Plenty of the terraced cottages are mid-row, with neighbours on both sides and a party wall at either end. That brings two constraints. There is no side wall to put a window in, so the roof carries the whole load of lighting and airing the top room. And the roof edge has to be treated with care next door, so the slates, flashings and the line of the party wall are all checked before a unit is sized.
The shape of the cottage matters too. Some pit cottages have a rear wing, a single-storey kitchen or a store that was added when the family grew. These lower roofs are good places for a small rooflight, because they bring daylight to rooms that the original builders never expected to be lived in. A kitchen that has relied on one window for a century can end up bright enough to need no lights on a dull afternoon, and with an opening unit overhead, steam leaves through the roof instead of settling on the walls.
Roofs that have lived a long time
A hundred-year-old roof frame is rarely square. The timbers have moved, the battens have been replaced in sections, and the slates, or sometimes tiles, may come from different decades. We go into the loft to see what is there, and pick a unit narrow enough to sit between the rafters without needing a major rearrangement. If a larger window is wanted, the rafters can be trimmed, but that is a bigger job and we say so at the outset.
A good number of these cottages already have a rooflight, installed years ago and now clouded between the panes or stiff on its hinge. A failed sealed unit is replaced by a new one, which keeps the old opening and often improves the way the window works. Our hub on misted and failed rooflights shows how to recognise the signs and what your options are.
Garden walls and neighbouring roofs can also affect the choice. On a terrace the next house may rise a little higher than yours, and its chimney can throw shade or turbulence across part of the slope. We note where the sun lands during the day and where the wind is channelled between roofs, because it decides which part of the slope will serve best for an opening window.
Where a complete unit comes out and another goes in, Building Control gets notified, and we take care of that as part of the job. Putting a fresh unit in the right place often means the room is used more, because it stops being the one nobody wants to sleep in during a heatwave.