameless glazed rooflight prices start at around £919 and go up to around £1869,” says Marienne.
“The smallest off-the-shelf window starts at £290, and prices can go up to tens of thousands for a bespoke version,” she continues. “The cost of a bespoke window is roughly £1,000 per metre, excluding additional structural support, installation cost or cranage.”
Mark says, “With flat rooflights, we recommend clients do some research prior to construction. Large savings can be made when producing the kerb details to accommodate an off-the-shelf option. Our 1000 x 1000mm flat rooflights start at £595, based on supply only.” All prices exclude VAT.
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“Velux roof window prices, based on standard sizes and glazing and manual openings, start at around £441 and go up to around £776. Frameless glazed rooflight prices start at around £919 and go up to around £1869,” says Marienne.
“The smallest off-the-shelf window starts at £290, and prices can go up to tens of thousands for a bespoke version,” she continues. “The cost of a bespoke window is roughly £1,000 per metre, excluding additional structural support, installation cost or cranage.”
Mark says, “With flat rooflights, we recommend clients do some research prior to construction. Large savings can be made when producing the kerb details to accommodate an off-the-shelf option. Our 1000 x 1000mm flat rooflights start at £595, based on supply only.” All prices exclude VAT.
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“Roof windows are probably the easiest to define, because they’re covered under British Standard BS EN 14351-1:2010,” Mark Warburton says. “This stipulates that roof windows must be installed in the same orientation and ‘in plane’ with the surrounding roof.” These can be fixed or openable.
By contrast, rooflights “are installed out of plane on a pitched roof or installed on a flat roof,” Angus Eitel says. “They’re usually fitted with an upstand or kerb to support the rooflight and keep it watertight.” These can give them a pop-up appearance.
A skylight is typically something bigger than a standard off-the-shelf product. “A skylight often features a combination of rooflights or roof windows side by side, which creates a large glazed opening, offering an expansive view of the sky,” Angus says.
He offers a good rule of thumb. “To put it simply,” he says, “I’d normally refer to a roof window as a single unit that you can access or reach while standing, a rooflight as a single unit that is more inaccessible, and a skylight as a wide expanse of glass that’s larger than a typical single product roof window or rooflight.”