Damp In A New Extension Or Converted Garage

What's happening?

An extension or a garage conversion has new walls and a new floor joined to an old house, and the joins are where damp appears. The new damp proof course needs to link properly to the old one. The new floor needs a damp proof membrane that is continuous and turned up at the walls, and many garage floors have none because a garage did not need one. Cavities need to be kept clear of mortar so water cannot cross, and ground outside must be kept below the damp proof course. Flat roof junctions and new window openings can add penetrating damp. New masonry and plaster also hold a lot of construction moisture that takes months to dry, so some damp early on is not a fault, while damp that persists or is on one wall is.

What usually causes it?

• A damp proof course in the new walls that does not link to the old one
• No damp proof membrane in a garage conversion floor, or one that was torn or not lapped to the damp proof course
• Cavity bridged by mortar or ground built up outside above the damp proof course
• A leaking flat roof or wall junction between the extension and the house
• New plaster and screed still drying, trapping moisture under finishes
• No ventilation, so condensation adds to the damp in a new, airtight room

Is it dangerous?

Low in the short term, but persistent damp ruins finishes and can rot timber floors and joinery. A damp floor under flooring can mean mould.

Can I fix it myself?

Note where the damp is, how high, and whether it is only in wet weather. Check outside ground levels and gutters and clear anything against the wall. Ventilate and heat the room evenly to help it dry. If the work was recent, find the paperwork: building control completion certificate, the builder's guarantee and any warranty. Do not seal the wall with damp-proof paint before the cause is known.

When should you call a damp proofer?

If the work is new, tell the builder in writing first and check the warranty and building control certificate. Then call a damp surveyor to identify the cause independently. Call a damp proofer for a missing membrane or a bridged cavity.

What will the damp proofer actually do?

• Survey the walls and floor and compare moisture readings with the old part of the house
• Check the junction between old and new damp proof courses and the floor membrane
• Check the cavity and ground levels outside for bridging
• Inspect the flat roof and wall junctions for penetrating water
• Fit a damp proof membrane or tanking where the floor or walls have none, or repair the bridged areas
• Make good the plaster and finish after the walls have dried

Who fixes this

• Damp survey
• Internal damp proof membrane
• Damp proof course installation

Questions people ask

Sources & References

1. Designing Buildings: damp proof membrane
2. Designing Buildings: rising damp
3. GOV.UK: building regulations approval
4. Property Care Association: rising damp