Guide · Skylight & Rooflight Installation & Replacement Chelmsford
Matching a replacement to an obsolete size
Rooflight openings outlive rooflight catalogues by a wide margin. A hole formed in a roof in 1974 is still exactly the size it was, while the range it was formed for went out of production three decades ago and the company that made it may no longer exist. That mismatch is the single most common technical obstacle on a replacement, and it is solvable in every case. It is just solvable in four different ways, and the survey decides which.
This page is about the sourcing problem specifically: how to work out what is up there, how to measure what is left behind, and what the realistic routes are when the size on the roof is not a size anybody sells. It sits under skylight replacement, which covers the wider job.
Why a size stops being available
Rooflight ranges are built around a grid of standard sizes, and that grid moves for reasons that have nothing to do with the roofs already carrying the old one.
Manufacturers rationalise their ranges every decade or so, dropping slow selling sizes and adding others. Glass thicknesses change as thermal standards tighten, which changes frame depths and sometimes the outer frame dimensions with them. Whole product families get retired when a new frame system replaces them. And the change from imperial to metric working, which ran through British construction across the nineteen seventies, left a whole generation of openings set out in feet and inches with no metric equivalent anywhere near them.
Establishing what is actually fitted
Before anything can be matched, the existing unit has to be identified, and that is a ten minute job from inside in most cases.
Roof windows carry a data plate, usually on the top edge of the sash where it is visible when the window is swung open, sometimes on the side of the frame. It gives the maker, the model and a size code, and occasionally a manufacturing date. That plate is worth more than every tape measurement combined, because it names the size in the maker’s own language and current charts can be read directly against it. Flat units and domes are less reliable. Some carry a label on the underside of the frame, many carry nothing at all, and those are measured rather than identified.
A photograph of the data plate on the sash edge answers more questions than any other single piece of evidence on a replacement.
Size codes, and how to read one that has aged
Roof window sizes are usually expressed as a two letter, two digit code rather than in millimetres, and the convention is consistent enough to be useful.
The letters denote a width band and the digits a height band, so a code identifies a rectangle from a published chart rather than describing it directly. Codes have been broadly stable for decades, which is why a like for like swap into an older opening is often straightforward. What has changed underneath the code is the sash depth, the glazing thickness and the flashing geometry, so the same code today produces a unit that fits the same hole and sits differently against the tiles. Different makers use different lettering, and cross referencing between two manufacturers’ charts is done on the millimetre dimensions, never on the codes.
Measuring an opening that was never square
Where there is no plate, or where the unit is a flat one, the opening itself becomes the reference, and the way it is measured matters.
A width taken once in the middle is not a measurement, it is an average. The opening is measured at both ends and in the middle, in both directions, and both diagonals are taken. A structural opening formed in 1979 with a hand saw, then subjected to fifty years of seasonal movement, will very often differ by ten or fifteen millimetres across its length and considerably more across the diagonal. The working dimension is the smallest of the three, because a frame has to pass through the tightest point, and the difference between the diagonals tells you how much packing the reveal will need.
Which number a supplier is actually quoting
Three dimensions describe a rooflight and they are routinely confused, which is how the wrong unit arrives on a Tuesday morning.
| Dimension | What it measures | Where it is taken |
|---|---|---|
| Structural opening | The hole in the roof structure | Between trimmers or inner faces of the kerb |
| Kerb outside size | What a flat unit sits over | Across the outer faces of the upstand |
| Unit or frame size | The product itself | Outer edges of the frame, usually overhanging the kerb |
Some manufacturers also publish a daylight opening, which is the visible glass and is smaller again. When comparing two quotations, the first job is establishing that both are quoting the same one of these four numbers. Very often they are not, and the difference reads as a price gap when it is really a specification gap.
Imperial roofs, and the numbers still in them
Openings formed before the mid nineteen seventies were set out in feet and inches, and the common sizes recur across Essex housing with striking consistency.
Two feet by three feet, three by four and four by four are the ones that turn up again and again on bathrooms, landings and rear extensions. Converted, those are 610 by 914, 914 by 1219 and 1219 by 1219 millimetres. Current standard grids sit at figures like 550, 780, 940, 1140 and 1340 in width, so an imperial opening typically lands somewhere between two standard sizes with fifty or sixty millimetres either side. That gap is the whole problem, and it is small enough that all four of the routes below stay open.
The four routes out of an obsolete size
Every one of these jobs ends in one of four places, and naming them early makes the survey conversation short.
The opening is eased slightly and the next standard size up goes in. The next size down goes in and the reveal is packed and re-lined. A unit is made to the measured size. Or the opening is properly enlarged, which brings structure into the job. The first three keep the work inside a day or two. The fourth changes the shape of the project entirely and is covered on when the opening has to change size.
Easing an opening by twenty millimetres
The most common answer, and the least dramatic, is that the opening gains a small amount and takes the next standard unit.
On a cut rafter roof with a trimmed opening, twenty or thirty millimetres is frequently available in the lining rather than in the structure. The plasterboard return and the timber sub-frame around the opening take up more space than people assume, and removing a lining batten can release the width needed without touching a trimmer. Where the structure itself has to give, a rafter roof will usually tolerate it with the trimmer repositioned and refixed, which is ordinary carpentry. What decides it is what the timber around the opening is doing, which is why it is established at survey and not on the day.
