Skylights and rooflights in Cambridge, planned around the light

Stand in a long kitchen extension on a terrace off Mill Road at nine in the morning in January. The sun is low in the south east, only a few degrees above the horizon (it barely reaches sixteen degrees even at midday), and the neighbouring wall throws the whole garden into shade. A rooflight over the far end of that room is the only source of real daylight the space will ever get. Now stand in the same room at three on a clear afternoon in late June. The sun is high and to the south west, around forty-five degrees up, and whatever glass you put in the roof will throw a bright, hot rectangle onto the floor that moves across the room as the afternoon goes on. Both of those rooms are the same room. The rooflight has to serve both of them.

That is the problem we solve before we quote. For every skylight or rooflight we install, we work out four things in order:

1. Which way the slope or the flat roof faces, and what shades it: a gable, a chimney stack, a neighbour's extension, a tree that loses its leaves in October.
2. Where the light patch will land through the day and across the seasons, so it falls on the worktop, the table or the desk you actually use, and not across the television or into your eyes when you sit on the sofa.
3. What the glass must do on that orientation: how much heat it lets in, how well it keeps heat in, and whether it needs a coating, a tint, or a blind.
4. How big the opening should be, which comes last, not first. A smaller rooflight in the right place usually makes a better room than a large one in the wrong place.

Cambridge makes this more important than it is in a hilly town. The city sits on flat land in the Cam valley at the edge of the Fens, with very little high ground to shade a roof. A rooflight here sees almost the whole sky and the whole arc of the sun, from a low winter path to a high summer one. That is good news for light and a real design question for heat, and it is why we look at the orientation of your roof before we look at a catalogue. Our light planning page shows the method in more detail, and the guide to which way a skylight should face covers each compass direction in turn.

None of this means a long or academic process. Most of the thinking happens in one visit, with a compass, a tape and a few questions about how you use the room. It means the choices are made on purpose and written down, rather than decided by whatever unit happened to be in stock.

Victorian and Edwardian terraces

The streets of Romsey, Petersfield, Mill Road, De Freville and Chesterton are lined with two-storey terraces in pale gault brick, with slate roofs and deep, narrow plans. The back rooms are dark because the house is long and the neighbours are close. A single-storey rear or side-return extension is the usual way to open these houses up, and a flat rooflight or a slim lantern over the new kitchen is often the only way to get daylight into the middle of the plan. The constraints are real: the party wall is right there, the gardens are small, and the roofs of the neighbours rise on either side. In loft conversions on these streets, a rear dormer with rooflights on the front slope is a common and accepted pattern, but the front slope faces the street and can fall inside a conservation area, which changes the kind of unit you can fit. Our guide to skylight replacement in terraced houses covers the replacement side of this.

Interwar and 1930s semis

Further out, in Cherry Hinton, parts of Chesterton, and along the arterial roads, the typical house is a 1930s semi with a hipped, tiled roof and a small back addition. The hips cut down the usable roof area, so the position of a roof window in a loft conversion needs care, and older additions often have shallow flat roofs that need a new upstand before a rooflight can go in. These houses respond well to a lantern over a new kitchen extension, because the gardens are usually longer and a lantern will not dominate the neighbours.

New estates

Eddington, Trumpington Meadows, Great Kneighton, Orchard Park, Cambourne and Northstowe have added thousands of new homes, many of them with large windows, trussed roofs and good airtightness. Owners here are often adding rooflights to a loft or a new extension, or upgrading a unit the developer fitted. The questions are different: trussed rafters cannot simply be cut, so a roof window has to be sized to fit between trusses or the structure has to be designed around it. These homes also hold heat well, which makes solar gain through new roof glass more noticeable in summer. The guide to adding skylights to new-build homes explains the trusses and the heat in more detail.

Village cottages and older houses

In the villages around the city, from Grantchester and Great Shelford to Histon, Linton and Cottenham, there are timber-framed and brick cottages, thatched and clay-tiled roofs, and a lot of conservation areas. The roof structure may be irregular, with old purlins and rafters that are not evenly spaced. Rooflights here tend to be conservation units in slim steel or aluminium frames, set flush with the tiles. The light question is often the reverse of a terrace: the cottage may already have low windows and deep walls, and a small rooflight on the rear slope can transform a dark upstairs room without changing how the house looks from the lane.

In every case the survey starts with the house itself. We look at the roof covering, the pitch, the structure we can see from the loft, and what is on the other side of each boundary. The light plan is only useful if the house can take the opening where the light wants it.

