Damp proofing in Londonfor period housingthat stays dry

The single most common job across London is a failed damp proof course in a solid-walled terrace or semi. The evidence is consistent: a tide mark at a fairly even height, salt deposits on the wall face, plaster that sounds hollow and crumbles when tapped, paint peeling at skirting level and skirtings that are soft at the back.

Treatment means injecting a new chemical damp proof course into the mortar bed using an accredited Sovereign or Wykamol system, then removing the salt-contaminated plaster and replacing it with a salt-retardant render and skim. Both halves are essential, an injection without the replaster leaves hygroscopic salts in the wall that keep the surface damp.

Before any of that is specified we check what else could be responsible, because in London the answer is often bridging rather than DPC failure. Raised patios, new paths, render carried down past the DPC and rubble in a cavity are all common, and correcting them can cost far less than a full treatment.

Damp that appears or worsens after heavy rain is water crossing the external envelope. In London that usually means eroded mortar joints on a flank wall, cracked or debonded render holding water against the brickwork, perished sealant around windows, missing drip details on sills, or a blocked or split rainwater pipe soaking one narrow band of wall.

High-level and roof details deserve their own mention because London's housing is full of them: parapet walls with defective copings, hidden valley gutters between terraces, and chimney stacks with failed flashings or open flues. Each produces a distinctive internal pattern: a ceiling stain, damp at the top of a wall, or a chimney breast that is wet on one side.

We correct the defect and then repair inside. Repointing in a mortar mix matched to soft London stock brick, patching or renewing render, replacing spalled bricks, resealing openings, and stitching cracked masonry where movement has opened a route for water.

London's cellars were built for coal and storage, not for living in. Turning one into a usable room means managing water arriving under pressure from the surrounding ground, which is a job for a cementitious tanking system or a cavity drain membrane with perimeter drainage and, where gravity discharge is not possible, a sump and pump.

Which system is right depends on the structure and how wet it is. Cementitious tanking bonds to sound masonry and resists water as a barrier; a membrane system accepts that water will arrive and manages it to a drain. Both are installed as accredited systems with the junctions, penetrations and returns detailed properly, because that is where basement waterproofing fails.

What does not work is waterproof paint. It cannot resist hydrostatic pressure, and every London cellar we see with peeling coatings on the wall is evidence of that.

London property changes hands constantly, and damp is one of the most frequently flagged defects in valuations and homebuyer reports. Buyers ask for retentions, lenders ask for specialist reports, and sales stall while everyone waits for a figure.

Our free survey produces the two things needed to move on: a clear statement of what is actually causing the damp, and a written fixed-price quotation for putting it right. Where treatment goes ahead, the 10-year workmanship guarantee and the 10-year QANW insurance-backed guarantee provide the documentation buyers, surveyors and lenders look for.

If you are selling, having that in hand before marketing keeps control of the conversation. If you are buying, it tells you what the real number is rather than the worst case a survey implies.

Most of our London work is in the west and north west of the capital: Harrow, Brent, Wembley, Ealing and the surrounding boroughs, and we work more widely across London on request. Outside the capital we cover Middlesex, Watford, Slough, Buckinghamshire, High Wycombe and Surrey.

Calls are taken Monday to Friday 8am to 8pm and Saturday 8am to 1pm, and surveys where a sale, remortgage or tenancy is waiting on the answer are prioritised. Tell us your deadline when you call and we will be straight about whether we can meet it.