In-plane rooflight replacement without a roof strip

A rooflight is either in-plane or out-of-plane. An in-plane unit sits within the thickness of the roof so that its glass follows the same slope as the roof surface, flush or nearly flush with the tiles or the flat membrane. The familiar pitched-roof window set into a run of slates is the domestic example. On commercial roofs the same idea appears as a flush glazed panel let into profiled metal or a built-up felt deck. An out-of-plane unit, by contrast, stands proud on a raised kerb or upstand, which is how most flat-roof lanterns and box rooflights are mounted.

The distinction matters because of how each one meets the weatherproofing. An in-plane rooflight is sealed to the roof by a flashing system that laps over the covering below the unit and tucks under it above, shedding water down the slope. An out-of-plane unit is sealed to the top of an upstand and the water is kept away from the glass by height. Replacing them is a different exercise in each case, and this page is about the in-plane kind.

A “roof strip” is exactly what it sounds like: lifting and removing an area of the roof covering, whether that is tiles, slates or a section of felt, to get at the timbers, the opening or the flashing. It is disruptive, it needs the roof left open for a spell, and it multiplies the labour and the risk. The whole appeal of an in-plane replacement is that the correct one rarely needs it. The old flashing releases, the unit lifts out of the timber frame it sits in, the new unit drops into that same frame, and a fresh flashing kit re-marries it to the covering that was never taken off.

A clean swap depends on the new unit being compatible with the opening and the covering that are already in place. Four things decide it.

The size stays the same. The structural opening in the roof, the hole framed by the rafters and trimmers, is the fixed constraint. If the new rooflight uses the same external frame size as the old one, it drops straight into that opening. Manufacturers keep the same size codes across generations for exactly this reason, so a unit fitted a decade ago usually has a current equivalent that occupies the same footprint to the millimetre. Match the size code and the timber never has to move.

The covering is intact and reusable. Slates and plain tiles that come away in one piece go back down over the new flashing. Interlocking concrete tiles are even more forgiving. A felt or single-ply field in sound condition takes a new flashing dressing without being cut back. The covering only forces a strip if it is too brittle or too far gone to be lifted and relaid, which is a question about the roof’s age and condition rather than about the rooflight.

The timber around the opening is sound. The frame lining and the trimmers carrying the load need to be dry and solid. Where a unit has been passing water for a long time before anyone replaces it, the timber underneath can be soft, and that has to be made good first. Sound timber means the new unit sits on a good bearing and a strip is avoided.

A compatible flashing kit exists for the covering. Flashing kits are made to suit the material they seal against: one profile for slates and thin coverings, another for plain tiles, another for deep-profiled or interlocking tiles, and recessed versions that sit the glass lower for a flush look. As long as a kit is made for the covering on your roof, the seal is remade without lifting the field.

When those four line up, an in-plane rooflight replacement without a roof strip is the straightforward, correct way to do the job, and it is how the great majority of failed in-plane units are dealt with.

Some jobs genuinely need the roof opened, and pretending otherwise stores up trouble. The honest cases are these.

You want a different size. Going larger, or changing the proportions, means enlarging the structural opening, which means cutting rafters, fitting new trimmers and taking the covering back to suit. That is a proper alteration, not a swap, and the covering has to come off around it. Going smaller has the same problem in reverse: the gap left around a smaller unit has to be re-framed and re-covered.

The covering is at the end of its life. If the tiles are laminating, the slates are nail-sick and snapping, or the felt has gone hard and is cracking, they will not survive being lifted and relaid. At that point the sensible route is to deal with the covering properly rather than fight it around a new unit, and the rooflight change rides along with that larger piece of work.

The rafters or trimmers have decayed. Soft, wet or rotten timber has to be cut out and renewed, and you cannot reach it without opening up. This is structural and it is not optional.

You are changing the type of unit. Swapping an in-plane rooflight for a kerb-mounted lantern, or the other way, changes how the unit meets the roof entirely. The flashing principle is different, the opening detail is different, and the covering has to be reworked to suit the new arrangement.

The failure mode to watch for is an installer squeezing a mismatched unit into an opening it was never meant for, packing the gap and dressing flashing over the compromise to make it look right on the day. It sheds water until the first driving rain finds the packing. A no-strip replacement is only sound when the unit actually fits; where it does not, the strip is the cheaper option once you count the return visits.

Whether the roof gets stripped or not comes down, more than anything, to the flashing. An in-plane rooflight is only as watertight as the flashing that laps it into the surrounding covering, and getting a fresh kit that suits both the unit and the covering is what lets the field stay in place.

Flashing kits are supplied to match the covering depth and profile. The main types you will meet are these.

The recessed option is worth understanding because it changes the look. A standard kit holds the glass a little proud of the tiles; a recessed kit drops the frame into the roof so the glass sits closer to flush, which reads as cleaner from the garden and from inside. Both are no-strip fittings on a sound roof. The choice is about appearance, not about whether the covering comes off.

One detail people miss: flashing has a minimum pitch. In-plane kits are rated down to a shallow slope, commonly around 15 degrees for many pitched kits, and below that the water does not shed reliably and a different detail or a kerb is needed. If your roof is very shallow, that is a conversation to have before the unit is ordered, not after.

