- A lead flashing not chased into the joint, so water runs behind the junction
- Cracks because extension and house settle differently
- A wall plate not properly fixed to the existing wall
- A cold edge between the new roof and the existing wall
Building detail · Roof
New roof against an existing wall
With an extension, the new roof meets the existing wall. Two structures that can move differently come together there. A good junction stays watertight, even if the extension settles slightly relative to the house.
- Roofing carried up the wall, covered with lead
- A wall plate along the existing wall or a separate supporting structure
- Room for differential settlement
Principle sketch
Build-up of the detail
Vertical section through the junction of a new roof with an existing wall. The numbers in the sketch refer to these parts:
- Existing wall
- Roof joists of the extension
- Roof deck with insulation
- Roofing carried up the wall
- Lead flashing, chased into the joint, over the roofing
- Wall plate along the existing wall
- Ceiling finish
Common mistakes
Where it often goes wrong
Leaks, condensation and cracks usually start at the junctions, not in the middle of a wall or roof.
In the drawing
What we look out for
We work out the detail for your home, based on a site survey and the chosen materials.
- The fixing of the wall plate and the condition of the existing wall
- The foundation of the extension relative to that of the house
- The height of the upstand and the lap of the lead
- Insulation that meets the existing wall
- Access to the junction for maintenance
A full-width rear extension often also means opening up the rear wall. Structural calculations
Detail FAQ
Frequently asked questions about this detail
Why is a lead flashing chased into the joint?
So that water running down the wall flows over the lead and not behind the roofing.
May an extension be fixed to the existing wall?
Yes, if the wall can carry the load. Sometimes a separate supporting structure or foundation is better. That follows from the structural assessment.
What happens if the extension settles?
If old and new are rigidly fixed together, cracks or leaks can occur. That is why we design the junction with room for movement.
Can I use this principle sketch for my own home?
The principle sketch shows the build-up, but it is not a working drawing. Dimensions, materials and fixings depend on your home. We work out the detail for your situation.
Detail library