Rooflight Installation in Marston

Mostly replacement, not new openings

The estates around Cherwell Drive, Marston Ferry Road, Edgeway Road and Northway are twentieth-century family housing, built and later extended. Rooflights arrived in that stock in two waves: some with the house or an early rear addition, many more during the run of loft conversions and kitchen extensions that carried on through the second half of the century and beyond. Either way, a large amount of Marston roof glazing has met every winter since it went in, and a fair share of it is at the end of what it was built to do.

That gives this part of Oxford a different centre of gravity from areas where the enquiry is about a first opening. Here the roof already has a hole in it, and the structural question was settled decades ago. What is open now is the condition of the unit, the condition of everything around it, and what can legitimately go back in its place.

Our scope is new installations and replacements, nothing else. When a unit has failed it comes out, and a new one goes in with new flashings and a sound upstand beneath it. The parts of a rooflight that fail are the parts that were doing the sealing, and a unit that has lost its seal has lost the thing that made it a roof.

How a first-generation unit fails

Misting between the panes. A sealed double-glazed unit works because the cavity between the panes is dry and stays dry. Once the perimeter seal goes, damp air enters, and the cloudiness between the sheets of glass is water with no way out. It never clears for good, and the glazing has stopped performing thermally whatever it looks like from the ground.

Perished gaskets. The rubber closing an opening sash against its frame goes hard, shrinks at the corners and stops compressing. Wind-driven rain then finds the head of the sash instead of being turned away.

Timber that has taken up moisture. Many older roof windows have timber frames. Where the finish has broken down and the frame has stayed damp through repeated winters, the timber softens, particularly at the bottom corners where water sits. Once rot is into a frame, the frame is no longer a reliable base for anything.

Polycarbonate that has yellowed. Dome rooflights on flat rear additions discolour under ultraviolet light, then go brittle. A yellowed dome also passes far less light, which is usually what the owner notices first.

Frames that have moved. Roofs settle and timber shrinks. A sash that catches or will not latch is telling you the opening has shifted around it.

Water running down the inside. Not always ingress. A cold unit in a warm humid room runs with condensation, and the owner reads it as a failing roof. Sometimes it is. Often the roof is sound and the unit has simply become the coldest surface in the room.

The like-for-like assumption

The most common misconception we meet in Marston is that replacing a rooflight is a swap. The old one comes out, an equivalent goes in, nothing else changes. It is a reasonable assumption and it is usually wrong, for three reasons.

The standards have moved. A rooflight fitted long ago was fitted to the expectations of its own day. Roof glazing is assessed on thermal performance when it is replaced, and the numbers that apply are the current ones. Replacing a rooflight is notifiable work. We handle the Building Control notification as part of the job, and the specification is written to satisfy it from the start rather than adjusted afterwards.

The product range has moved. Frame depths, modular sizes, flashing systems and glazing build-ups are not what they were. A unit of the same nominal size may have a different frame section, sit at a different depth in the roof, or need a different flashing kit for the covering. Where the original size is no longer made, the answer is a nearby modular size with the opening adjusted to suit, or a made-to-measure unit.

The building’s status may have moved. Designations are made over time. A house that carried no constraint when its rooflight was fitted may now sit inside a conservation area, and in Marston that is a live question rather than a theoretical one. Conservation area status is a matter of public record, and it can change what the replacement has to look like.

What the old unit was hiding

The condition of the opening is only fully visible once the old unit is out of it, and that is the part of a replacement homeowners are least prepared for. What we look at, the moment the unit lifts out, is the timber trimmer at head and sill, the rafters either side, the kerb or upstand on a flat roof, and the underlay where it was cut. A frame letting water past slowly has been feeding it into that timber for years. On a flat rear addition, a softwood kerb may have softened beneath the dome. On a pitched roof, the felt may have torn back where the original flashing was fastened through it.

Making the opening good is part of the job, not an extra discovered halfway through. Sound timber is cleaned off and reused; timber that is not is cut out and renewed. Where the kerb cannot carry a modern unit at the right height above the finished roof, a new kerb is formed. Setting a new rooflight onto an unsound base carries every existing defect into new work.

The covering is then reworked around the unit.

