Guide · Skylight & Rooflight Installation & Replacement Chelmsford
What is found when the old unit comes out
The moment an old rooflight is lifted out is the first time in decades that anybody has seen the construction around it. Everything up to that point has been inference: a survey reads what it can from above and below, a bradawl tells the truth about a few square centimetres of timber, and photographs show a finished surface. Then the unit comes off and the actual condition is visible.
Most of the time what is underneath is unremarkable. Sometimes it is not, and the findings fall into a fairly short list that recurs across the Essex housing stock with almost boring reliability. Knowing that list in advance is what stops a discovery becoming a crisis. This page belongs with the practical detail behind skylight replacement.
Why anything is found at all
A rooflight is the one place in a roof where a hole was deliberately made, and holes attract everything. Water that gets past the covering anywhere upslope finds the trimmed timber first. Insulation gets cut around the opening and rarely gets reinstated as neatly as it started. Cables and ducts are routed towards openings because that is where the voids are accessible.
Add twenty five years, several changes of ownership and at least one previous alteration nobody documented, and the perimeter of a rooflight becomes the most eventful two hundred millimetres of the whole roof.
Soft decking at the corners, and what put it there
The most common finding on a flat roof is timber decking that has gone soft at one or two corners of the opening, while the rest of the board is sound.
The reason is geometry. Water that crosses the detail runs down the inside face of the upstand and collects where the kerb meets the deck, and it collects fastest at the low corner because the roof has a fall. The top face of that decking can look completely normal while the underside has been wet for years. The test is mechanical: a screwdriver pushed into the board at the junction gives the answer in two seconds. Where the board is soft, it comes out to the nearest joists and new decking goes in, which is an hour of work if it is caught and a much larger conversation if it is not.
Wet insulation, and how far the water travelled
On a warm deck flat roof the insulation sits above the deck and below the covering, which puts it directly in the path of anything getting past the perimeter.
Rigid boards do not hold much water themselves, but the joints between them are a highway. Water entering at a kerb can travel two or three metres along a board joint before it finds a route down, which is why the wet area of a ceiling is so often nowhere near the rooflight. When the covering comes back around the opening, the extent is checked by lifting a board rather than by looking, because saturated insulation has lost most of its thermal value and putting a new unit on top of it changes nothing about the room below.
Wet insulation does not dry out under a waterproof covering, which is why it is removed rather than left to recover.
Chipboard, plywood and OSB decks compared at a cut edge
The material of the deck decides how a discovery is handled, and the three found across Essex behave very differently once wet.
| Deck material | Typical era | Behaviour once wet |
|---|---|---|
| Softwood boarding | Pre-1960s extensions and outbuildings | Rots locally, individual boards replaceable |
| Chipboard | 1970s and 1980s | Swells, delaminates, loses all strength |
| Plywood | Throughout, better grades from the 1990s | Delaminates at the plies, edges first |
| OSB | 1990s onward | Swells at cut edges, holds up better in the field |
Chipboard is the one that produces the unwelcome findings. It was widely used, it has no useful wet strength, and a board that felt firm underfoot can be crumbling at the opening edge.
A unit bedded in mastic and nothing else
A recurring find on older flat rooflights is a frame sitting on a bed of mastic with a handful of nails through the flange, and nothing else holding it down.
Mastic was doing the entire job: fixing, weathering and levelling. It works for about a decade, then it hardens, shrinks away from one surface and cracks. What is underneath is usually a kerb top that was never planed level, which is why the mastic was there. A new frame is set on a level, dry top and mechanically fixed at the specified centres, with the weathering done by the covering dressed up the kerb rather than by a bead of anything.
Frames fixed straight through the covering
The second common historical shortcut is a frame screwed down through the felt or the membrane and into the deck, with a bead of sealant over the screw heads.
Every one of those fixings is a hole through the waterproof layer, in the flattest part of the roof, with water lying on it. They survive as long as the sealant does. When the unit comes out, the giveaway is a line of small dark stains on the deck at fixing centres, each one directly under a screw. The correct arrangement puts all fixings above the water line, through a flange bearing on the top of an upstand, so nothing penetrates the covering at all.
An upstand that was never actually built
Occasionally the unit comes off and there is no kerb at all. The frame was sitting directly on the deck with the covering dressed up against the outside of it.
This turns up on garages, outbuildings and rear extensions across the county where the work was done informally, and it explains a great deal about the condition of everything around it. There is no height between the water on the roof and the underside of the frame, so any ponding, drifted snow or wind driven rain reaches the seal directly. Building an upstand at that point is not an extra so much as the first time the detail has existed, and the height is set from the finished surface for the position rather than from a general figure.
Underlay cut short around a roof window
On a pitched slope the equivalent finding is the underlay. It should be cut, dressed into the frame and supported so that any water getting past the tiles runs onto the flashing and away.
