Rooflights fitted properly, and made to last.

Skylight is the everyday word. It covers anything glazed set into a roof, it is what people type into a search box, and it tells an installer almost nothing about the job. The other three are specific.

A roof window sits in the plane of a pitched roof, in line with the covering, and opens on a centre pivot or a top hinge. It comes in fixed manufactured sizes with a matching flashing kit for the covering it is going into. VELUX, Fakro, Keylite and Roto all make them, and the size codes are how a survey gets turned into an order. Most are rated for pitches from about 15 to 90 degrees. Details are on the roof window installation and VELUX window installation pages.

A rooflight, in trade use, is a unit for a flat or nearly flat roof. It sits on a kerb built up above the roof surface, the glass is usually one plane set at a shallow angle, the frame is slim aluminium, and the sizes are commonly made to order rather than picked off a list. That is the flat roof rooflight job.

A roof lantern is a framed structure with several sloping glass planes rising to a ridge or an apex. It also sits on a kerb, but it adds height and volume to the room below, and it is heavier and more involved to set out. See roof lantern installation.

A conservation rooflight is a fifth thing, and Gloucestershire has more of them than most counties. Cast or pressed steel frame, one central glazing bar, low profile, designed to sit almost flush in a thick slate roof. The reason it exists is buried in the way Cotswold roofs are built, and that is covered further down.

The reveal is the plastered box between the frame and the room. Built well it doubles the amount of daylight that reaches the floor. Built badly it grows mould in the top corners.

The convention is a vertical lining above the window and a horizontal lining below it. Vertical at the head throws light deeper into the room. Horizontal at the cill lets warm air from a radiator underneath rise straight up the face of the glass, which is the cheapest condensation control there is. Splaying the two sides widens the beam of light. It costs another half day of carpentry and plastering, and on a north-facing room it is the difference between a bright room and a lit one.

Insulation runs continuously around the frame, tight to it with no voids, and the vapour control layer is taped to the frame rather than trimmed short of it. A 20mm gap of nothing behind the plasterboard at the head of a roof window is a cold bridge, and cold bridges are where black spotting appears in February.

This is the biggest technical difference between Gloucestershire and almost anywhere else, and it is the reason a Cotswold job is priced differently from an identical unit going into a semi in Longlevens.

Cotswold stone slate is not a repeating module. The slates are graded by size and laid in diminishing courses: the largest and thickest at the eaves, getting progressively smaller and thinner up the slope to the ridge. Every course is a different depth from the one below it, and the gauge changes continuously. The old yard names for the sizes survive, running from Cussomes at the eaves through Muffities and Wivetts to the small slates near the ridge. Underneath, they are hung on riven battens and the joints between them are commonly sealed from below with a lime mortar torching mixed with animal hair, often with no underlay at all.

What that means in practice is that an opening cannot be cut through the roof and closed up around a frame. The courses above the opening have to be stripped back up the slope, the frame set, and the courses then re-laid downwards, re-establishing the diminishing gauge so the finished roof reads correctly. Slates come out broken, so reclaimed stone is sourced to match thickness and weathering colour. New stone stands out against a two-hundred-year-old roof for a decade or more, which is why matched reclaimed material is worth the wait.

The slates are also thick, commonly 20 to 30mm, and the roof loading runs well over 100 kilograms per square metre. A standard roof window frame and its flashing kit are made for a covering a fraction of that thickness, so a conventional unit sits proud and reads as a modern object bolted onto an old roof. A conservation rooflight has a slim cast frame, a single central glazing bar and a profile designed to sit nearly flush with a thick slate, and that is the whole reason the product exists. The conservation rooflight page covers the sizes and finishes.

Torching is the other conversation. Once disturbed it cannot be put back exactly as it was, so the replacement detail, usually a breather membrane on counter battens, is agreed with the conservation officer rather than decided on the scaffold. Roofs like this run through Painswick, Bibury, Northleach, Chipping Campden and the wolds villages between them.

Coleford, Cinderford, Mitcheldean and Lydney are built of local sandstone, much of it in former mining terraces with thick walls and steep roofs at 40 to 45 degrees. The covering is usually Welsh slate in a small format with a generous headlap, sometimes over sarking boards rather than open battens.

Two things follow from that. First, the Forest is wetter and windier than the vale, and driving rain arrives from the south west with force behind it, so head laps get increased over the manufacturer's minimum and side flashings are taken wider. Second, a steep pitch changes the geometry inside: the reveal becomes a taller box, the top of the frame sits further above floor level, and reaching an opening handle from standing height stops being realistic. That is where a rod-operated or electrically operated unit earns its place.

A steep pitch also means more of the unit is visible from the road below than on a shallow roof, which matters for the terraces inside the Forest's conservation areas.

Longlevens, Hucclecote, Tuffley, Warden Hill and Charlton Kings are mostly interwar and post-war semis and detached houses with hipped roofs at 30 to 35 degrees, covered in concrete interlocking tiles. Structurally these are cut roofs with purlins and struts, so rafters can be trimmed, which makes them among the most straightforward roofs to work on.

