Why is there damp where my extension joins the house?

This is one of the most reliably diagnosable damp faults in a house, because the geometry gives it away. The wall above a lower roof is exposed to rain over its whole height, and the water it absorbs runs down inside the cavity. Below the abutment that cavity is now inside your extension. Without a tray to intercept the water and eject it above the flashing, it simply arrives in the room.

What you'll need

• Binoculars
• Torch
• Camera on a pole for the abutment (optional)

Step by step

1. Look at where the damp is.A horizontal band on the inside of the main house wall, at or just above the level where the extension roof meets it, is the signature. It follows rain and it may be worse at one end where the roof falls.
2. Look at the abutment from outside.There should be a lead flashing dressed into the wall and over the roof covering, and above it a line of weep holes or open perpends in the mortar joints. No weep holes above a flashing usually means no tray.
3. Check the flashing itself before blaming the tray.A flashing pulled out of its chase, splitting at the folds, or replaced with a cement fillet that has cracked will leak at the same place and is far cheaper to fix.
4. Find out when the extension was built.Cavity trays over abutments have been required practice for a long time, but they are among the most frequently omitted details, particularly on porches, conservatories and DIY-era extensions.
5. Look for the same problem at other abutments.A bay roof, a garage roof against the house, a porch, a boiler flue penetration. Any lower roof against a cavity wall needs the same detail.
6. Understand what the repair involves.Retrofitting a tray means cutting into the outer leaf in short sections above the roof line, inserting preformed tray units, and rebuilding with new weep holes and a new flashing. It is bricklayer's work, done in stages so the wall is never unsupported.
7. Do not have the wall injected or the inside tanked.Neither addresses water arriving inside the cavity from above. Tanking the internal face will simply move the damp to the edge of the tanking.
8. Check the extension's own damp course while you are at it.The tray in the main wall is one detail; the extension will also have a barrier at its own base and a vertical barrier at the junction between old and new brickwork, and both are commonly missed.

Tips

• Weep holes are small vertical slots or plastic inserts in the mortar joints. If somebody has repointed or painted over them, water backs up behind the tray and the tray becomes useless.
• If the extension has a flat roof, check the upstand height at the abutment as well. An upstand that is too shallow lets water over the top in heavy rain regardless of any tray.
• This is a defect worth documenting properly if the extension is recent, because it may fall under a builder's liability, a warranty, or a claim against a previous owner's undisclosed works.

Common mistakes

• Sealing along the top of the flashing with mastic — It is the standard quick fix and it fails within a couple of years, and while it lasts it can also block the weep holes that were the wall's only drainage.
• Assuming a leaking abutment means the roof covering has failed — The roof is usually fine. The water is coming out of the wall above it, which is why replacing the roof covering does not fix it.

Questions people ask