Conservation rooflight installation in SouthamptonSat low in the old slates.

Before there were roof windows, there were rooflights: small cast-iron frames bedded into the slates of workshops, stables and attic rooms, with a single pane or a pair of panes divided by a bar. They sat flat, they were made of thin section metal and they were painted dark. The modern conservation rooflight borrows that appearance on purpose, with modern glass and a modern seal behind it.

Three features give the type its name. The unit sits low, with the glass only a little above the slates rather than standing proud of them. The frame is slim, in steel or aluminium, so more of the opening is glass than metal. And there is a vertical bar down the middle that divides the glass into two panes, which is what a historic rooflight would have had and what makes the opening read as old from the street.

A standard roof window, by comparison, has a deeper frame, a wide flashing surround and a single large pane. Both are good products. They simply suit different roofs, and the guide to choosing a conservation rooflight or a standard roof window lays the two side by side.

Why the detail matters to the council

When a roof is protected, what the council weighs is how the building looks from outside. A flush, dark, barred unit in the right place can be close to invisible from the road, while a deep, bright-framed window in the same slope draws the eye. That is why conservation officers talk about profile, proportion and finish, and why those are the words we use on the survey.

None of this makes the glazing bar compulsory in every case. Some roofs and some officers prefer a single pane; some makers offer both. The answer page on whether a conservation rooflight needs a glazing bar explains the choice.

On every job, the order of work is the same. We survey the roof and record its covering, then we draw the proposed unit, then the roof is checked with the council where that is needed, and only after that is the rooflight ordered. Nothing is made until the position and the finish are agreed.

Part of our area is inside a National Park, and those parks have their own planning authorities. Inside the New Forest National Park the planning authority for most applications is the Authority for the Forest itself, not the district council. Inside the South Downs National Park it is the South Downs National Park Authority. Their offices, rules and published guidance are different from the council next door.

National Park land also has reduced permitted development for some kinds of work. Rooflights can still be installed, but you check before assuming a right exists, and that a drawing and a short application may be needed where a district council would not ask for one. We look at the address on the survey and tell you which authority applies and what we have found.

What the Park authorities look for

The two parks care about different things in the detail. In the New Forest, the interest is in village character, the pattern of cottages and the dark skies at night, which makes the choice of a small, low unit more than a matter of taste. On the South Downs, the interest is in the character of the villages and the way the buildings sit in the countryside, as well as the dark night sky. In both, a rooflight on a rear slope that cannot be seen from the road is easier to justify than one on the front.

Dark skies are a good reason to think about the glass and the interior lighting under the unit. A rooflight in a bedroom lights the room by day; in the evening it can show light to the sky, so a blind or a shaded fitting helps in a Park. Our overview of rooflights in the New Forest and South Downs National Parks sets out what each authority publishes.

You can find each Park's planning pages on the New Forest and South Downs authority sites. Both explain how to contact a planning officer before applying, which is worth doing for an unusual roof.

A slim frame leaves little room for glass thickness, and for many years conservation rooflights were single glazed to fit the profile. That has changed. Makers now offer sealed double glazed units thin enough to sit in a slim section, so a new unit can look like a historic one and perform much better. Whether a particular rooflight is double glazed is a question of the product, and the quote says which build-up you are getting. Our answer on whether conservation rooflights are double glazed gives the background.

The thermal rules depend on how the unit is classed. For an existing home in England, Part L 2021 caps the rooflight figure at 2.2 W/m²K; roof windows are judged as windows, at 1.4 W/m²K or Window Energy Rating Band B. Makers declare their own figure for each product, and the quote shows it next to the limit that applies.

On an older roof, the roofer is the lead trade. They strip the slates or tiles from the opening, cut the felt or membrane, deal with the battens, and dress the flashings. We fit the rooflight into that opening, or where the roofer prefers, fit it alongside them on the day. We agree which approach with them beforehand.

The sequence is short but exact. The covering is stripped back and set aside. The rafters are checked against the trimmer positions we gave. The unit is placed on the battens, squared, fixed to the maker's instructions and tested for level. The flashings go on, working up from the sill, and the covering is brought back to them in courses. Last come the finishing details at the head.