The opening you already have

On a replacement, size is not a free choice. The existing opening was formed to suit the rafter or truss spacing in that roof, and the timber either side is carrying load. What can go back without structural work is broadly what fits that clear width.

Making an opening taller is usually the easier direction, because it runs with the rafters rather than across them and needs the head and sill trimmers repositioned rather than a rafter cut. Widening is a different matter. It means taking out part of a rafter, and on the trussed roofs common across the Marston estates it can mean cutting a structural member of a truss, which is not something to do without a designed alteration. The webs inside a trussed rafter are holding the roof in shape, and they are not spare.

Where somebody wants substantially more light than the original opening gives, there are better answers than fighting the structure. Two units in adjacent rafter bays deliver more daylight than one wider unit and need no structural change. A taller unit in the same bay brings light deeper into the room, because on a pitched roof it is the height up the slope that governs how far light reaches the back wall. Where genuine widening is right, the load path is worked out and it is priced as the structural job it is.

Loft conversions of earlier decades

Many Marston houses gained a room in the roof during the conversion waves of earlier decades, and in a good number of those rooms the roof window is now the weakest element in the space. The floor is fine. The stairs are fine. The insulation may well have been improved since. The window is original.

That shows up in ways the owner feels before they diagnose. The room runs cold in winter near the glass and the heating never quite wins. It overheats in summer, because early glazing had little solar control and the blind that came with it has long since given up. Condensation gathers on the frame and runs onto the plasterboard reveal, staining it.

There is a second consideration in a converted loft that does not apply to a landing or a bathroom. Where a roof window serves as the means of escape from a habitable room in the roof, it has to meet the requirements set for that: clear opening area, the dimensions of the opening, and handle height above floor level. An older window may not meet what is required now, and on a replacement that is assessed rather than assumed. It sometimes drives the choice of unit, and occasionally the size of the opening.

The reveal deserves attention at the same time. Insulating the plasterboard splay properly, with the vapour control layer continuous around the frame, removes the cold line where condensation forms. Renewing the unit and leaving a thermal bridge around it solves half the problem.

Old Marston and its conservation area

Old Marston is a distinct village core rather than an extension of the estates around it, with stone and rendered cottages, the church of St Nicholas, and lanes running towards the Cherwell. It carries conservation area designation and contains listed buildings, and both change what a replacement rooflight has to be.

In a conservation area the appearance of an alteration becomes a material consideration, which is why the flush conservation rooflight exists as a category. Such a unit sits down in the plane of the roof rather than proud of it, is finished dark, and traditionally carries a single vertical glazing bar so it reads as a Victorian cast-iron light. Fitting one flush into stone or slate means re-dressing the covering around it, not sitting a frame on the courses.

Where a building is listed, listed building consent is separate from planning permission and one does not stand in for the other. The designation covers the whole building, including roof slopes nobody sees from the lane, and a replacement is not automatically permitted merely because something is already there.

The planning authority here is Oxford City Council. We are installers rather than the council, and we do not present ourselves as the people who decide. What we can tell you is which specification usually satisfies a building of that kind. Where an architect or heritage consultant is involved, we specify to their drawings.

How a Marston enquiry runs

A rooflight installation in Marston starts with the roof rather than the product.

What is there now. Age, type, timber or plastic, dome or roof window, and what it is doing: misting, running with condensation, jammed, discoloured or letting water past. Photographs from inside and out tell us a good deal before anybody climbs.

The roof around it. Covering, pitch, structure, rafter or truss spacing, and whether there is an underlay. On a flat rear addition, the kerb height and the covering where it turns up against it.

The room below. How the space is used, whether it is a habitable room in the roof, and where condensation is forming. This is where the escape requirement and the reveal detail get settled.

The address. Whether any designation attaches to the building, which takes minutes on the estates and matters far more in the Old Marston core.

Then the specification, and the price last. We install VELUX, Fakro, Keylite and Roto units and specify the one the roof and the position require, with glazing chosen for the orientation of that slope. A figure quoted before those four steps is a guess, usually a low one that moves once the old unit is out.

We have been installing rooflights for fifteen years, our workmanship carries a ten-year guarantee with coverage and exclusions set out on its own page, and we handle the Building Control notification. Marston sits well inside the twenty five mile road radius we work from Oxford.