Sizing answer
What sizes do conservation rooflights come in?
Unlike a standard roof window with its code, a conservation rooflight is generally built to fit the opening. The size comes from the roof structure and the room, and it is settled at the survey.
The answer
They come in a wide range, and many makers will build to your measurements, so the real limits are on your roof. The spacing of the rafters, the line of the slope, the weight of the glass and the room fix the practical size. A narrow unit between two rafters is the simplest case. We confirm the size at the free survey.
Why there is no single size chart
Makers publish ranges, and many will cut a unit to order. The roof, not the catalogue, decides which size is workable on your home.
A standard roof window has a code. In MK04, the letters carry the width and the digits carry the height. A conservation rooflight rarely works that way. It is a made-to-order unit whose frame, glass and bar are built to a given width and length, within a range set by the maker.
What this means for you
You do not choose from a numbered list and hope it fits. You start from the opening that suits the roof and room, and the maker builds to it. That is also why the same rooflight line can differ from one supplier to the next, with different smallest and largest units. Compare makers on what they will build for your opening, not on a headline range.
Frame size and glass size
A figure quoted for a rooflight may be the outside of the frame, the hole in the roof, or the visible glass. The glass you see is smaller than the frame, and the frame is smaller than the hole the roofer cuts. Ask which is being given, so you compare like with like. The same care applies to VELUX size codes, where the code is a nominal size.
How the rafters and the slope set the size
A unit that fits between two rafters needs no cut timber. A wider one cuts a rafter and needs new framing round the opening.
The simplest conservation rooflight fits the gap between two neighbouring rafters. Nothing structural is cut, and the opening is trimmed with short timbers top and bottom. Old roofs have uneven spacing, so the width available differs from one bay to the next. We measure it on site.
Going wider
A unit that outruns the gap means cutting a rafter, with doubled timbers and a trimmer carrying its load round the opening. That is ordinary roofwork, but it adds to the job and has to be done to current Building Regulations. Cutting a new opening in a roof is building work and is notified to Building Control, and we handle the notification.
Length along the slope
Length is limited by the room, the position of the eaves and ridge, and how the covering laps round the unit. A long unit also puts more glass on the roof, which brings weight and a heavier frame. The balance between a strong frame and a slim one is the maker's to settle.
Projection
None of this changes the 150mm limit on a house. A flush unit sits well inside it. On a flat, a planning application is needed at any size.
What stops a conservation rooflight being very large
Glass weight, frame strength, the roof structure and the look of the slope cap the size. A pair of smaller units can light a room better than one large unit.
Bigger is not always better, and it is rarely free.
Weight and strength
A big sealed unit with a laminated inner layer weighs a lot. The frame has to carry it and the roof has to carry the frame. Makers set their own limits, and past them the answer is two units.
Look and proportion
On a period roof, one big pane can dominate the slope and break the pattern of the covering. Two or three narrower units, set at equal spacing and in line, keep the character of the roof and spread the light. For the choice of bar to go with them, see our answer on glazing bars.
The room and the consent
The size of the room sets how much light is useful. With a listed building, a large unit gives the officer more to question when consent is decided. For the product overview, start at the conservation rooflights page.
Questions to settle before a size is chosen
- How wide is the gap between the rafters where the unit will go?
- Is the size quoted the frame, the roof opening, or the visible glass?
- Does the maker build to your measurements, or only to set steps?
- Would two smaller units suit the slope better than one large one?
- Is the house a flat, or a listed building, needing consent first?
Sizing queries that arise
Is there a smallest size?
Makers set a minimum, because below a certain frame width the unit cannot be built to the same strength. A small rooflight for a landing or a bathroom is possible, and the survey will tell you where the limit lies for your choice.
Can a conservation rooflight replace an old cast-iron skylight in the same opening?
Often the old opening can be reused, as the unit is measured to it. The timbers round it are checked, and the work is notified to Building Control. Say at the survey if there is an existing opening you want to keep.
Do I pay more for a made-to-measure size?
Bespoke sizing and finishes add to a quote, and so do wider openings that need new framing. The free survey is followed by a written quote, which stays as it is while the job does.
Do the sizes differ between makers?
Yes. Each maker has its own ranges and limits, and some build to measure while others work in steps. Compare what each will build for your opening and the whole-unit U-value, not the headline range.