A straightforward in-plane replacement follows the same sequence every time. Access is set up, usually a tower or a roof ladder for a single unit, more where several are grouped or the roof is high, as it often is on a commercial building. The covering immediately around the unit is eased, not stripped: a few slates or tiles at the head and sides are lifted just enough to release the old flashing. The old flashing comes away. The old sash and frame are removed from the timber lining they sit in.

The lining is checked and made good if needed. The new frame is fixed into the opening, set for square and for weathertightness. The new flashing kit is built up around it, soakers and aprons dressed into the covering below and above so water is led down the slope and over the tiles beneath. The few tiles or slates that were eased are relaid over the flashing, and internally the plaster reveal is made good where it meets the new frame.

For a single unit on an accessible slope, that is commonly half a day to a day, weather allowing. A group of units, or a high or awkward roof needing more access, runs longer. The times below are industry-typical guidance for a like-for-like in-plane swap with no strip and no timber renewal.

Weather is the real variable. An open roof, even a small one, wants a dry window to work in, and a wet or windy day pushes the work back rather than forcing it through. That is a scheduling point rather than a cost one on a no-strip job, because the opening is small and briefly exposed.

A replacement is the one moment the glass is genuinely open to change, and it would be a waste to put the same specification back without thinking about it. The old unit that is coming out was very likely clear double glazing with a G-value up around 0.6, which is the proportion of the sun’s energy that passes through as heat. On a north-facing slope that is fine and even welcome. On a south-facing slope in this part of Essex it is the reason the room got too hot in the first place.

Southend faces south across the Thames Estuary, with an open horizon and reflected light coming off the water onto every south-facing roof plane along the coast. An in-plane rooflight on a south slope collects hard through the middle of the day. Glare on a screen, a loft bedroom that will not cool down by bedtime, a kitchen that becomes unusable between two and six on a July afternoon: these are solar-gain problems, and the number that governs them is the G-value of the glass, not the U-value everyone quotes. Since the unit is coming out anyway, matching the glass to the way the slope faces costs little and settles the problem for good.

The rough shape of the options looks like this.

Reading the U-value column, the spread is narrow. Reading the G-value column, it runs from 0.60 down to 0.26, and on a sunny south slope that difference is worth hundreds of watts arriving through the glass all afternoon. A same-size in-plane rooflight replacement without a roof strip lets you keep the tidy job and quietly fix the overheating in the same visit, which is the whole reason we specify glass by orientation. There is more on how these numbers behave on our solar-control glazing and pitched-roof pages.

A like-for-like replacement is still a controlled job. Replacing a rooflight is notifiable under the Building Regulations, and the work has to meet current standards even though the opening is not changing. Two parts apply.

Part L covers thermal performance. It sets a maximum U-value for replacement rooflights in a dwelling, and any current unit worth fitting beats that limit comfortably. This is the reason a replacement cannot simply reuse an old single-glazed or first-generation double-glazed sash: the new glass has to hit the current figure.

Part K covers safety glazing and guarding. Glass overhead has to be laminated on the inner pane so that, if it ever breaks, it holds together rather than falling into the room. Toughened outer panes are standard on rooflights. This is not optional and it is one of the things a proper replacement quietly puts right if the old unit predates the requirement.

Someone has to tell Building Control the work has been done. We make that notification to Southend-on-Sea City Council, or to the relevant local authority for your address, as part of the installation, so the paperwork exists for when you come to sell. As for planning permission, a same-size in-plane replacement that does not project further than the old unit is generally permitted development on a house, though flats, listed buildings, conservation areas and any Article 4 zone are the exceptions. The Leigh and Leigh Cliff conservation areas carry an Article 4 Direction covering windows and roof materials, so a slope there can need a planning application where an identical slope elsewhere would not. Treat that as general guidance and check with the local authority for your own address before assuming.

Cost on a no-strip replacement is driven by three things: the unit and its glass, the access, and the flashing kit. As industry-typical guidance rather than a fixed price, a single in-plane rooflight replacement on an accessible slope, supplied and fitted with a new flashing kit, commonly sits in the low four figures, with solar-control or triple glazing adding to the glass line, and access adding where a scaffold is unavoidable. A group of units or a high commercial roof scales up from there, mostly on access and labour rather than on the units themselves.

Before you agree to anything, a few questions sort a clean job from a bodged one.

1. “Can this be done without stripping the roof?” On a same-size swap with sound covering and timber, the answer should be yes. If it is no, ask which of the honest reasons applies: size change, covering condition or timber. Vagueness there is a warning.
2. “Is the new unit the same external size as the old one?” Same size means same opening means no structural work. A different size is a different job and should be quoted as one.
3. “Which flashing kit suits my covering?” The answer should name the covering, slate, plain tile, profiled tile or membrane, and the kit made for it. That tells you they have looked at the roof, not just the rooflight.
4. “What is the G-value of the glass?” A number should come back. If the slope faces south or west and the answer is a clear-glass figure around 0.6, you are about to put the overheating straight back in.
5. “Who notifies Building Control?” The replacement is notifiable. We make that notification for you. If nobody is making it, the job is not finished on paper.

An in-plane rooflight replacement without a roof strip is, for most failed units, the right job and the tidy one: the same opening, the same covering, a fresh flashing kit and better glass than came out. If you want to know whether your own unit qualifies for a clean swap, and what glass the slope it sits on should carry, ask us for a specification before you commit to anything.