Rooflight advice
Measuring for a Roof Lantern: Five Details to Check
Before ordering a roof lantern, turn the site measurements into a clear, agreed drawing. These five checks help prevent the common mistake of ordering the right numbers against the wrong measurement reference.
1. Name the measurement reference
Identify exactly what each measurement describes: the structural opening, internal kerb or finished external kerb. Match that reference to the current manufacturer drawing for the chosen range. Record dimensions in millimetres and label both sides. A note saying only width and length does not tell the supplier which surfaces were measured.
2. Record the finished build-up
Account for the finished construction rather than an incomplete stage of the build. Ask the installer to identify the insulation, roof membrane and other layers that affect the specified measurement. Do not add a guessed clearance. The product’s instructions determine which dimensions and allowances are relevant.
3. Check the diagonals
Have the installer check both diagonals as well as the side measurements and support levels. Compare the results with the tolerance in the selected product’s instructions. A rooflight is not a way to disguise an out-of-square or unsuitable support; resolve discrepancies before the order is placed.
4. Show the orientation
Mark the ridge direction, nearby walls and any obstructions. Include a section through the support so that the rooflight installer and roofer can see how their details meet. For a flat skylight, use its own span, slope or width labels instead of borrowing lantern terminology.
5. Keep one agreed version
Keep the checked drawing with the configuration, finish and glazing choices. If the opening changes, contact us before manufacture and ask whether the order can be amended. Do not assume that updating a site drawing changes an existing order. Retain the written confirmation of any agreed amendment.
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