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Home & garden.

1 October 2021

How much does it cost to repair damp?

See the typical cost of different types of damp treatment and damp proofing so you know how much you should be paying to stop damp in its tracks.
SI
Sarah Ingrams
Pile sterling 435876

Repairing a damp problem can be expensive. Here we look at typical damp treatment and damp proofing costs plus expert advice to make sure you don't end up spending more than you need to.

Damp proofing costs can run into the thousands of pounds and be very disruptive, as the most extreme works significantly affect your home's structure. Make sure you're prepared for the potential costs – and don't overpay for work – by reading our guide below.

How much does damp proofing cost?

We've worked with the Royal Institution of Chartered Surveyors (RICS), which publishes average building work and repair costs, to bring you the average cost for a range of different damp treatments - from repairing a damp-proof course with silicone to laying an entirely new damp-proof membrane.

To give a more realistic idea of how much various treatments might cost you, we've provided prices for terraced, semi-detached and detached houses, as well as the costs for treating just one wall or one area.

Which? members can log in to see all the prices in the tables below, as well as findings from our damp investigation, which revealed bad practice from a number of specialist damp companies. 

If you're not a Which? member, try Which? to gain instant access to all the damp proofing costs.

Problem: the existing damp-proof course is below ground level

Solution: excavate as much soil as is needed to ensure the ground level is at least 15cm below the damp-proof course.

The costs below are for treatment a minimum width of one metre from the wall.

Work needed
Terraced house
Semi-detached house
Detached house
Excavate soil only - one wall
Excavate soil only - whole house
Excavate soil and lay new gravel path - one wall
Excavate soil and lay new gravel path - whole house
Excavate soil and lay concrete paving slabs - one wall
Excavate soil and lay concrete paving slabs - whole house
Excavate soil and lay 10cm-thick concrete path - one wall
Excavate soil and lay 10cm-thick concrete path - whole house

Problem: walls are damp at a low level because the damp-proof course is damaged or non-existent

Solution: create a damp barrier with silicone or insert new damp-proof course.

To add a new damp course (DPC), two courses of brickwork will be cut out and a bitumen-coated hessian damp-proof course inserted. Then the brickwork will be made good, including the mortar pointing, so that it looks like it did before. 

Injecting silicone involves drilling into the walls.

Work needed
Terraced house
Semi-detached house
Detached house
Inject silicone damp-proofing - one wall
Inject silicone damp-proofing - whole house
Insert new damp-proof course - one wall
Insert new damp-proof course - whole house

Table notes: the damp-proof course  in this case is bitumen-coated hessian.

Problem: damp is penetrating a cavity wall

Solution: clear blocked cavities.

This can be done from the outside, by cutting out three bricks, clearing the blocked cavities and any mortar droppings and then renewing the bricks so that they match the rest of the wall.

Work needed
One area
Two areas
Three areas
Five areas
Clear out cavities in isolated areas less than 3 metres above ground level
Clear out cavities in isolated areas higher than 3 metres above ground level

Problem: damp is penetrating walls at different heights, around windows and doors, and at ceiling height

Solution: insert a damp proof course in the walls at different levels to create a barrier to damp.

Options include: a cavity tray - used to stop water from the outer wall reaching the inner wall

The cost of inserting a cavity tray or damp-proof barrier includes removing bricks, replacing them and making them match the existing brickwork. 

Work needed at low level (no scaffolding)
1-metre wall
2-metre wall
3-metre wall
5-metre wall
Insert cavity tray 
Insert stepped cavity tray 
Insert new damp-proof barrier/course where there is a 'weakness' in the wall 
Work needed at high level (scaffolding)
Insert cavity tray
Insert stepped cavity tray
Insert new damp-proof barrier/course where there is a 'weakness' in the wall 

Table notes: The damp-proof barrier/course is added in the coping area. The costs for the cavity tray quoted are made from polypropylene. Where stepped, the cavity tray is made from polypropylene and lead.

Problem: damp is penetrating the walls because exterior surfaces are damaged

Solution: apply waterproof paint to exterior walls to stop damp coming through.

This can be painted onto brickwork, concrete and rendered surfaces.

Work needed
Area 1sq m
Area 2sq m
Area 3sq m
Area 5sq m
One coat of exterior paint in isolated areas less than 3 metres above ground level
One coat of exterior paint in isolated areas higher than 3 metres above ground level

Problem: internal walls are damp

Solution: 'seal' the walls (often called tanking) with damp-proofing material.

The material used to seal the walls in the prices below is asphalt, a thick liquid. The prices include removing the old plaster, clearing out the joints in the brickwork/ blockwork, applying three quotes of asphalt tanking, rendering the walls (with cement and sand), refixing or installing new skirting boards to match the existing ones and decorating the wall.

