Conservation rooflight installation in Exeter

A conservation rooflight is a roof window designed to disappear into a natural slate or tile roof rather than stand proud of it. The frame is thin, the glass sits almost level with the roof covering, and the glazing is usually split by one slender bar running up the slope, the same layout as the cast iron foundry rooflights fitted to Victorian outbuildings, mills and stables. Most are fixed. Some open, on a hinge at the top or a pivot, and the opening versions are chosen for rooms that need air.

The difference from an ordinary roof window is easy to see from the street. A standard window has a deep frame, a bulky external cover and a visible rim, and it stands a clear step above the slates. A conservation unit has a profile only a little deeper than the slate itself, so the shadow line around it is shallow, and the metal is a dark, matt colour that reads as part of the roof. Our page on what makes a rooflight a conservation rooflight goes through the features one by one.

The parts, in the order you meet them from the outside, are these.

The frame

A narrow steel or aluminium surround that holds the glass and fixes to the roof structure beneath the slates.

The glazing bar

A single slim bar, normally on the centre line, that divides the glass into two tall panes and gives the unit its period character.

The glass

A sealed unit in the better systems, held in the frame by a bead and sealed against the weather.

The flashing

The metal dressing that joins the frame to the slates around it, shaped to follow the courses so water is carried away.

Two metals dominate the market, and the choice between them is less about looks than about how the frame is made and how it ages. A galvanised steel frame is the traditional material, closest in section to the old cast iron, and it gives the thinnest, most solid-looking bars. An aluminium frame is lighter, resists corrosion on its own and can be thermally broken, which improves how well it holds heat.

Both are powder coated, and the colour is chosen to blend with the slate. Dark grey and black are the usual choices, in a matt or low-sheen finish, because a glossy frame shines in low sun and draws the eye. Some makers will colour match to an existing cast iron rooflight on a neighbouring building, which is worth asking for if the roof already has an original one.

Steel gives a slightly heavier, more handmade look and a very strong frame.

Steel gives a slightly heavier, more handmade look and a very strong frame. Its weakness is that if the coating is scratched through, the steel beneath can start to rust, so the finish and the care taken in handling and fixing matter. Aluminium needs less thought on that point and tends to give a lighter unit that is easier to carry onto a steep roof. It can also be made to open more easily on bigger sizes.

Neither is better in every case. On a listed building the conservation officer may have a view, and some prefer the traditional material on principle. On a house by the Exe estuary, where salt in the air is a fact of life, a well coated aluminium frame has an edge on maintenance. We set the two side by side, with bar widths and finishes, in steel or aluminium conservation rooflights, and we will say plainly which one we would choose for your roof.

A conservation rooflight is fitted into a slate roof in a particular order, and it is slower than fitting a standard window because the slates must be cut and re-laid by hand. Our guide to conservation rooflights on slate roofs follows the whole process, and this is the short version.

The unit is fixed to a timber frame between the rafters, so the glass sits at the height of the slate and not above it. The slates around the opening are lifted, trimmed and re-laid on the battens so the courses run cleanly past the frame. The flashing is dressed over and under the slates so rain is carried over the edge of the frame and on down the roof. A rooflight that looks fine from the ground can still let water in if the flashing is rushed, which is why we take most care on this part of the work.

Old slate roofs are not uniform. The slates may be graded in size from the eaves to the ridge, laid in diminishing courses, and some are fixed with nails and pegs that are decades old. A good installer works with what is there. Slates that are cracked are replaced with matching second-hand ones where the roof calls for it, and where the slates are in poor condition, we say so before quoting rather than discovering it halfway up the roof.

Rafters on old roofs are rarely in the places a modern drawing would put them. Spacing can vary from one bay to the next, and some are hand-sawn or even round timbers. The width of the rooflight is often set by the gap between two rafters, and where it is wider, a rafter is trimmed and the load carried by a new doubled timber either side. That is structural work, and it affects how the ceiling below is finished.

Planning and Building Regulations are two distinct regimes, and an old house gets no exemption from the second. A new rooflight cut into an existing roof is building work: the rafters are trimmed, which is a structural alteration, and the new window must meet Part L, the part of the regulations that deals with the conservation of fuel and power. That means Building Control approval is required, and the notification to them is something we do for you.

Replacing a whole rooflight, frame and glass together, is also notifiable, because a window is a controlled fitting and the replacement has to meet Part L. Replacing only a sealed glass unit within a sound frame is not notifiable. That matters on a roof that already has an old conservation rooflight with a failed unit, since a glass-only swap can be simpler. Our page on Part L and replacement rooflights explains the line between the two.

Both new and replacement rooflights are bound by the limiting U-values in the 2021 Volume 1 edition of Approved Document L, so a slim conservation system has to be chosen with that in mind. The sealed double glazed unit does the heavy lifting here. The regulations also recognise that historic buildings need special consideration, and the guidance allows some flexibility for listed buildings and those in conservation areas where full compliance would unacceptably alter the character. Whether that applies to your roof is agreed with Building Control and the conservation officer, not assumed.

Fire and escape rules sometimes matter too. A rooflight near a boundary can be affected by the rules in Part B, and a roof window serving a loft room can act as an escape window if it meets the size and position rules in Approved Document B. A fixed conservation unit does not do that job, so where a loft room needs an escape route, an opening unit of suitable size is specified. Whether a loft room needs one is settled at the survey, since it depends on the room.

You get the completion certificate at the end of the job where one is issued, and it is worth keeping with the deeds.

A new opening follows a set order, and the order matters more on an old roof than on a new one. Everything that can go wrong in an old house goes wrong early, so the survey does most of the work.

We begin by looking at the roof from outside and the ceiling from below. We measure the rafters, note their spacing, check the slates for condition and size, and look at the ceiling to see whether it is lath and plaster, lime, board or something else. We also ask about designation: whether the house is listed or in a conservation area, and whether anyone has already spoken to the council. That visit gives us what we need for the written quote and the finish schedule.

Next comes the unit. Conservation rooflights are made to order, so there is a lead time, and this is the stage to settle any consent. Where listed building consent or planning permission is needed, nothing is ordered and no date is fixed until it has been granted. Where Building Control needs notifying, we do that.

On the first day the room is sheeted and screened. The slates around the opening are lifted and stacked for re-use, the roof structure is opened and the rafters are trimmed and doubled where the plan calls for it. The ceiling is supported and the opening cut from below. At the end of each day a temporary weatherproof cover is fixed over the hole, and the room below is protected until the next morning.

The frame is then fixed, the flashing dressed and the slates re-laid to the courses, with matching slates added where any were broken in lifting. The glass goes in last, to keep it clean. From that point the roof is watertight, and the work moves inside to the lining, which is boarded, beaded and plastered to the shape in the schedule.

The plaster needs time to dry before it is painted. The reveal is handed over either bare for your own decorator or painted, as agreed, and the room is cleaned. A single rooflight takes a few days on site, with drying time on top, and a pair or a trimmed rafter adds a little to that. For what to expect in the house on the day, our answer on how much mess fitting a rooflight makes may help.

If you want to start the process, ask for a quote and tell us a little about the house.

A good double glazed conservation rooflight with a warm-edge spacer and low emissivity coating performs well

If it is the right size, in the right place and in a matt, dark colour, yes. The unit sits close to the slate, the frame is thin and a single central bar echoes the old cast iron design. The part that goes wrong most often is the position: a rooflight that lines up with the windows below and follows the slate courses looks settled, and one that does not looks added on.