Can rooflights be fitted to a barn conversion?

Planning is settled before the roof is

On a barn, the rooflight question is a planning question first. Agricultural buildings converted under prior approval are frequently granted with conditions on the number, size, position and appearance of roof openings, and some consents restrict them severely on the elevation visible from a lane or a village. A barn inside a conservation area, or one that is listed or curtilage listed, is tighter again.

So the first thing to do is read the decision notice and the approved drawings, not the product catalogue. If a scheme shows three rooflights on the rear slope and none on the front, that is what can be built, and adding one on the front later is a fresh application rather than a site decision. We will fit what your consent allows, and we will not fit what it does not. Our page on rooflights in barn and outbuilding conversions goes into how the schemes are usually shaped.

Flush conservation units and what makes them look right

Most barn consents in Oxfordshire ask for conservation pattern rooflights: a slim frame, usually black or dark grey, with a central glazing bar on the larger sizes, set so the glass sits close to the plane of the covering rather than standing proud of it. On a stone barn in the Cotswold fringe villages that is the difference between a roof that reads as an old building and one that reads as a conversion.

The look comes from the installation as much as from the unit. A conservation unit fitted at standard depth stands out from a hand-made clay tile roof, so the frame is set lower and the flashing is dressed tight into the courses. On a stone slate roof the diminishing course sizes mean the unit’s width should be chosen to work with the slate widths at that height on the roof, not just to hit a catalogue size. Getting that right is quiet, unglamorous work and it is what makes the finished roof look untouched.

The structure you find under a barn roof

Barn roofs are not built like house roofs. A traditional stone or brick barn typically has heavy principal rafters or trusses at wide spacings with common rafters or purlins between, often hand-cut, often not parallel, and frequently with rafter spacings that vary along the building. Later steel-framed agricultural buildings converted under prior approval have a portal frame with purlins on a regular grid, which is a different job again.

What this means practically is that a barn rooflight position is set by the structure, not by the drawing. The width available between two hand-cut rafters may be 380mm in one bay and 470mm in the next. We measure the real bays before ordering, because a unit sized off a drawing on a barn is a unit that arrives and does not fit.

Where a unit can go without touching a truss

The best barn installations avoid cutting structure altogether. A unit sized to sit between two existing rafters needs no trimming and no structural work, which keeps the cost down and keeps historic timber intact, and on a listed barn keeping the timber intact may be a condition of consent rather than a preference.

Where a larger opening is genuinely wanted, a rafter can usually be trimmed with new timber above and below, but on a hand-cut roof carrying loads in ways that are not obvious, that is a job for a structural engineer to look at rather than a decision for the roof. Cutting a principal rafter or a truss member is not something we will do on our own judgement. We say so at survey, and often the better answer is two units in adjacent bays rather than one big one.

Insulating the slope, and what it does to the opening

A converted barn roof carries a thick insulation layer that was never there when the building was agricultural, and that depth changes the rooflight detail. The unit has to be set in relation to the finished internal surface rather than the rafter, the insulation has to be carried across the reveal so the perimeter is not colder than the slope around it, and the vapour control layer on the warm side has to run unbroken and be sealed to the frame.

Where that is not done, condensation appears at the reveal on cold mornings and the rooflight gets the blame for a detail failure 100mm away from it. The reveal is worth splaying, particularly at the head and sill, so the opening widens as it comes through the ceiling. In a tall barn volume a splayed reveal also throws light much deeper into the room.

Stone slate, clay peg tile, corrugated sheet: three different flashings

The covering decides the flashing kit and the labour. A Stonesfield or Cotswold stone slate roof is heavy, laid in diminishing courses, and every slate around the opening is cut and hung individually; it is the slowest and the most skilled of the three. A clay peg tile roof takes a standard tile flashing but the tiles are brittle and often irreplaceable, so units are set to work with the existing gauge rather than forcing it.

A profiled fibre cement or metal sheet roof, common on later agricultural buildings, takes either a matching in-plane sheet or a kerb-mounted unit with a purpose-made flashing to the profile. It is the quickest of the three, and on a prior approval conversion where the sheet is being retained, it is often what the consent expects.

Sizing for a large open volume

Barn interiors are tall and deep, and daylight behaves differently in them than in a house. A rooflight high in a tall volume lights the whole space but leaves the floor dimmer than you expect, because the light spreads over a much bigger area. Two smaller units spread along the slope give more even light than one large one in the middle, and they usually sit better on the elevation as well.

The other consideration is glare and heat. A big south-facing barn slope with generous glazing gets very warm by mid-afternoon, and a tall volume takes a long time to cool. Where the consent allows the units on the north slope, take them, and where it does not, a solar control coating is worth the cost.

Getting a barn job priced properly

What we need is the decision notice and the approved roof plan, the covering type, whether the roof structure is hand-cut timber or a steel frame, and whether the barn is listed. From that we can send a range by email, and after a survey where the real bays are measured we give a fixed price that does not move.

Barn conversions are steady work for us across the villages within our 25 mile radius, out towards Stanton Harcourt, Islip and the Otmoor edge. We install VELUX, Fakro, Keylite and Roto units including conservation pattern models, installations and replacements only. Call 01865 704245 or email info@heritageskylightsoxford.co.uk. Our workmanship carries a ten-year guarantee and we have fifteen years in the trade.