How Many Skylights Are Right For Your Space?

March 2026

Skylights can transform dark, dull rooms into bright, airy spaces. A roof window brings natural daylight into areas where conventional windows don’t work and can even provide ventilation when you choose an opening model. However, adding too many skylights can lead to glare and unwanted solar gain while too few fail to deliver the light and comfort you expect. Determining the right number for your room depends on several factors beyond personal taste. This guide explains how skylights work, how to calculate the right total glazed area, and how to plan your layout so your new roof windows enhance rather than compromise your home.

What do skylights do, and why does quantity matter? 

A skylight, also called a roof window, channels daylight through the roof into the space below. Because it is angled to the sky, a roof window lets in up to three times more light than a vertical window of the same size. This high light output can reduce dependence on artificial lighting and improve wellbeing. Operable skylights also provide ventilation, releasing warm air that collects near the ceiling. 

Skylight size and number have a big impact on comfort. Large glazed areas admit more daylight but can also increase heat gain in summer or heat loss in winter. The U.S. Department of Energy advises that the total skylight area should never exceed 5 percent of a room’s floor area when there are many other windows, and no more than 15 percent when the room has few or no windows. Exceeding these proportions may cause glare or overheating, while undershooting them results in under lit rooms. Balancing these considerations helps you enjoy all the benefits of daylight without the drawbacks.

Key factors affecting how many skylights you need 

Room size and floor area 

The first step is to measure your room. Multiply the length and width of the space to calculate the floor area. As a rule of thumb, the total glazed area of all skylights should be between 5 and 15 percent of the floor area, depending on how bright the room already is. Professional advice suggests that roof windows should account for at least 10 percent of the floor space in a loft conversion, and more glazing is often desirable in dark spaces. Once you know the desired total glazed area, divide it by the area of a typical roof window to determine how many you need. Standard sizes offered by major manufacturers such as VELUX and RoofLITE+ include 55 by 78 cm, 66 by 118 cm, 78 by 98 cm, 78 by 118 cm and 78 by 140 cm. These sizes equate to roughly 0.4 square metres to 1.1 square metres of glazing. 

Ceiling height and room proportions 

Skylights deliver light deep into a room because they are angled towards the sky. High ceilings or large rooms may require multiple windows to achieve even lighting. The distance between skylights should be about 1.5 times the height between the floor and the roof. For example, if the vertical distance from floor to roof is 3 metres, space the skylights roughly 4.5 metres apart to spread the light evenly. In very tall rooms, two or three smaller skylights arranged in a line can provide more uniform illumination than a single large unit. 

Orientation and sunlight 

The direction your roof faces influences how much light and heat your skylights admit. According to energy saving guidance, north facing skylights provide consistent but cool illumination. East facing skylights capture morning sun and warmth, which can be welcome in bedrooms or breakfast rooms. West facing skylights admit strong afternoon sunlight and can cause significant heat gain on summer afternoons. South facing skylights deliver the most passive solar gain in winter but may lead to overheating in summer. Placing skylights under the shade of deciduous trees or fitting interior or exterior blinds will help control heat gain. When planning multiple skylights, vary their orientation where possible to spread light throughout the day. 

Roof pitch and slope 

Roof angle governs which skylight models you can install. Centre pivot roof windows, one of the most common types, typically suit roof pitches between 15 and 90 degrees. Top hung roof windows open outward and are ideal for lower roof pitches, usually 15 to 55 degrees. For roofs below about 15 degrees, choose products specifically designed for low pitch applications. The slope or tilt also affects solar gain. To maximise winter heat gain and minimise summer heat gain, the skylight’s slope should be equal to your geographical latitude plus 5 to 15 degrees. A professional installer can advise on the appropriate pitch and flashing kits. 

Room use and lifestyle 

How you intend to use the space also influences how many roof windows to install. Living rooms, home offices and loft conversions benefit from generous daylight to improve mood and productivity. Kitchens and bathrooms may need smaller or fewer skylights to avoid excessive heat and glare while still removing cooking moisture or steam. Bedrooms may only need one or two roof windows for soft morning light. Because skylights let in so much more light than vertical windows, it is usually best to err on the side of fewer units in rooms used primarily for relaxing or sleeping. 

Calculating skylights, a practical example 

For a 20m² open-plan kitchen diner with very little natural light, use the 15% rule. That means you’d want around 3m² of roof glazing, which typically works out at around three standard 78 x 118cm roof windows. 

If the same room already has large patio doors or good natural light, use the 5% rule instead. That means you’d only need around 1m² of glazing, roughly one standard roof window or two smaller units. 

For a 12m² bedroom with a normal-sized window, the 5% rule is usually enough. That’s around one medium roof window or two small ones, depending on the level of light you want. 

As a general guide: 

• Use 15% for darker rooms 

• Use 5% for rooms that already have good natural light 

• Choose fewer windows for softer light 

• Choose more (or larger glazing) for brighter spaces

Additional design considerations 

Spacing and layout

Keep skylights 1.5 times the room height apart to avoid pools of light and darkness. Align multiple windows symmetrically to create visual harmony. 

Ventilation and operation

Decide whether you need manual, electric or solar powered opening. Operable skylights can purge stale air and reduce overheating. For high ceilings, consider electric or solar models with remote control. 

Glazing and energy efficiency

Look for low emissivity coatings, multiple glazing layers and insulated frames to minimise heat loss and reduce condensation. Laminated or toughened glass provides safety and durability. Triple glazed units offer excellent insulation and are ideal for bedrooms and living rooms. 

Building regulations and emergency escape

Building regulations may require larger roof windows that double as escape routes in loft conversions. Products labelled as emergency escape or means of escape windows meet these requirements and are usually top hung models. Always check local regulations.

Planning your project 

Before committing to purchases, measure your space carefully, assess how light moves through the room at different times of the day and think about where furniture will be placed. Use the 5 to 15 percent rule to estimate the total glazed area and adjust for room use. Consult an architect or installer if your roof structure is complex or if you plan multiple openings. Guidance on important things to consider before installing skylights covers planning permission, structural considerations and other practicalities.

At Sterlingbuild we supply a wide selection of pitched roof windows from trusted brands such as The VELUX Group, RoofLITE+, Fakro, Keylite and The Rooflight Co. Our range includes centre pivot, top hung, electric and solar powered models, conservation style windows and low pitch options, with sizes from 55 by 78 cm up to 134 by 140 cm. We also stock combination flashings that allow you to install multiple roof windows side by side, such as the VELUX 3 in 1 system.