Skylight and rooflight installation in Colchester, surveyed before it is quoted.

The historic core and the river villages

Inside the town walls, through the Dutch Quarter, and out along the Colne and the Stour to Dedham, Nayland, Stoke-by-Nayland and Bures. Conservation areas, listed buildings, clay pantile and plain tile over timber frames that have moved for four hundred years.

Here a rooflight is a planning question before it is a glazing question. What sits flush, what keeps a single central glazing bar, and what the conservation officer will accept before anything is ordered. The answer is almost always a conservation rooflight: slim black steel, one bar, sitting nearly level with the covering.

A standard domestic roof window standing proud of the tiles on a visible slope will be refused, and reasonably so.

The suburbs and the market towns

Lexden, Prettygate, Shrub End, Stanway, Myland, Greenstead, and out to Braintree, Witham and Halstead. Inter-war and post-war stock, generous plots, and the rear extension as the standard improvement.

Nobody here is asking about listed building consent. They are asking how many rooflights a flat roof extension needs, what fall it has to be laid to, and why the last one leaked. That is flat roof rooflight territory, and the answer is usually two smaller units rather than one large one.

The coast

Mersea, Brightlingsea, Wivenhoe, St Osyth, Clacton, Frinton, Walton and up to Harwich. Salt air, and a specification that has to survive it.

A standard architectural powder coat that is perfectly adequate in Braintree will chalk within three or four years on a seafront: the surface goes matt and powdery and the colour lightens. A marine grade coating costs a little more at order and does not do that. Fixings want to be stainless rather than plated. This is the single most useful thing we can tell a coastal customer, and almost nobody asks about it.

Most installers are fluent in one of those three markets. Being fluent in all three is what this business is for.

On a flat roof

Flat roof rooflights are the most common installation in this area by a wide margin, because the rear extension is the standard improvement and a flat roof is what most of them have. The unit sits on a timber kerb raised above the water line so the covering can be dressed up its face and terminated under the frame.

A roof lantern is a pitched glazed structure on that same kerb rather than a flat pane. It catches light from four directions, sheds water and dirt by its pitch, and gives a room real height above the ceiling line. It is the right answer over a kitchen extension or a main living space, and over a utility room it is money spent on the wrong room.

A walk-on rooflight is structure that happens to be transparent, used where a roof is also a floor: a terrace over a habitable room, or a light well that is also the route to a garden.

On a pitched roof

A roof window sits in the plane of the covering and is weathered by a flashing kit rather than a kerb. That kit is specific to the covering, and using the wrong profile is the single most common defect we are called out to correct.

VELUX is the name most people arrive with, and most of that work is replacement rather than new openings: reading the size code off the data plate, matching an obsolete size, specifying the right flashing kit for the roof you actually have.

By what you are trying to solve

Something has failed, and it is misted, leaking or simply thirty years old. That is replacement, and it is the most common call we take.

The room is too hot in summer or too cold in winter. That is a glass specification question and it lives on energy efficient rooflights.

You cannot reach it to open it, which on a flat roof is true by definition. That is electric and opening rooflights.

The room has no roof of its own, because there is a floor or a loft between it and the sky. That is a sun tunnel, and it is the only product that reaches an internal hallway or a windowless bathroom.

The kerb, on a flat roof

The upstand the unit bolts to. It has to be square, level along its top even though the roof beneath falls, about 150mm above the finished surface at its lowest point, insulated on its outer face, and it has to be measured before the unit is ordered.

A made to size rooflight will not sit on a kerb 20mm out of square, and it cannot be adjusted. On most jobs the kerb is built by whoever builds the flat roof, which is frequently not the company supplying the rooflight, and that division of responsibility is where jobs go wrong.

The flashing kit, on a pitched roof

Four elements working together: an apron below, soakers interleaved up each side, a head flashing above, and an underlay collar connecting the roofing membrane to the frame.

The kit has to match the covering profile. Colchester’s older stock is largely clay plain tile and pantile; the post-war estates are almost all profiled concrete interlocking tile. A flat profile kit fitted to a profiled tile leaves a gap under every roll. It will not leak on the day. It leaks in the first driving rain arriving from the wrong direction, and on this coast that is weeks rather than years.

Where the water goes

Flat roofs are not flat. They are laid to a fall so water leaves, and a rooflight sitting on a kerb is an obstruction in that path. Positioned where water arrives already moving, it sheds past. Positioned where water arrives slowly, or on a roof with inadequate fall, it ponds along the upslope face and sits against the termination for days.

Standing water finds any weakness eventually, and it is heavy enough to cause further deflection, which deepens the pond.

The vapour control layer

The invisible one, and the reason for a large share of the leaks that are not leaks. The layer that stops warm moist room air entering the roof build up has to be carried up the kerb and sealed to the frame. Where it is not, moist air condenses inside the insulation and appears as a damp patch on the ceiling.

It looks exactly like a leak. The distinguishing question is simple: does it happen in cold dry weather? Rain leaks follow rain. Condensation follows cold.