Work needed
Terraced house
Semi-detached house
Detached house
Bitumen-coated polyethylene membrane
Mastic asphalt tanking

3 x 3 metre room
4 x 4 metre room
8 x 4 metre room
Bitumen-coated polyethylene membrane
Mastic asphalt tanking

Table notes: Membrane used in this case is 500 gauge bitumen-coated polyethylene. Prices by room size are a range between price quoted for bitumen and mastic.

Problem: damp floors

Solution: 'seal' the floor (often called tanking) with damp-proofing material.

There are two options: bitumen-coated polyethylene membrane or mastic asphalt tanking. Both jobs priced below involve removing the flooring, the screed (concrete under the floor) and skirting from the room. Then either bitumen-coated polyethylene membrane and priming or two coats of asphalt tanking are laid before reinstating the screed and refixing the skirting boards.

Work needed
Terraced house
Semi-detached house
Detached house
Bitumen-coated polyethylene membrane
Mastic asphalt tanking

3 x 3 metre room
4 x 4 metre room
8 x 4 metre room
Bitumen-coated polyethylene membrane
Mastic asphalt tanking

Table notes: Membrane used in this case is 500 gauge bitumen-coated polyethylene. Prices by room size are a range between price quoted for bitumen and mastic.

Problem: kitchen or bathroom walls are damp

Solution: install an extractor fan in an external wall.

Work needed
Domestic
Commercial
Install extractor fan

Table notes: This work includes forming a hole through the cavity wall or window glass.

Damp-proofing cost data

To calculate the average prices above, RICS uses cost data from its Building Cost Information Service (BCIS) database, where costs are collated from a variety of sources and analysed.

Everything needed to carry out the work is included in the costs such as labour, materials, any preliminary work, contractor overheads and profit, and VAT.

Materials costs are based on the best trade prices from a range of suppliers across the UK, which are then benchmarked to reveal the best national average. Labour rates are based on the current Building and Allied Trades Joint Industrial Council wage agreement. RICS then uses this data in a standardised model of the average double glazed window and door types and sizes. Data copyright RICS 2020 and reproduced with permission. Data is current as of October 2020. 

Damp treatment jargon buster

We've explained some of the common damp treatment terms below, but it's also worth visiting our pages on condensation, rising damp treatments and how to get rid of penetrating damp for more details.

Word or phrase
Explanation
Damp-proof course (DPC)
A horizontal strip, often made from plastic or bitumen felts, built into the wall at the height of at least 15cm above ground level.
Damp-proof membrane (DPM)
A sheet of material, impervious to water, laid under the concrete floor. This should be connected with the damp-proof course, so that the house is effectively sealed and protected from groundwater.
Silicone damp-proof barrier
Instead of inserting a new damp-proof course, silicone is sometimes injected into a wall to create a barrier.
Cavity tray
Used in a cavity wall to drain away any water from the inner wall to the outside through 'weep' holes (holes in the outer wall). These are usually used around places where something 'interrupts' the wall, such as windows. These come in many different forms, depending on where they are in the wall, and are made of many different materials.
Tanking
Covering an entire damp area, such as a whole wall or floor, in damp-proof material (either a membrane or liquid), to effectively seal the area and protect it from moisture.

Don't pay for unnecessary damp treatment 

Damp can be a complex issue, and there can sometimes be more than one solution. The most expensive cure might not be the only option, or you may be able to do something yourself that costs very little. 

For example, condensation damp is reasonably easy to sort out and can often be dealt with by simply opening windows more often, not drying clothes indoors, turning your heating on more, fitting vents or installing bathroom and kitchen extractor fans. 

With penetrating damp, something as simple as clearing your gutters can also help.

We've heard from many Which? members who have tackled damp problems themselves, particularly condensation and some types of penetrating damp. 

One said: 'I cleared the blocked gutter myself. It was a 10-minute job.' Another said: 'It was a simple issue of a tile that had come loose and I could fix that myself.'

Rising damp can be the most expensive type of damp to fix. Fortunately, it's one of the least common kinds of damp. 

Without letting your damp problem become too serious (damp can wreak havoc on building structures), it's worth taking a little time to monitor it so you can work out what the cause might be. You could use this time to try out a few cheap or free treatments before you call in a specialist. 

Read our guides to how to stop condensation, rising damp treatments and how to get rid of penetrating damp for more details. - the more you know, the better armed you'll be if you do need to call in the professionals.

Professional damp-proofing specialists

If you do decide to call in a professional, get quotes from at least three companies (some do charge for a damp report, but you get this back if you use the company) and seek advice from an independent damp specialist if there are inconsistencies in the work that's been suggested. 

That way, you'll find out whether or not the treatment is absolutely necessary, and will be able to weed out any firms that try to make you pay over the odds.

In our undercover damp-proofing investigation (see below), we found that households could be spending hundreds of pounds on unnecessary damp-proofing treatment.

Use Which? Trusted Traders to find a reliable, reputable damp expert near you. Alternatively you can use our Trusted Trader search tool below to find a local damp specialist near you. All traders we list have been through our stringent assessment process before they can be called a Which? Trusted Trader.