Two numbers matter. The U-value measures heat loss through the glazing: lower is better, and the Building Regulations set a limiting value of 2.2 W/m²K for rooflights, with better performance expected in a new home. The g-value measures how much of the sun's energy passes through the glass into the room: a g-value of 0.6 lets in a lot of solar heat, and a solar control glass with a g-value closer to 0.3 lets in roughly half as much. Roof glass takes more sun than a wall window of the same size because it faces the sky, so on a flat roof or a south slope the g-value deserves as much attention as the U-value.

The choice of glass follows the orientation:

A north-facing rooflight gets almost no direct sun in a Cambridge summer and none in winter. The light is steady, soft and cool all day, which is ideal for a study, a studio or a bedroom. Here the priority is a low U-value, since the glass gains little heat, and clear glass is usually the right choice because you want every bit of daylight.

A south-facing rooflight gets several hours of direct sun on a clear day, even in December. In summer that sun is high and strong, and on a flat roof it strikes the glass almost straight on. Solar control glass, an external awning blind, or both, are usually part of the plan. We also think about where the bright patch will land, because a hot patch on a dark stone floor will warm the room long after the sun has moved.

East and west slopes are the glare slopes. The sun comes in low in the morning or the evening, at an angle that reaches deep into the room and straight into your eyes. An east-facing roof window in a bedroom will wake you at five in June unless it has a blackout blind. A west-facing rooflight over a living room can put low sun across a television screen on summer evenings. Neither is a reason to avoid the slope; both are reasons to plan the blind and the position at the same time as the window.

Safety is the last part of the glass. Glazing above your head should have a laminated inner pane, so if it ever broke the pieces would stay bonded to the interlayer rather than fall. A walk-on rooflight needs heavier laminated glass rated for foot traffic, with a slip-resistant finish. Self-cleaning coatings on the outer pane help on a low-pitched roof where dust and pollen settle. If glare is already a problem in a room you have, our answer on how to stop glare from a skylight sets out the fixes in order of cost and effort.

We start inside, not on the roof. We want to know how you use the room: where you cook, where you sit, where the television goes, whether the space is used mostly in the morning or the evening, and whether anyone sleeps under the new glass. Then we take the orientation of the roof with a compass, note what shades it and when, and measure the room, the ceiling height and the roof pitch.

From the loft, if there is one, we look at the rafters or trusses, the insulation and any sign of old damp. On a flat roof we check the covering, the falls and the height of any existing upstand. If a conservation area or a party wall is involved, we note it then and say what it means for the design.

The light plan itself is short, because it only needs to answer the questions that matter:

• Position: where the opening goes, and why that position puts light where you need it through the year.
• Size: the dimensions, and how they relate to the room and to the structure.
• Glass: the U-value, the g-value, whether it is solar control, and whether the inner pane is laminated.
• Shading and control: which blind, if any, and whether the unit opens by hand, by pole, or electrically.
• Consents: whether the work needs planning permission or listed building consent, and confirmation that we handle the Building Control notification where it is notifiable.

The price comes with the plan, so you can see what each choice costs and change it before any work is booked. If you want to understand what drives the price before we visit, our costs page explains the factors in general terms. When you are ready, you can book a light survey or message us on WhatsApp.

A survey is also the point to ask about the things that worry people about roof glass: whether a rooflight will be noisy in heavy rain, whether a bedroom will be too bright in summer, whether a lantern will feel cold in winter. Each of these has a practical answer in the choice of glass, frame and blind, and the answers go into the plan rather than being left to the day.

The guarantee is written, and it comes with the paperwork for the job: the Building Control completion certificate where the work was notifiable, the manufacturer's product details, and the light plan itself, so you have a record of exactly what was fitted and why. It is a workmanship guarantee from us and is not insurance backed. The full terms are on the guarantee page.

Rooflights need very little looking after, but a little helps them last. Clean the outside of the glass once or twice a year if it sits on a low pitch where dirt collects, or less often with a self-cleaning coating. Keep the gutters and the area around a flat rooflight clear of leaves, so water runs off the upstand freely. Open an opening rooflight every so often to keep the hinges and seals moving, and test the rain sensor on an electric unit once in a while. Our guide to rooflight cleaning and care covers the details, including what not to use on coated glass.

If anything about the installation is not right, message us and we will come back to look at it. The point of the plan and the guarantee is the same: you should know what you are getting before we start, and be able to rely on it for years after we finish.