Custom Skylight Design

When bespoke is necessary

Bespoke is a specification decision, not a grade of quality. A purpose-made rooflight is a unit built to dimensions and to an appearance that cannot be bought, which is worth paying for only when the roof or the consent position requires it. There are a limited number of situations where it genuinely does, and they come up more often in Oxford than in a city built mostly after the war, because so much of the stock here predates standardised carpentry and sits inside a designation that constrains how the finished thing may look.

The openings that force it

The clear width falls between catalogue sizes. Rooflights are made in a ladder of widths designed to drop between rafters set at regular modern centres. A hand-framed roof does not oblige. Where the space between two rafters is wider than one standard unit and narrower than the next, the choice is to cut structure, to accept a smaller unit, or to have the frame made to the space that exists.

The opening is not a rectangle. Trapezoidal slopes, triangular ends, hipped planes, mansard junctions and roofs that taper on plan all produce apertures no manufacturer stocks.

The roof is out of square. A building that has moved over a century presents an opening whose diagonals differ, and a rigid rectangular frame announces that along one edge.

An existing historic opening has to be matched. Where an aperture is original to the building, keeping it and making the unit to suit is usually preferable to enlarging it, structurally and for consent.

The required appearance is not on sale. Proportion, glazing bar arrangement, frame depth and finish sometimes have to answer to a specific historic precedent rather than to a manufacturer’s range.

The unit is larger than anything standard, or the junction is unusual. Above a certain size the catalogue stops, and against a party wall or into a valley a standard flashing kit may have nothing to work with.

When standard will serve

The honest half of this page is the half that talks people out of it. Specifying a purpose-made unit where a catalogue one would have sat correctly is a waste of the client’s money, and the easiest way for a supplier to enlarge a job without improving it.

A standard unit carries real advantages that a one-off gives up. Its flashing arrangement has been developed and tested for slate, plain tile, pantile and flat coverings, and it is the most reliable part of the installation. It costs less, arrives sooner, and can be matched again years later if it ever has to be taken out and a new one installed.

It also matters that conservation-pattern rooflights are made as standard products. A unit sitting flush to the roof plane, finished dark, with a single vertical glazing bar, is a catalogue item from more than one manufacturer. Where one of those sizes suits the rafters and the elevation, it will often satisfy an officer without a bespoke frame entering the conversation at all. We install VELUX, Fakro, Keylite and Roto units alongside the specialist conservation ranges, and a standard unit is not a lesser outcome.

Where a standard unit will sit correctly and read correctly, that is what we specify, and we say so even though the bespoke route carries the larger invoice.

Rafters in an old roof

The size charts published by every rooflight manufacturer rest on one assumption: that the roof was framed to regular centres by somebody working from a drawing. In a house built in the last few decades that holds, and the chart widths are chosen so a unit drops between two rafters without either being cut.

Period Oxford roofs were not framed that way. Rafters were cut on site, spaced by eye and by the length of timber available, and adjusted around chimney stacks, valleys, hips and purlins. Spacing frequently varies along a single slope, and it can vary between the two slopes of the same roof. The timbers are often not uniform in section, having been sawn rather than machined. The clear width between two adjacent rafters is therefore a measured fact particular to that pair of timbers, and it is unlikely to correspond to a number in a catalogue.

The three options, and what each costs

Trim the structure. Cut one rafter, install trimmers above and below the opening, and transfer the load to the neighbouring rafters. This is ordinary carpentry on an ordinary roof. It is a different proposition on a listed building, where the historic roof structure is part of what is protected and cutting original timber may itself require consent rather than being a matter for the builder’s judgement.

Accept a smaller unit. Choose the next catalogue size down and live with a margin of roof either side of the glass. On a modest slope a smaller light often reads better anyway. Elsewhere it means giving up meaningful daylight in a room that had very little to begin with.

Make the unit to the opening. The frame is fabricated to the measured clear width, the existing timbers stay where they are, and the glass area is as large as the roof will allow. This is where custom skylight design in Oxford is doing real work rather than adding a premium, and it is the option a conservation-sensitive roof most often ends up at.

