Flat roof rooflights fitted in Liverpool

A flat roof in this part of the country is rarely a single clean construction. Terraces built in the late nineteenth century had flat-topped outriggers and bay roofs added over the decades, and the coverings have been replaced more than once. Extensions on 1930s semis in Crosby, Maghull and Wallasey were mostly added long after the houses were built. Estates around St Helens, Warrington and Ellesmere Port have flat-roofed garages and porches that have turned into rooms. What lies under the covering matters more to us than its age.

Felt. Built-up bituminous felt, in two or three layers, is the covering we meet most on older extensions. It can be dressed up a new kerb neatly with a strip of the same material, bonded to the existing layers and lapped so the water meets only folds that point downhill. Our page on fitting a rooflight to a felt or rubber flat roof covers the compatibility questions in more depth.

Single-ply membrane. EPDM rubber and PVC or TPO sheets are the usual newer coverings. Each is dressed up the kerb in one sheet where it can be, with a purpose-made corner detail, and each comes with its own rules about adhesives and joints, which we follow.

GRP, or fibreglass. A moulded resin roof is stiff and has no joints, and it takes a rooflight well because the kerb can be laminated into the deck as part of the same material. It does need a careful hand, because a resin joint cured in damp or cold weather does not grip as it should.

Warm and cold decks. In a warm deck, the insulation lies on top of the structure, under the weatherproof layer. In a cold deck, it sits between the joists with a ventilated void above it. A rooflight in a cold deck needs the void kept clear around the kerb, otherwise condensation collects in the timber. We look at the edge of the roof and the ceiling at survey to see which one we have.

Whatever sits under the covering, there are three things we look for before we price: the state of the deck, the direction of the fall, and the position of the joists. Rotten boards or a sagging deck need dealing with, and we say so in the written quote rather than hiding the work in a vague allowance. Water leaves the roof along the fall and passes the kerb on the way, and the opening can go only where the joists allow without a structural change.

Most rooflight leaks are not leaks through the glass. They happen at the edge of the kerb, where the roof covering meets the frame. So that is where the effort goes. A good kerb has four properties. It is high enough to stand above any water that lies on the roof. It is insulated, so that warm air from the room does not meet a cold surface and condense. It is square and true, so the glazing sits flat with an even seal. And it is clad in a material that the roof covering can bond to.

The height of the upstand is set by the roof covering manufacturer's guidance as well as by good practice. If a kerb is too low, the covering cannot be turned up far enough above the water line to be reliable. If it is too high, the wind pulls at the overhang and the unit looks awkward from the garden. We build the kerb to the height the covering requires and no higher. The full detail on this is in kerbs and upstands for flat roof rooflights.

Prefabricated kerbs come in two forms. A timber kerb is built on site from treated boards and then insulated and clad. A GRP or insulated composite kerb arrives made to measure and is fixed to the deck and the joists. Composite kerbs are lighter, quicker to fit, and come with a continuous thermal break. Timber is flexible where the roof is out of square. Either can be right, and we choose according to the roof, not to a favourite.

The covering is then dressed up the kerb. On felt, that means a base layer, a cap sheet and a metal or felt drip at the top. On EPDM, it is a single piece of membrane, bonded and folded at the corners without cuts. On GRP, it is laid in and laminated in resin. Whichever it is, the covering must finish above the level where water lies, with a drip or flashing that carries it clear of the glass seal.

Finally, the sealing between glass and kerb. A good rooflight has a compression seal, a drainage channel and an overhanging cap, in that order, so that water that gets past the first line meets the second and is sent back out. We do not rely on a bead of sealant for a long life.

There is also a quiet job in the kerb: ventilation of the void. On a cold deck, the air path between the joists is kept open to the eaves or parapet vents, and we leave a gap around the kerb for it. It is dull work and invisible once done, and it stops the damp patches that otherwise appear around a rooflight in the third winter.

We do not publish a price list, because the same rooflight can mean two very different jobs. A fixed double-glazed unit dropped between two existing joists on a sound felt roof is a short, clean job. The same unit on a rotten deck, with a cut joist, a new kerb and a re-laid cap sheet, takes longer and uses more material. The glass is the same in both. The quote is not. Our page on what a flat roof skylight costs goes through each element, and our costs page sets out how we price across the services.

The things that move the figure are the same on every job. Size and glazing specification come first: a larger unit, a laminated or solar-control pane, or a triple-glazed unit each add to the cost of the product. Opening mechanisms add a motor, a rain sensor and an electrical supply. Structural work comes next, because every cut joist needs trimmers. The condition of the covering round the opening matters, since a worn roof makes the new tie-in harder to guarantee. Access also plays a part: a rear extension reached only through the house takes more time than one with a side gate.

What the quote includes is just as important. We survey first and then give a price in writing, and you pay that figure unless you alter the job. It lists the unit, the kerb, the structural work, the covering, the making good inside and the Building Control notification. A survey comes before any price, because a price from a photograph is a guess.

Because the quote is fixed, there are no surprises on the day. If the roof is opened and the deck is worse than the survey could show, that is a conversation and a change we agree before anything more is done, never a bill that appears at the end.

It is also worth asking what is in a competing quote. A cheaper figure often leaves out the kerb, or the insulation, or the paperwork, or allows only for the glass to be fixed to the existing felt with a bead of sealant. These are the details that fail first. A like-for-like comparison is one in which every line of ours has an equivalent in theirs.

Many of the flat rooflights we fit are not first-time installations. Older houses on the Wirral and in Liverpool carry plastic domes from the 1980s and 1990s that have yellowed, cracked or clouded over. Some are timber-framed boxes that have swollen with damp. Others are double-glazed units that have misted between the panes because the seal has failed. Whatever the unit, it comes out and a new one goes in. When the glass has misted, our page on misted rooflight replacement explains what is going on and what we fit instead.

A replacement is usually quicker than a new opening, because the hole and the kerb are already there. We take the old unit off, inspect the kerb and the covering round it, and decide whether the existing upstand is sound or whether it needs rebuilding. An old kerb is often too low by modern standards, uninsulated, or built of timber that has rotted. In that case we replace it. If we only swapped the glass, the weakest part of the roof would be left in place, and the new unit would inherit its problems.

Sizes also matter. A new unit that is slightly larger than the old one needs the opening altered, and that is structural work. One that is slightly smaller leaves a gap that has to be framed out. We measure the old opening and can usually supply a unit made to match it, which keeps the work simple and the ceiling undisturbed. If the room has been refitted since, we can help choose a better position at the same time.

Replacement is also the moment to improve performance. A modern unit with low-emissivity glass and a warm-edge spacer loses far less heat than an old dome, and a self-cleaning outer pane means the roof needs climbing less often. If the room has become too hot in summer, solar-control glass is a sensible upgrade. Some owners also take the chance to add an opening mechanism, so that the room can be ventilated from above.

A second question we get is whether a rooflight can go on top of a roof that is itself old. It can, as long as the covering will hold a bond or can be renewed locally, and our answer on felt or rubber flat roofs covers that. The survey tells us where the line falls on your roof.

Tell us which room is dark, we look at the roof, and a firm price arrives in writing, with the work backed by our guarantee for 10 years in writing. Phone or WhatsApp both work, and a photo of the roof and of the ceiling gives us enough to offer a sensible first steer. If you want to know which roofs are typical for your kind of house, the house types page is the place to start. Our guarantee page sets out what the written cover says. Get a free quote