Can a Skylight Go Into Any Roof?

Fitting a rooflight into an existing roof means cutting through the covering, the underlay and the timbers, then rebuilding the edges around a new unit so that water runs round it. Four things decide whether that is straightforward.

• The covering. Is it a plain tile, an interlocking tile, slate, a flat membrane, or something else?
• The structure. Are the rafters or joists where the opening needs to be?
• The pitch. Is the slope steep enough, or shallow enough, for the unit you have in mind?
• The space below. Is there a room, a loft or a stair at the spot that needs light?

The first three are about the roof. The last is about the spot you want to light, which is where we start. Why that comes first is the subject of starting from the spot you use.

Plain clay or concrete tiles

These are common roof coverings on Derby's older houses and some newer ones. The tiles are lifted around the opening and a flashing kit is fitted so the new unit sits flush with the surface. Roof windows are made with flashings to suit this and the work is routine.

Interlocking tiles

Larger profiled tiles, common on interwar and later houses, need a flashing designed for the profile. The unit is chosen to match the depth of the tile, and the surveyor notes the profile before ordering.

Natural and fibre-cement slate

Slate can take a rooflight with a slate flashing kit. The slates around the opening are removed and re-laid with care. Older slate is brittle, so the work is slower. A conservation-style unit is often the better look on slate, and our page on conservation rooflights on slate roofs goes through this.

Flat roofs

A flat roof takes a rooflight on a kerb or upstand, with the waterproof layer dressed up and over it. The roof needs to drain around the unit. A flat roof also needs a fall for water to run off, which is worth checking early. See whether a flat rooflight needs a fall.

Metal sheeting and profiled sheets

Garages, outbuildings and some extensions have corrugated or box-profile sheets. Rooflight sheets in matching profiles can replace some of the existing sheets, or a kerbed unit can be fitted into a cut opening. The method depends on the sheet.

Thatch, stone-slate and unusual coverings

Thatched roofs and stone-slate roofs need specialist treatment, and a rooflight on them is a design decision as much as a building one. We would look at the roof in person before saying yes or no. Where the building is old or protected, planning matters too.

The covering is only the skin. Underneath, the roof is a frame of timbers carrying the load, and a rooflight has to work with it.

On a pitched roof, the opening usually sits between rafters. Where the unit is wider than the gap, a rafter is cut and its load is carried round the opening by new timbers. That is ordinary work. Where the roof has trusses, as many newer houses do, the timbers are part of an engineered frame and cannot simply be cut. Trusses are made to work as a set, so cutting one weakens its neighbours. A unit sized to fit between trusses, or a position that avoids them, is usually the answer, and sometimes a structural engineer is asked to design the support.

Older roofs with purlins and common rafters are easier to alter in this way, though the purlin may shift the position. Flat roofs have joists, and the opening runs with them where possible.

If the roof turns out to be less accommodating than the survey expected, the plan is adjusted on paper first and the fixed written quote reflects it before you agree anything.

Pitch

Roof windows are made for a range of pitches, and the range depends on the product. A very shallow pitch, close to flat, may need a flat rooflight on a kerb in place of a roof window. A steep pitch may need a taller light well. The surveyor measures the angle on site.

Space below

A rooflight above a loft that is not used needs a light well to carry daylight down through the loft to the ceiling. If the loft is full of insulation, the well is lined and insulated so the heat stays in. Where the space below is a stairwell or hall, a sun tunnel may be simpler, as it carries daylight down through a small ceiling circle.

Planning and Building Regulations

For a house, a rooflight is usually permitted development with conditions, and our planning guide explains where flats, listed buildings and conservation areas differ. Cutting into the roof structure makes a new opening building work, with approval from the local authority or a registered building control approver. Where the job needs it, the notification is made by us.

English houses face no Building Regulations test for daylight, so a rooflight is always a choice. A loft bedroom brings escape rules into play. Approved Document B accepts a roof window as the way out only on storeys no higher than 4.5 m up, and the opening must be 0.33 m² clear with no side under 450 mm.

You can learn a lot before a survey. Look at the roof from the road or garden and note the covering. Look in the loft and see whether the roof has rafters and purlins or trusses. Photograph the room and mark the spot you want lit.

Then ask for a free survey. A price is named only after the surveyor has seen the spot, the ceiling and the roof above it, and the fixed written quote you receive holds on the day. Roof time for a simple single unit is usually one or two days. A 10-year workmanship guarantee, in writing, stands behind the installation.

Got a photo of the room? Send it to 07880 284133 on WhatsApp, or use the survey form. For the full range of units we install, see the home page. If you are looking at a unit that is already up there and tired, see how often a skylight should be replaced.