The constraint is the shape. On a hip, the slope narrows towards the corner, so the usable rectangle is much smaller than the roof looks from the garden. Add a purlin across the middle at about mid-height and a binder in the ceiling, and the clear zone on a typical interwar rear hip is often around 1.2 metres of vertical run across two rafter bays. People arrive expecting one large window and the roof will only take a shorter one.

The good answer is usually two units in adjacent bays rather than one wide one. Two openings between existing rafters need no structural design, deliver more glass in total, and light the room more evenly than a single unit in the middle. It also avoids the situation where the only way to fit the window somebody has already chosen is a designed trimmer and a Building Control structural submission.

A sealed unit is two or three panes of glass separated by a spacer bar, with a desiccant inside the spacer to absorb the small amount of moisture trapped at manufacture, and two seals around the edge: a primary butyl seal that stops vapour, and a secondary structural seal that holds the sandwich together. When the primary seal fails, humid air migrates into the cavity, the desiccant saturates, and from then on the cavity fogs whenever the outer pane is cold. That is what misting is.

Rooflights reach that point sooner than vertical windows, for reasons that are all about their position. The glass takes direct sun for hours, so the seal cycles through a bigger temperature range every day. It sits nearly horizontal, so rain lies on it and the edge seal spends more of its life wet. And ultraviolet exposure on a roof is simply higher. Fifteen to twenty-five years is a normal service life for an older unit in a roof, and the failures often arrive in a group because the whole roof was glazed on the same day.

There is no way to dry the cavity out again, and any product that claims to is selling a hole drilled in a piece of glass. The real choice is a new sealed unit into the existing frame, which works where the frame is sound and the glass is a standard size, or a new complete unit. On a timber roof window past twenty years old the gaskets, the sash and the flashing kit are all at the same stage as the glass, so the whole unit is usually the better outcome and rarely much more work.

A replacement is the one moment when a glass upgrade is nearly free of extra labour, because the frame is coming out anyway. The difference over twenty-five years of product development is large. A 1990s double-glazed roof window sits somewhere around 2.8 to 3.0 W/m2K. A current manufactured roof window runs around 1.0 to 1.4, and a good made-to-size flat roof unit can be near 1.0, with triple glazing lower again.

Coatings do the rest of the work. A low-emissivity coating on the inner pane reflects long-wave heat back into the room. A solar control coating on the outer pane cuts the heat coming in, which matters on any south or west facing roof and matters enormously on a loft room, where the glass is overhead and the sun is on it at the hottest part of the day. Solar control units bring the g-value down from around 0.5 to roughly 0.25 to 0.3, and on a west-facing conversion that is the difference between a usable summer bedroom and an unusable one.

Safety glazing is not optional in a roof: a toughened outer pane, and a laminated inner pane wherever there is habitable space, a stairwell or a bath underneath. Laminated glass has a second benefit that surprises people, which is rain noise. A 6.8mm laminated outer on a shallow flat roof unit noticeably reduces the drumming that a single sheet of toughened glass makes in heavy rain. Where a unit is high above a stairwell and cleaning it is unrealistic, a self-cleaning outer coating is worth specifying. The energy efficient rooflights page goes further into the numbers.

Where a roof has three or four units of the same age, replacing them together is materially better than replacing them one at a time. One scaffold, one Building Control notification, one set of visits, and the glass and frames match across the elevation instead of drifting through three product generations. The sequence runs from the top of the slope down so the roof is closed each evening. Where one unit has already misted, the others glazed on the same day are usually within a couple of years of the same fate, and a second scaffold costs more than the units.

A finished loft room needs a different approach because there is plaster, paint and often fitted furniture below the opening. The work is done from outside wherever it can be: the covering back, the flashings off, the frame out through the roof, the new one in from above. Done that way a like-for-like swap leaves the internal lining and decoration completely alone. The internal reveal is only opened where the opening size changes or where insulation behind the plasterboard is found to be missing, and if that is likely the survey says so in advance. Floors are protected, the room is sheeted, and the loft hatch is closed and sealed while any cutting happens.

Gloucestershire has an unusual concentration of both, and the two are not the same thing. Conservation area status controls what the outside of a building looks like and often removes permitted development rights through an Article 4 direction. Listing protects the whole building, inside and out, and covers alterations that nobody outside will ever see.

In a conservation area the pattern is fairly consistent. A unit on a road-facing slope is usually resisted. A conservation rooflight on a rear or hidden slope is commonly accepted, with conditions: a slim cast frame with a central glazing bar, a black or dark finish, set flush or nearly flush, sized modestly, and lined up with the openings already on the elevation rather than placed where the room would prefer. Matched reclaimed slate for the surrounding courses is a normal condition on a stone roof.

On a listed building, listed building consent is required for anything affecting character, and that includes the internal reveal, the loss of historic riven battens, and the removal of mortar torching. Do this the sensible way round: pre-application advice from the conservation officer, with photographs of the roof and a note of exactly what is proposed, before anybody orders a unit. Officers in this county have seen a great many rooflight proposals and will usually say early on what would be acceptable, which is far cheaper than a refusal after the scaffold is up.