What is often found instead is underlay slashed with a knife and left, so the cut edges hang loose and water arriving from further up the slope drops straight onto the ceiling behind the window. Modern units are supplied with an underlay collar precisely to make this detail repeatable. On an older installation the collar is simply absent, and fitting one as part of the replacement usually settles a mark on the reveal that everybody had blamed on the glass.
Trimmers that were never doubled
Structural findings are less common than water findings, but they carry more consequence.
Where a roof window was cut into a slope by somebody working quickly, the rafter is sometimes cut with a single trimmer at head and sill and no doubling of the members either side. The roof does not fall down, because roofs are forgiving and the loads are modest, but the opening will have spread slightly and the frame will be sitting in something that is no longer square. The evidence is a diagonal measurement across the opening that differs by ten millimetres or more. Correcting it means doubling the flanking members and refitting the trimmers, which is carpentry rather than engineering at this scale.
Loose fill, old fibre and what is lying in the void
Anything opened into a roof void in a house built or altered before the year two thousand can contain material that needs identifying before it is disturbed.
The specific ones are vermiculite loose fill, which was used as loft insulation and can carry contamination, old mineral fibre that has been rained on and compacted into a dense mat, and early insulation board used as a lining under an opening or as a soffit inside a light well. None presents a hazard while it is undisturbed. All of them become a controlled matter the moment somebody starts cutting. Where suspect material is identified, it is sampled and handled by a specialist licensed for that work before the opening is touched.
An extract duct discharging into the roof space
This is the finding that explains the most and surprises people the most. A bathroom or kitchen extract fan connected to a length of flexible duct that stops somewhere in the void instead of terminating through the roof or a wall.
Warm wet air is pumped into a cold space every day. It condenses on the underside of the deck, drips onto whatever is below, and produces marks that look identical to a leak and follow the household routine rather than the weather. Where a rooflight opening gives access to it, the honest thing is to sort the duct while everything is open. It is an hour’s work at that moment and a considerable job at any other.
Cables, junction boxes and what crosses an opening
Roof voids carry services, and openings are exactly where they are found because openings are where the void has been accessible.
The usual items are lighting circuits crossing the ceiling on the shortest route, junction boxes sitting on the plasterboard rather than fixed to a joist, aerial and satellite cables threaded through, and occasionally an old cold water pipe run that nobody drained. Anything that has to be diverted around a new opening is a matter for an electrician working to Part P where it is a circuit alteration. It is identified before the saw comes out, which is one of the reasons the survey looks into the void from below wherever a hatch allows.
Birds, wasps and the space above a lined ceiling
A void behind a sloping ceiling or above a flat roof lining is an attractive place to nest, and finding out on the day is inconvenient for everybody involved.
Old nesting material is combustible, holds moisture and blocks any ventilation path the roof was designed to have. Active nests are a different matter entirely, since wild birds and their nests are protected while in use, and the work waits rather than proceeds. Wasp nests in a warm south facing void are common through late summer across the Essex villages, and they are dealt with before anyone opens the roof rather than after.
Two coverings, one laid over the other
Flat roofs are frequently overlaid rather than stripped, so what looks like one covering can be three generations of felt sitting on each other.
The signs are visible at the opening once the unit is off: layers in section at the cut edge, a build-up that is far thicker than a single covering should be, and an upstand that has effectively got shorter each time a new layer went on. That last point is the one that matters, because an upstand built to one hundred and fifty millimetres in 1985 may now stand at ninety. Establishing the true height above the current finished surface, rather than the height it was built to, is the measurement that governs whether the kerb can stay.
How findings are dealt with without stopping the job
The organising principle is simple. The roof is closed and watertight at the end of every working day, whatever has been found, and no discovery changes that.
In practice that means the sequence is planned so that a finding is assessed while the opening is still small, materials for the likely findings are carried as standard, and the decision point comes before anything irreversible. A soft board, a wet section of insulation, an absent upstand and a short underlay are all handled within the same visit if they are found in the morning. Anything that genuinely needs a specialist, such as suspect material or an active nest, means the opening is decked, weathered and closed properly rather than sheeted while it waits.
What is priced as provisional, and what is not
An honest replacement quotation separates what is known from what is conditional, and says which is which in writing.
Known items are the unit, the flashing or covering reinstatement, the access arrangement, the notification and the internal making good. Conditional items are the ones that depend on what is underneath: decking replacement priced by the square metre, insulation replacement by area, a kerb rebuild as a stated figure, and any electrical diversion as a provisional sum. Setting those rates before the work starts means a finding produces a known number rather than a negotiation on a ladder. What a written quotation should set out in full is covered on what a replacement quotation should contain, and how the ranges are built is on the costs page.
Keep reading
More on skylight & rooflight installation & replacement chelmsford
Sizing a rooflight for the room
Read the guide →Roof window or rooflight, and the difference
Read the guide →Flat roof against pitched roof rooflights
Read the guide →What happens on installation day
Read the guide →Cleaning and maintaining a rooflight
Read the guide →All about skylight & rooflight installation & replacement chelmsford →
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