Rooflight and skylight installation in Braiswick

More units rather than larger units

The most common specification error we correct on properties like these is a single very large rooflight where two or three smaller ones would do a better job.

One large unit gives one bright area and leaves the rest of a big room noticeably darker by contrast. Several units spaced across the slope give even daylight across the whole space, read as ambient light rather than as a feature, and are usually easier to fit because each can sit between structural members without trimming.

They also fail independently. If one seal goes in twenty years, you replace one unit rather than a very large and very expensive pane.

Lanterns and vaulted spaces

The rear extensions on these plots are frequently large enough for a lantern to sit in proportion, and this is where they belong.

A lantern collects light from more than one direction, so the room stays bright as the sun moves round rather than having one good hour. It adds height and shape to a flat ceilinged room. And on a large extension it reads as architecture rather than as a hole in the roof, which a single big flat unit does not.

What it needs is structure. A lantern is heavy and the opening has to be trimmed to carry it, which on a new extension is designed in and on a retrofit means an engineer. It also needs a properly built and insulated kerb, and the kerb on a lantern is doing more work than on a flat unit because it carries the frame as well as weathering the opening.

Where a ceiling is being vaulted rather than left flat, roof windows in the pitched plane throw light across the whole room in a way a flat unit cannot, and on a large space that is usually the better answer.

Glass, and where the money should go

On a big glazed area the specification matters more than on a small one, because the consequences of getting it wrong are proportionally larger.

Solar control on south and west. Not optional on a large unit. A big rooflight facing the afternoon sun without it produces a room nobody uses in July, and the fix afterwards costs nearly as much as the original installation.

Nothing extra on north. No direct sun at all, so specify for thermal performance alone. Solar control on a north facing rooflight makes a good room slightly darker and delivers nothing.

Warm edge spacers throughout. The bar holding the panes apart is where a surprising amount of heat leaves. A traditional aluminium spacer conducts well and shows as a band of condensation around the edge of the glass on cold mornings, which gets mistaken for a failed unit. A stainless or foam warm edge spacer runs several degrees warmer and the edge misting largely disappears.

Triple glazing, usually not. It takes the U-value from about 1.2 to about 0.8, which is real and modest. It is heavier, which affects the actuator on an opening unit. It transmits noticeably less daylight, which works against the reason you fitted a rooflight. The same money on solar control glass, an insulated kerb and a properly sealed vapour barrier does more for how the room feels.

Trees

A genuine consideration on these plots and one that catches people out.

Mature trees drop leaves, and leaves collect around a flat roof rooflight kerb and in the perimeter drainage. Standing water against a seal is what kills a flat roof unit early, so on a wooded plot the clearing is a twice yearly job rather than an annual one.

They also affect access. Many are protected, and a scaffold or crane position may need to work around a canopy rather than through it. Worth raising at survey rather than on the day.

On the other side, tree shade is free solar control, and a rooflight under a deciduous canopy gets shading in summer and full light in winter, which is exactly the right way round.

Planning, for larger glazing

Rooflights on a house are generally permitted development provided they project no more than 150mm beyond the plane of the roof and sit below the highest part of it. That covers almost every roof window in a slope.

A lantern is a different matter, because it stands proud of the roof by considerably more than 150mm. On the flat roof of a rear extension that is usually dealt with as part of the extension’s own permission or permitted development allowance rather than separately, but it is not automatic, and on a large or prominent lantern it is worth confirming rather than assuming.

The extension itself brings Building Regulations regardless of planning. The rooflights form part of the thermal calculation for the whole structure, and there is a limit on total glazed area as a proportion of floor area, so a heavily glazed extension may need a calculation rather than an assumption. Getting the glass specification right first is cheaper than compensating for it in the walls afterwards.

Lead times, and what actually delays a job

The fitting is rarely the long part.

A like for like replacement into a sound opening is a day. Two or three in one visit is two to three days. A new opening with making good and plastering runs to three or four days. A lantern on an existing extension, including the structural work, is usually a week.

What lengthens the programme is the order. Stock size roof windows are quick. Made to measure units and lanterns are manufactured to your dimensions and run to several weeks, sometimes longer for a large or unusual configuration. Where a structural engineer is involved, their drawing has to come before the order, not alongside it.

The sensible sequence is survey, structural check where needed, then order, then book the fitting to the delivery date. Booking the fitting first and hoping the unit arrives is how jobs slip.

What to send us

A photograph of the roof from the garden, one of the room from inside showing the ceiling, and a rough measurement of the extension if there is one. For a replacement, the data plate on the existing unit. From those we will give you a realistic range by email before anyone visits, and a fixed itemised price after survey.