A flat extension roof needs a coordinated design for structure, insulation, drainage and junctions. Discuss edges, the existing wall connection and any rooflights together. The selected waterproof covering is only one element of the complete roof assembly.
What must the extension roof support and connect?
Establish whether the roof simply encloses the extension or must accommodate other intended features. Discuss possible future uses now rather than assuming they can be added later. The rear wall connection, windows above and the room below all influence the proposal.
Request a section showing the complete roof assembly. This allows structure, internal clear height and junctions to be considered together. A product name for the uppermost layer provides insufficient information, particularly where the extension meets several existing materials or different levels on the original building.
How do the roof layers work together?
Insulation, airtightness and moisture control must suit the structural arrangement. An existing roof and a completely new one do not automatically need the same approach. Establish the layers, their functions and the treatment of penetrations.
Include relevant system guidance in the detail development. Milieu Centraal highlights the importance of an appropriate approach to insulating flat roofs; adding individual layers without assessing their interaction can create an unsuitable assembly. Review the entire junction rather than treating insulation thickness as the only meaningful choice in the specification.
Which route will rainwater follow?
A roof described as flat still requires a designed drainage route. Consider falls, outlet positions and the connection to the wider drainage system. Rooflights, upstands and interruptions can affect that route.
Ask how outlets remain accessible for maintenance. The drainage question does not end at the roof edge: show where water goes after leaving the outlet. A detail drawing helps prevent the roof, wall and terrace being designed independently with conflicting assumptions about the same rainwater connection and the space required to make it work.
Which edge details and maintenance information should be recorded?
Edges and junctions deserve as much attention as the main roof area. Specify the connection to the house and the finish visible from the garden and upper windows. Request inspection and maintenance information for the selected materials.
Establish the work included in the offer, including adjacent reinstatement. Keep relevant details with the building records. They are useful if another contractor later changes a rooflight, installation or wall component and needs to understand the existing roof assembly rather than guessing from its visible surface.