Openings that are not rectangular

A rectangular hole in a flat plane is the easy case. Old roofs regularly present something else, and a shaped unit is the only thing that will sit in it without a compromise showing from the ground.

Tapered and trapezoidal slopes. Rear outriggers and back additions were frequently built up to a boundary that was not parallel to the main house. The roof over them narrows along its length, and a rooflight set into it either follows that taper or sits visibly askew within it. The eye reads the relationship between the edge of the glass and the edge of the roof immediately.

Hips and triangular ends. Where a hipped end is the only slope with the orientation the room needs, the usable area is a triangle. A rectangle placed inside it wastes most of the plane. A shaped unit following the hip lines uses the geometry instead of fighting it.

Mansard junctions and valleys. A mansard has a steep lower slope and a shallow upper one meeting at a change of pitch, and glazing crossing that junction has to be made as two planes with a weathered joint. Where two roofs meet, the unit has to stop clear of the valley by a proper margin so the covering can be dressed, and the shape of what remains is often not square.

Roofs that have moved

Every old roof has settled. Purlins deflect under a century of tile and slate, wall plates roll outwards, rafters spread at the foot, and the plane develops a gentle dish or twist that is invisible until something rigid and rectangular is placed against it.

A manufactured rooflight frame is flat and square and stays that way. Set into an opening whose diagonals differ, it leaves a tapering margin along one edge and a widening one along the other. Set into a plane that is twisted, it either rocks on two corners or forces the covering to change direction around it, which is where water eventually finds a path.

What the survey records

On any roof of age the survey takes more than a width and a height. Both diagonals of the proposed opening are measured, because equal diagonals are the only proof that a rectangle is a rectangle. The plane is checked with a straight edge or a laser line in both directions. Rafter twist is recorded, because a timber that has turned along its length presents a bearing surface that is not parallel to the roof surface. Purlins, ceiling ties and previous alterations are noted, since these determine what can be done to the opening at all.

From that record there are two workable responses. Where the structure allows and the building’s protection does not forbid it, the opening is trimmed and lined so it presents a true, square, flat aperture, and the discrepancy is absorbed in the lining rather than the frame. Where the timbers must stay as they are, the unit is made to the geometry the survey recorded, including a frame that is not quite rectangular.

Matching an existing opening

Much bespoke work is not about creating something new. It is about putting a new unit into an aperture that already exists and must not change.

Old openings are worth keeping for several reasons at once. The structure around them has been carrying its loads for a long time. The covering has been dressed to them, and on a stone slate or handmade clay tile roof the pieces immediately around an opening are cut and not easily reproduced. Where the building is listed, the aperture and its surround may be part of the historic fabric in their own right. In a conservation area, an opening that has always been there occupies a settled position in the roofscape, whereas a new one has to be justified.

Matching means recording rather than estimating. The survey captures the structural opening rather than the plastered reveal, since the two are rarely the same and ordering to the internal finish is a classic way to arrive at a unit that is too large. It records the depth from the roof surface down to the structure, which sets how the new frame must be built up to finish flush, and how the covering currently terminates at each edge.

Where a failed unit is being taken out and a new one installed in its place, the old one is measured as an object before it is removed, not after. Its face dimension, its sightline, its bar position and the height it stands above the covering all disappear the moment it is off the roof, and all of it is likely to be wanted by whoever assesses the replacement.

Replicating a Victorian light

The reference object behind almost every conservation rooflight specification is the nineteenth century cast iron light. It is worth being precise about what made it look the way it did, because replicating the appearance means copying specific characteristics rather than gesturing at a period.

It sat almost in the plane of the roof. A cast iron light was set low, with the covering dressed close around it, so it read as an interruption in the slate rather than an object on top of it. Depth of projection is the characteristic that most reliably separates a conservation unit from a modern one at a glance.

It was taller than it was wide. The proportions followed the space between rafters and the length of glass that could be handled. A landscape-format unit reads as modern immediately, whatever its finish.

It carried a single vertical glazing bar. One central bar, dividing the glass into two upright panes, is the detail most people recognise without being able to name it. It exists because single glazing could not span the whole width, and it survives as the visual signature of the type.

