Where the light is lost, and where it is not
Daylight enters through clear glass, so the useful question is how much clear glass a unit gives you inside a given hole in the roof. A conservation rooflight gives a little less than a plain pane of the same size, mainly because of the bar down the middle.
Take two openings of the same outer size. One has a single unbroken pane, the other has a vertical glazing bar splitting it into two lights, as most conservation units do. The bar is a strip a few centimetres wide running the full height of the glass, and everything behind it is in shadow. On a pane around half a metre wide, that is a narrow band of the width. You lose it, but you do not lose a large share of the window.
The frame matters too, and here the conservation unit often does better than people assume. Traditional steel sections are slim, because steel is stiff enough to span without bulk, so the border of frame around the glass can be thinner than on some timber or uPVC roof windows. How the sightlines compare depends on the maker and model, so it is worth checking the drawings of the actual units rather than relying on the category. The comparison of steel and aluminium frames goes into profile in more detail.
The glazing bar, looked at closely
A glazing bar costs light in two ways: the strip of glass it covers, and the shadow line it throws across the room when direct sun comes through at a low angle.
The first is fixed. Whatever the bar's width, that strip of glass is dark all day. The second changes with the sun. When the sun is high and nearly square to the glass, the shadow is barely wider than the bar. When it comes in at a raking angle, the bar's depth starts to matter, and the shadow stretches into a longer stripe across the floor or the wall. You will notice it as a line through the sun patch, which some people like and some do not.
Whether the bar is integral, meaning it is part of the frame and separates two sealed units, or applied to the face of a single pane, changes what an officer thinks of it more than how much light it blocks. An applied bar can be slimmer, but it may not be accepted on a listed building. The guide to integral and applied glazing bars covers that choice.
Size is the real limit
The biggest cost to your daylight in a conservation setting is usually the size of opening the planners will support, not the design of the unit.
Officers tend to prefer rooflights that are modest, vertically proportioned and few in number, set so they do not crowd the ridge or the eaves. On a small front slope of a Victorian terrace in Romsey or off Mill Road, that can mean a unit noticeably smaller than you would have chosen for the room. A smaller hole lets in less light in simple proportion to its glass area, and no choice of glass or bar makes that back.
So when you are weighing up the light, compare the size you can realistically get consented, not the largest size in a catalogue. The page on conservation rooflight sizes and proportions sets out how size is judged, and what conservation officers look for covers the rest of their checklist.
The slope and the Cambridge sun
Which way the slope faces changes the light far more than the frame does. In Cambridge the sun at noon is only about 14 degrees above the horizon in midwinter and about 61 degrees in midsummer.
On a south slope pitched at around 40 degrees, even that low December sun meets the glass at a reasonable angle, so the room gets direct winter sun and the glazing bar throws a clear shadow line through it. In June the same slope takes the sun far more squarely and gets hot, which is where solar control glass in a slim frame earns its place.
On a north slope of the same pitch, the winter sun is too low to reach the glass at all. The room gets steady light from the sky, softer and more even, with no shadow line from the bar because there is no direct beam to cast one. In high summer the sun clears the pitch and grazes the glass for part of the day. A north-facing conservation rooflight will always feel calmer and less bright than a south-facing one, whatever the frame. Many of these choices come down to the front or rear slope, and the trade-off between light and planning is set out in front or rear slope placement.
| Factor | Effect on light | What helps |
|---|---|---|
| Glazing bar | Small, plus a shadow line in sun | Slim integral bar |
| Frame border | Small; often slim in steel | Check the maker's drawings |
| Consented size | Large | Best position on the slope |
| Orientation | Large | Plan for the slope you have |
| Inside lining | Large | Splayed, pale reveals |
Getting the light back inside the room
Once the size and slope are fixed, the lining of the opening does more for the room than any detail of the frame.
A rooflight in an older roof often sits at the top of a deep reveal through rafters, insulation and plasterboard. If that reveal runs straight down, it acts like a tube and narrows the light. Splaying it, with the head cut closer to horizontal and the sill closer to vertical, spreads the light across the ceiling and lets it reach further into the room. Finishing the reveal in a pale, matt colour bounces more light back down instead of soaking it up.
- Set the unit where it lights the part of the room you use, such as the stair, the desk or the bed, not simply the middle of the slope.
- Keep the glass clear unless privacy or heat calls for something else, and ask the maker what each glass option does to the light before choosing.
- If the room needs more light than one consentable unit gives, two smaller units in a regular rhythm may be more acceptable than one large one.
This is the kind of question we work through at the survey. Every quote comes with a written light plan that sets out the position, size, glass and lining, and why. See how we plan the light, read more about conservation rooflight installation in Cambridge, or book a light survey.
Questions people also ask
Is a conservation skylight darker than a standard roof window?
Size for size, slightly, because of the glazing bar. In practice the bigger difference comes from conservation units often being consented at a smaller size than a standard roof window would have been. Compare the glass area of the units you are actually choosing between.
Can I have a conservation rooflight without a glazing bar?
Some makers offer conservation-style units with a single pane, and the bar is optional on some sizes. Whether that is acceptable depends on the building and the officer. On a listed building or a prominent front slope, expect the bar to be requested.
Does flush fitting reduce the light?
No, flush fitting is about how the frame sits against the roof covering outside. It does not shrink the glass. The depth of the reveal inside the room has far more effect on how much light reaches the floor.
Will triple glazing make a conservation rooflight darker?
Each extra layer of glass and coating lets through a little less light, so there is some cost. Many slim conservation frames are designed around double glazing, so check what the maker offers and weigh the light against the heat loss for your slope. The guide to thermal performance of conservation rooflights covers the glazing options.
