Do flat rooflights and skylights come in standard sizes?
Yes. Every manufacturer offers a range of standard sizes, which are cheaper and available faster. What happens in practice is that an opening formed twenty or thirty years ago rarely matches one, so most replacements are made to size orders with a lead time attached.
What a standard size means
A size the manufacturer holds or produces in batches rather than making individually.
It costs less, it arrives sooner, and the range covers the sizes most commonly wanted, generally in a series of steps rather than a continuum.
Why an old opening rarely fits one
Because the opening was formed to suit whatever unit was going in at the time.
Sizes have changed, manufacturers have come and gone, and a great many older kerbs were built to a dimension somebody made up on site.
Which means the opening dictates the unit rather than the reverse.
What made to size actually is
A unit manufactured to the dimensions we measure, rather than a bespoke design.
It is the same product in a different size, not a special. What it costs is a modest premium and a lead time.
The lead time
Several weeks is normal, and it is frequently the longest part of the process.
Which is why a date is given after the order rather than at the quote, and why we hold a unit until the morning it goes in rather than delivering it to stand in a garden.
How the dimensions are taken
From the kerb, on four sides and both diagonals.
Measuring two sides and assuming the other two match is how a unit arrives that does not fit, and on an old kerb the four sides are frequently not equal.
The diagonals are what reveal whether it is square.
Structural opening or kerb size
The distinction that causes ordering errors.
Manufacturers quote sizes against a defined dimension, which may be the kerb outside face, the structural opening or the clear glass. They are not the same number.
Which is quoted is confirmed with the manufacturer rather than assumed, and it is the commonest way a unit is ordered wrong.
What happens when a kerb is out of square
A made to size unit is ordered to the measured dimensions rather than the nominal ones.
Where the racking is modest that works. Where it is substantial, or where the extension is still moving, the kerb is corrected first.
The detail is on what is a builders kerb.
Building a new kerb to a standard size
The obvious answer on new work, and it is worth doing deliberately.
Where an extension is being built, the kerb is formed to suit a standard unit rather than to an arbitrary dimension, which saves cost and lead time.
It requires the size to be chosen before the deck is built, which is why we ask to be involved early.
Adapting an existing opening
Sometimes cheaper than a made to size unit and it depends on the numbers.
Where an old opening is close to a standard size, it can occasionally be adjusted so a standard unit fits, which is carpentry rather than a premium.
Whether that is worth doing is arithmetic we do at survey.
Size steps within a range
Worth knowing, because they are not fine.
Standard ranges step in intervals rather than millimetres, so the nearest standard size to an existing opening can be noticeably smaller or larger.
Smaller means adapting the kerb inward; larger means enlarging the opening and the structure.
Maximum sizes
There is a limit, and it is set by several things at once.
Glass manufacturing limits, the weight anybody can safely handle, the structure carrying the opening, and what will physically get to the roof.
Beyond a point, two units is the answer rather than one.
Getting a large unit to the roof
The constraint that decides many size questions.
A unit that will not come through a house or round a side passage needs hoisting, and that is a cost and a method rather than an inconvenience.
It is measured with a tape at survey, including the turn at the bottom of the stairs.
Whether shape options exist
Rectangles overwhelmingly, with circles and other shapes available at a premium.
On a domestic flat roof a rectangle is almost always right, and unusual shapes bring cost, lead time and a harder weathering detail.
Matching an existing unit
Worth attention where a second is being added to a deck.
Two units on one roof should match in size, profile and glass, and matching an older unit means identifying its range rather than approximating its dimensions.
Where the original cannot be matched, replacing both is frequently the better answer.
Ordering from a photograph
Something we will not do, and it is worth explaining why.
A photograph shows a covering and an approximate size. It does not show whether a kerb is square, what its height is above the finished covering, or which dimension a manufacturer’s size refers to.
An indicative range from photographs is honest. An order from them is a unit that does not fit.
What happens if a unit arrives wrong
It goes back, and the job waits for another lead time.
Which is why the measurements are taken carefully once rather than quickly twice, and why we confirm the size basis with the manufacturer before ordering rather than after.
How the measuring is done on a flat roof
We measure four sides and both diagonals, confirm with the manufacturer which dimension their size refers to, and on new work specify a standard size and have the kerb built to suit it.
The whole picture is on the flat roof rooflight installation page.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.