Questions covered in this guide
The opening size is only one detail
VELUX distinguishes mounting types and specifies roof-pitch requirements for skylights and flashing. A rough indoor measurement is useful context, but it is not enough to select a replacement product and its matching installation components.
Identify what is already there
Look for product information that can be read safely from inside and save existing manuals or invoices if available. Photograph the interior appearance and describe whether the unit opens or stays fixed. Ask the installer to verify the mounting arrangement, required dimensions, roof slope, and surrounding condition on site. Keep homeowner observations separate from verified ordering measurements so a casual note does not become the supplier’s manufacturing instruction.
Create an order-release worksheet
Use one line each for the existing model, replacement model, mounting type, installer-verified dimensions, compatible flashing identification, and any curb or interior modification. Add the name of the person verifying each technical entry and the date. Ask the supplier to identify accessories included in the package and those supplied separately. If a substitution is proposed, have the installer repeat the compatibility check for that actual product instead of approving it because the visible size looks similar.
Confirm the whole opening scope
Request a written description of any interior finishing or surrounding roofing work before approving the order. Decide who will resolve a discrepancy discovered after removal and how the house will be protected while a corrected product is obtained. The national roofing calculator does not verify skylight dimensions or include a precise replacement package. Retain the completed worksheet and final model documents; they will be more useful for future service than an unlabeled photograph of the finished ceiling.
Putting it into practice
This is an ordering-control guide, distinct from deciding whether to retain a skylight. It separates homeowner observations from installer-verified dimensions and requires compatibility to be reconsidered when a product changes.
Sources and supported claims
VELUX distinguishes deck-mounted and curb-mounted replacement skylight installation.
Source ↗ · national · Checked 2026-10-12
VELUX specifies roof-pitch requirements for skylights and associated flashing products.
Source ↗ · national · Checked 2026-10-12