Rooflight and skylight installation in Great Horkesley

The question that sets the price

Cut roof or trussed rafters. It decides more about what a rooflight costs than the choice of unit does, and you can answer it yourself in two minutes with a torch.

Rafters running from eaves to ridge with a horizontal purlin partway up and clear space between them is a traditional cut roof. Rafter, purlin and ceiling joist each do a separate job, so forming or widening an opening is ordinary carpentry. Wide rooflights are possible and the loft is convertible.

A repeating pattern of triangular frames with diagonal members crossing the middle of the space, joined by flat pressed steel plates, is trussed rafters. Each truss is a single engineered component in which the rafter, the ceiling tie and the webs are designed to work together, and cutting any member sends the load it carried somewhere it was not meant to go.

With trusses at the usual 600mm centres, a unit narrow enough to sit between two of them without touching either goes in with no structural work at all. That is the quick, inexpensive job and it is what most enquiries here turn into. Anything wider needs a structural trimmer designed by an engineer, which is routine and which is the point at which the price steps up.

Broadly, properties here from before the 1970s are cut roofs and anything later is likely trussed. A photograph looking up in the loft settles it without an appointment.

Coverings and the flashing kit

Concrete interlocking tiles dominate, on pitches typically between 30 and 35 degrees, with clay and slate on some of the older property.

Interlocking tiles are profiled and need a deep flashing kit rather than the flat kit used on slate. An interlocking tile has a raised profile with a channel running down it, and a flat kit designed to sit against a smooth slate leaves open voids under those channels where it crosses the tile. Water gets under them within a season, and because it enters below the flashing it appears inside as a leak that seems to come from nowhere.

This is the most common installation fault we find on other people’s work, and it is exactly the place a cheap quotation saves money, because the kit is a separate order line and nobody can see which one was used once the job is finished.

Ask any quoting installer which kit is specified for your covering by name and part number. A quotation that cannot answer that has not been written from a survey.

What we fit here

Roof windows into rear slopes for loft rooms and landings, which the cut roof properties take very well. Flat roof rooflights and lanterns over rear extensions, which a great many houses here have gained. Sun tunnels into internal bathrooms and hallways. And a growing amount of replacement work, because a lot of units went in during the 1980s and 1990s and sealed units last fifteen to twenty five years.

On any replacement, photograph the data plate on the sash rather than measuring. It is on the top edge of the sash or the frame beside the hinge, and on a centre pivot window rotating the sash brings it into view. From that photograph we can usually tell you what the unit is, whether the size is still made, and what the job costs, without anyone visiting.

Extensions and the kerb

Where an extension is involved the kerb decides whether the installation lasts. At least 150mm of upstand, the deck falling away from it so water cannot pond against the seal, and rigid insulation on the outer face of the kerb itself.

That last item appears on almost no quotation and matters more than the glass specification does. A 1200 by 1800 rooflight has six metres of perimeter, and an uninsulated timber kerb punching through the roof insulation presents close to a square metre of poorly performing envelope. It costs a sheet of board and an hour, and it disappears under the covering, which is exactly why it is the first thing dropped.

Permissions and access

Most properties here are permitted development: no more than 150mm projection beyond the plane of the roof and not above the highest part of it, with additional restrictions on side elevations facing a boundary. Individually listed buildings need consent regardless of slope or visibility.

Access is good, with off street parking on most of the village and space to stand a tower at the rear. Two storey rear slope work needs scaffold, but it can stand on private land, which avoids the highway licence.

How much

Glazed area somewhere around 15 to 20 percent of floor area gives a bright room without turning it into a greenhouse. The common mistake is going straight to the largest unit that fits.

On a south or west facing slope, a very large rooflight without solar control glass produces a room that is uncomfortable on a July afternoon and expensive to heat on a January night. The fix afterwards costs nearly as much as the original installation, because the labour and the access are the same either way.

More moderate glazing with better glass, plus an opening unit at the highest point so heat can leave, gives a better room than more glass does.

Which way

South. The most sun, through the middle of the day when it is strongest. Solar control is not optional.

West. Frequently worse in practice than south, because late afternoon sun is low and arrives when the building has already been warming all day. Low angle sun is also where internal blinds struggle most.

East. Morning gain, shorter and cooler. Fine over a kitchen. Difficult over a bedroom, where the issue is light rather than heat and blackout provision is the answer.

North. No direct sun at all. Even, cool, glare free light all day, and the best light in the building for a work surface or a studio. Specify for thermal performance alone and spend the saving elsewhere, because solar control here would only make a good room slightly darker.

Getting heat out again

Glass specification reduces how much heat arrives. It does nothing about heat already in the room, and that is what an opening at high level is for.

Warm air rises, so an opening at the highest point lets it leave and draws cooler air in through anything open lower down. The useful version is night purge: open it late, let the accumulated heat out overnight, close it in the morning before the outside air warms. On a motorised unit with a rain sensor that happens without anyone thinking about it, which is the real argument for a motor over a pole.

None of it works if the rooflight is the only thing open. The inlet at low level is half the system.