It was dark and slender. Thin cast iron sections and dark paint gave a small dark rectangle with a fine bar, a very different object from a broad white frame.

Reproducing that in a modern frame

A new unit has to do all of this while also being thermally broken, double or triple glazed, and weathertight to current expectations. Those requirements pull against slenderness, because an insulated glass unit is thicker and heavier than a single pane, and the frame carrying it has to be deeper than cast iron ever was.

The answer is to put the necessary bulk where it is not seen. The section visible above the covering is kept as slim as the glass make-up allows, with the depth taken downwards into the roof build-up rather than upwards into view, and the thermal break sitting within that depth. The glazing bar is applied to the outer face of the glass and aligned with an internal spacer, so it reads as a structural bar rather than a strip stuck to the inside of the pane, which is what gives cheaper imitations away. The finish is specified against a named colour standard rather than described as dark, so what is fabricated is what the officer agreed.

Some compromise is unavoidable and better stated than hidden. A modern insulated unit will not have the sightline of a single-glazed cast iron light. What it can have is the same proportion, bar position, low profile and colour, and those four together carry the appearance.

Satisfying the conservation officer

A bespoke unit is assessed differently from a catalogue one. A named product from a known range comes with published drawings and a body of precedent. A one-off comes with neither, so the specification itself has to demonstrate what will be installed. A bespoke proposal is therefore presented rather than described, and what goes in front of an officer, or in front of the architect who will submit it, is a set of documents rather than a sentence in a covering letter.

What the specification contains

An elevation of the roof slope showing the unit in position, with its relationship to the eaves, the ridge, the party walls and any other openings. Position and proportion are usually assessed before anything else, because a well detailed unit in the wrong place is still the wrong answer.

A section through the unit and the covering showing exactly how far it stands above the roof surface and how the covering is dressed to it. This is the drawing that demonstrates a flush installation rather than asserting one.

Frame and material details giving the visible section dimensions, the glazing bar arrangement and its position, the finish stated against a colour reference, the glass make-up, and the flashing metal chosen to suit the covering rather than defaulted.

We are installers rather than the local planning authority, and consent is the council’s to give. What we can do is produce a specification that answers the questions an officer assessing a roof addition normally asks, in a form that can be submitted. Where you are working with an architect, a conservation consultant or a structural engineer, we specify to their drawings and coordinate with them.

Drawings before fabrication

The defining practical fact about a purpose-made rooflight is that it cannot be adjusted on the day. A catalogue unit that turns out to be slightly awkward can often be accommodated by trimming the opening, because the unit is the known object and the roof is the variable. A bespoke unit reverses that, and once it has been fabricated the variable is spent. The process therefore front-loads the decisions.

The four stages

Survey. A physical inspection inside and out: structure, covering, rafter positions and condition, diagonals, plane, the existing opening if there is one, access, and the elevation the unit will be seen from. Nothing is ordered from a photograph or from dimensions supplied over the phone.

Measured drawings. The survey is drawn up as a record of what exists, with the proposed unit set into it. This is the document that goes to a conservation officer or an architect, and the one that catches the problems: an opening that is not square, a purlin in the way, a chimney too close for the flashing to be dressed properly.

Shop drawings. The fabricator produces drawings of the unit itself, with every dimension, the frame sections, the bar position, the glass specification and the finish, checked against the survey line by line.

Sign-off. The shop drawing is agreed in writing before anything is cut. That signature is the last point at which a dimension can be changed without a cost, and it exists so the client knows what is being made rather than discovering it on delivery.

How bespoke goes wrong

The failures in one-off rooflights are consistent enough to list, and every one is a process failure rather than a manufacturing one.

The commonest is a frame with no considered flashing arrangement. A standard unit arrives with a flashing kit developed for the covering it is going into. A bespoke frame arrives as a frame, and unless the weathering detail has been drawn and the flashing metal specified for that slate, tile or membrane, somebody improvises it at roof level. That is where one-off units acquire their reputation.

Close behind is a frame with no drainage path. A well designed frame carries incidental water down a channel in the rebate and discharges it above the covering. A one-off drawn as a picture rather than as a section has nowhere for it to go.