Going a size down, and what that costs in daylight
Fitting the next standard size below the opening is the least popular option and it is sometimes the right one.
The frame is set into the opening, packed and mechanically fixed to sound timber, and the difference is taken up in the reveal, which is re-lined to meet the new frame. The loss is real. Dropping from a 1219 wide opening to a 1140 wide unit loses about eighty millimetres of width, which on a bathroom rooflight is barely noticeable and on the main window of a loft bedroom is. The gain is that the structure is untouched, the programme is short and the unit is a stock item. Where the opening sits in a truss bay with no width to give, this route and the bespoke one are the only two on the table.
Made to measure, and what it actually covers
Most manufacturers of flat rooflights and several roof window makers will produce a unit to a measured size, and the flexibility is greater than people expect.
For a flat rooflight, made to measure is close to routine, because the unit is a frame around a sealed unit and both are cut to order. Sizes are usually available in single millimetre increments within a range, with minimum and maximum dimensions set by the glass rather than the frame. For roof windows the picture is tighter, because the sash, the flashing kit and the hinge geometry are all engineered around the standard grid, so bespoke roof windows exist but come from fewer makers and cost proportionately more.
Bespoke on a trussed roof, and why it is often the answer
Where the roof is trussed, the calculation changes entirely, because the option of easing the opening quietly disappears.
A truss is an engineered component with its webs and bottom chord carrying load in tension and compression, and cutting one is not comparable to cutting a rafter. On the estates at Great Notley, South Woodham Ferrers, Beaulieu Park and across the Basildon and Harlow housing, rooflight openings sit between trusses and the width is fixed by the truss centres. Twenty millimetres cannot be borrowed. On those roofs a made to measure unit is very often cheaper than the structural design work that easing the opening would require, and the longer lead time is a fair trade.
Models discontinued by makers still trading
The easiest version of this problem is a range that has been superseded by the same manufacturer, because the succession is documented.
Makers publish replacement charts showing which current model supersedes which discontinued one and whether the outer frame dimensions carried across. Where they did, the swap is genuinely like for like on size and the only new components are the unit and the flashing kit. Where they did not, the chart names the difference in millimetres, which converts directly into how much lining work follows. Either way the answer exists in writing rather than being estimated, and it is worth having that reference in hand before the order goes in.
Units from makers that no longer exist
The harder version is a unit from a company that closed thirty years ago, and Essex roofs carry a good number of them from the regional flat roofing suppliers of the period.
These are treated as unidentified and matched by measurement alone. The unit is measured in place where possible, the kerb is measured after the unit comes off, and the replacement is specified against the kerb rather than against a code. On a flat roof this is usually comfortable, because the kerb is very often being rebuilt anyway and a new kerb can be built to suit a standard unit rather than the other way round. That single move, sizing the kerb to the unit instead of the unit to the kerb, resolves a large share of obsolete size problems on flat roofs.
Adaptor flashings, and what they can and cannot do
Where the new frame is slightly smaller or shallower than the old one, the flashing is the component that closes the difference on a pitched roof.
Flashing kits are made in variants for different covering depths and for recessed or standard installation, and there is a useful amount of adjustment in how the apron and the side soakers are dressed. That will absorb a modest change in frame position or depth. What it will not do is bridge a genuine size gap. An apron stretched across a fifty millimetre shortfall sits proud, holds water at its upslope edge and produces a slow mark on the reveal two or three winters later, and by then the cause is a long way from obvious.
Tolerance, and how much slack a frame really has
Fitting instructions specify a gap between the frame and the opening, and that gap is a working allowance rather than spare capacity.
Typically it is around ten millimetres per side for a roof window, which is there for adjustment, insulation and thermal movement, not for absorbing a mismatched size. Fill it with packing on one side and the frame goes in out of square, which shows up as a sash that binds at one corner or a seal that only compresses along three edges. The frame is set square, plumb in its plane and out of wind, and packed evenly, with the insulation collar formed around it afterwards. A window that opens sweetly on the day and drags after one summer was almost always fitted twisted.
Lead time, and keeping the roof shut while it is waiting
The practical difference between the four routes is mostly measured in weeks rather than in difficulty.
A standard size in a mainstream range is often stocked or supplied within a fortnight. A made to measure flat rooflight typically runs two to four weeks. A bespoke roof window can be longer, and a specialist glass build inside a bespoke frame longer again. What holds the programme together is a simple rule: nothing is stripped until the new unit is on site, unwrapped and checked against the measured opening. The old rooflight, whatever its condition, is keeping the weather out until the moment the new one is ready to go in.
What to send so a size can be matched
Four things settle almost every obsolete size question by email, before anyone climbs anything.
A photograph of the data plate if there is one, taken with the sash open. A photograph of the unit from inside showing the reveal and the frame. The opening measured at the ceiling in both directions with a tape, both figures taken at both ends. And what the roof covering is, in whatever words come naturally: flat and black, grey tiles, slate. From there a range can be given and the route narrowed to one or two options, and the survey confirms the structure and turns it into a fixed price. The costs page explains how those ranges are built.
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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