Distinguish glazing size from the structural opening
The visible glass size, product dimensions and required opening are not always identical. Obtain complete interface drawings and identify which dimension is shown on your sketch. Include weight and support information. This avoids the engineer and supplier working from different assumptions. Selecting a different product later may require renewed coordination of the opening and interfaces.
Consider the roof around the opening
An opening may interrupt structural elements and change load distribution. Its position relative to beams, walls and the external wall therefore matters. Discuss upstands and additional finishes as well. The structural design must coordinate with the building detail, but it does not automatically cover waterproofing or insulation. Assign coordination of these interfaces so the structural detail becomes part of a complete roof detail.
Include the installation stage
For an existing extension, establish which roof information is reliable and what investigation is required. Before cutting an opening, agree who assesses temporary works and construction methods. For a new extension, product information can be incorporated early. Keep final supplier drawings with the structural drawing. Check that revisions and the intended opening still correspond before ordering.
Practical checklist
- Confirm the required structural opening
- Share final supplier information
- Coordinate roof layers and upstands
- Check revisions before ordering
A rooflight’s external dimensions are not automatically the required structural opening dimensions.
Preparing your own project
Would you like this assessed for your home? See our information about house extensions. It explains which information is relevant to that brief. The project check helps you prepare your enquiry.
Unsure about a term? The structural glossary provides plain-language explanations and technical context. To understand how the documents are organised, explore our fictional example project .
Background and sources
Sources checked on 3 October 2026. This guide is not a project-specific assessment.
Editorial guide • updated 3 October 2026. General information; not a project-specific assessment.