Then there are the dimensional failures, usually ordering to the plastered reveal rather than the structural opening; the thermal failures, where a one-off frame without a proper break performs worse than the unit it replaced; and the appearance failures, where nobody specified the bar position or the finish, so the fabricator applied a default and the bespoke unit ends up looking generic.

Why lead times lengthen

A bespoke unit takes longer than a catalogue one, and it is worth understanding why, so the programme can be planned around it rather than pressed against it. The steps are sequential and none of them can begin early. The survey has to happen before the drawings, because the drawings are the survey. The drawings have to be agreed before fabrication, because fabrication is irreversible. Where a consent application forms part of the route, that runs on the council’s timetable, and the specification has to be settled before it is submitted.

Fabrication has its own sequence. Frame sections are cut and assembled to the drawing, glass is made to size for that unit specifically rather than drawn from stock, and where the frame is powder coated to a specified colour it waits for a batch in that colour.

None of this compresses by pushing. What it does reward is starting the conversation early. On a wider refurbishment the rooflight specification belongs near the front of the programme alongside the structural decisions, not at the end with the finishes. The sequence that must never happen is opening the roof before the unit exists, and we plan the works so the roof is only ever open when there is something to put in it.

Structure, glass and weight

Once an opening exceeds standard sizes, two conversations start that a catalogue installation rarely needs.

The structural conversation

A standard rooflight usually fits between existing rafters or requires one to be trimmed, which is routine. A larger opening means interrupting several structural members and carrying their loads around the opening on trimmers, back into the members either side, and down into whatever supports them. On a pitched roof it also means considering how the loss of rafters affects the roof’s resistance to spreading, particularly in an older structure already carrying its load in ways the original carpenter would not recognise.

Where an opening is large enough for this to be a calculation rather than a judgement, it belongs to a structural engineer. We coordinate with the engineer, build to the design that comes back, and make sure the rooflight dimensions and the structural design describe the same opening. On a listed building this runs alongside the consent conversation, because the structural solution has to be one the building’s protection will accept.

Glass and what it weighs

Glass weight scales directly with area, and a bespoke unit is often bespoke partly because it is large. Each additional pane adds weight, so a triple glazed unit is heavier than a double glazed one of the same size, and a laminated inner leaf, the sensible choice over any space where people sit, adds more.

That weight bears on the trimmed opening, which is part of the structural design; on the frame, whose sections have to be sized for the glass rather than for the appearance somebody wanted; and on the installation, since a unit above a certain weight cannot be carried up a ladder or through a staircase. How it gets onto the roof is established at survey rather than discovered on the morning.

The usual make-up overhead is a toughened outer leaf with a laminated inner one, which holds together if it ever breaks and is quieter under heavy rain. Solar control is necessary on south and west-facing glass of any size. Shaped and curved panes are made individually, and some coatings are harder to obtain in non-rectangular shapes, which is a question to settle at drawing stage rather than after the frame is made.

Where the cost sits

A bespoke unit costs more than a catalogue one, and the difference is not a single premium applied at the end. It is spread across several items, and knowing which apply to your job is the only way to compare two quotations.

The sequence we work in

The building first. What it is, what designations apply, and what the roof slope is seen from. Much of this is established before anybody visits.

The survey second. Structure, covering, rafter positions, diagonals, plane, the existing opening where there is one, and access. This decides whether bespoke is necessary at all, before any product is discussed.

The route third. Standard or purpose-made, stated with the reasons. If a catalogue unit will sit correctly and read correctly, that is the recommendation.

The drawings fourth. Measured drawings, the specification for consent where consent is engaged, then shop drawings signed off before fabrication.

The price with the drawings, not before them. A figure quoted for a bespoke unit before the roof has been measured is a guess, and usually the low guess that gets revised once somebody goes up a ladder.

Custom skylight design in Oxford is mostly an exercise in finding out whether it is needed. We have been installing rooflights for fifteen years, our workmanship carries a ten-year guarantee with its coverage and exclusions set out on its own page, and we cover Oxford and up to twenty five miles by road. We install new units and replacements only. If you have an opening that nothing in a catalogue seems to suit, the useful first step is a survey rather than a look at a range.