Roofing
Skylight / Rooflight Opening Calculator
Calculates structural opening size and flashing for rooflights
Updated 13 May 2026 · Live
What this tool does
Calculates structural opening size and flashing for rooflights.
Skylight / Rooflight Opening Calculator — enter the skylight width, height and tolerance per side ({skylight_width_mm}, {skylight_height_mm}, {tolerance_mm}) to get the required rough opening size, {result}.
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Formula Used
How the skylight / rooflight opening calculator works
Calculates structural opening size and flashing for rooflights. The calculator takes your dimensions and supplier rates, applies a standard UK trade formula, and returns a quantity with an indicative cost. Every figure is an estimate — site conditions always move the final number.
Typical UK roofing wastage
Standard UK roofing wastage is 5–10% for plain tiles and 10–15% for slates or interlocking profiles, rising on complex hipped roofs and around valleys. Our defaults reflect common UK trade allowances, and can be adjusted upwards for non-standard geometry or downwards where experience supports a lower figure.
What this tool does not do
It does not replace a professional quote, factor regional pricing, assess structural adequacy, or confirm Building Regulations compliance. Those remain the responsibility of a suitably qualified designer, surveyor, or your building control officer.
On-site considerations for skylight / rooflight opening
Working at height needs proper edge protection and a risk assessment under the Work at Height Regulations 2005. Roof safety systems are a legal requirement on commercial work and strongly recommended for domestic.
Building Regulations and compliance
Roof coverings, insulation upgrades, and structural changes to rafters fall under Approved Documents A, C, and L. A re-roof that exposes more than 25% of the roof slope typically triggers thermal upgrade obligations. When in doubt, a pre-application enquiry to the local authority can give early clarity, which tends to be less costly than retrospective correction.
Before you order
Tile colour shifts between production runs, so a single batch number across the whole roof keeps the finish consistent. Keep a few dozen spares for future repairs. Cross-checking the calculator’s output against a supplier quote helps catch differences in pricing assumptions — ask for exact product specifications (grade, finish, batch number) and confirm delivery timescales against your programme.
Adjusting the defaults
Every input in this calculator is editable. Enter your own dimensions, supplier prices, and wastage allowance — the output recalculates instantly. If the defaults feel off for your region or project type, your own numbers always override them.
Using this skylight / rooflight opening calculator alongside other BuildMetricLab tools
This calculator works best as part of a planning workflow. Pair the quantity with our project contingency, labour-hours, and material-cost calculators to build a complete estimate before you pick up the phone to a supplier. All BuildMetricLab tools run entirely in your browser — no sign-up, no data sent anywhere, and every formula is shown on-page so you can audit the maths.
Sources & methodology
Calculates structural opening size and flashing for rooflights. Every result is calculated from the values you enter, and all inputs are editable.
Frequently asked questions
Are skylight / rooflight opening calculator results accurate enough to order materials?
Use them as a starting estimate only. Verifying the final quantity with your supplier or contractor before ordering is good practice — site conditions, wastage and cut-offs all affect the true figure.
How much tolerance should I allow around the skylight?
Manufacturers typically specify a tolerance of 30–50mm per side to allow for the flashing kit, packers and adjustment during installation. Check your rooflight manufacturer's fitting instructions for the exact figure, as this varies by product and roof finish.
Does this tool replace professional advice?
No — this tool only estimates the rough opening size. Any work that involves cutting or trimming roof rafters affects the structure, so the opening size and trimmer detailing should be checked with a suitably qualified professional, such as a chartered structural engineer or experienced roofing contractor, and verified against Building Regulations.
Does the rough opening size include the flashing kit dimensions?
No. The calculated rough opening is the structural hole in the roof, based on your skylight size plus tolerance. The flashing or weathering kit sits over this opening and its own coverage dimensions are set by the manufacturer — check the flashing kit's fitting guide separately.
Spotted something off?
Calculations or display — let us know.
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