Damp management and remediation

Brick replacement method: softer bricks are traditionally cut out at the damp course level, and new harder (e.g. engineering) bricks inserted as replacements. Two courses of engineering bricks are sometimes used for damp coursing both habitable and non-habitable buildings. This can be an advantageous method, as modern plastic dpcs can provide too much of a slip plane (some free-standing walls have literally been knocked or blown off their dpcs). Saw-slot method: in the past slots have been cut out at damp course bed joint level, and metal sheet dpcs installed. Today, plastic and occasionally lead sheet is more commonly the preferred dpc. Massari method: a polyester resin is introduced into the full thickness of the subject wall to form a dpc bonded to the masonry above and below. This could well be an effective method, as long as the resin damp course is not prone to cracking if the wall settles or subsides. Physical damp course insertion - saw slot method: if a building is detached and needs a dpc to control or stop dampness above ground level, a physical dpc is probably the most suitable option. Most people feel reassured seeing evidence of a physical dpc in a wall, such as a line of plastic or lead edging. Physical insertion of a dpc is often given a rather negative vote of confidence in the industry. It is usually deemed to be too expensive and disruptive.

A 1962 edition of BRE Digest 245 devoted several pages to physical dpc insertion. Later editions of BRE Digest 245 (1981 and 2007) contained less information on physical dpc insertion, although their value was emphasised, for example:

'The only certain way in all circumstances of introducing an effective dpc into a wall is to insert a new physical membrane. Techniques for doing so have improved over recent years and the cutting of an old lime/sand mortar bed joint through the entire thickness of a 225mm (9 inch) solid wall should, in most cases, present no problems.'

In the 1981 digest, chemical injection was termed a non-traditional approach, whereas physical dpc insertion was considered traditional. The chemical dpc option appears to be more forcefully promoted than in earlier digests, for several reasons:

• Chemical injection is advised when physical insertion is not possible (e.g. to random coursed masonry).
• Thick masonry walls were considered too difficult to physically damp course.
• Agreement certificates were by then available for tested chemical dpc products.
• The presence of settlement cracks in structures could mean it was dangerous to attempt physical dpc insertion.

However, BRE 245 further added the point that:

'The advantage of the physical techniques is that the membrane can be extended internally to form a vertical dpc between any solid floors and the horizontal course.'

Cutting methods have developed from the early days in the 1950s, when circular and hand-held saws were used. Disc cutters have been used incorporating dust extraction, but caused problems if the masonry was excessively damp, clogging the vacuum bag. The efficiency of hand-held power chain saws improved during the 1970s, making physical dpc installation more competitive.

Nowadays, low-density polythene is commonly used for the dpc, a distinct improvement from the 24 or 26 SWG copper that was once used. The polythene can either be linked to a solid floor dpm or, for a timber-suspended floor, simply tucked down between walling and floorboards.

As equipment for cutting slots in masonry walls improves, greater speed and ease of installing a physical retrofit dpc must surely tip the balance more towards this method. One of the biggest problems, however, lies in the lack of expertise in the UK in installing physical dpcs.

Advantages and disadvantages of physical dpc insertion Advantages: Easier to link with other physical damp courses.  Suited to coursed brick/stonework. Only damaged/defective internal plaster needs renewing. Uses more traditional building skills. Can be visually inspected (but not entirely). Can be expensive to install, but a long-term remedy. Backed by BRE. Method can be used to repair a section of existing dpc visibly. The physical damp course can be linked to floor dpms by simple lapping. Materials used for the damp course will usually have a long expected life. Specialised cutting machines can be used to cut slots in thin jointed masonry – even when engineering bricks are present. The plastic damp course material is available in 900mm width rolls, making it versatile and able to be installed to masonry of changing alignment and thickness. Masonry walls up to 700mm thick are commonly (and successfully) physically damp coursed. Once installed, the dpc should last for years, making it unlikely that a later reinstallation of a failed retrofit dpc will be necessary. If hard and stubborn plasters and renders are retained, there will not then be a need for potentially damaging hacking off. No need to introduce large volumes of water-based treatments into the masonry. Suitable for historic framed buildings, where an effective physical damp course is needed under the timber cill plate, typically on a dwarf support wall. The physical damp course material is manufactured to British Standards. There are less likely to be problems of split responsibility, as a physical dpc does not rely on specialist replastering (especially by another contractor) to work. Advantageous for walls that do not contain internal plaster or where walls will be stripped and left as bare brickwork.

Figure 1: A 1 B brick wall shown with a new physical dpc, which links to an existing solid floor dpm. The floor screed has been renewed along the wall perimeter to help achieve a reliable dpc/dpm linkage

Figure 2: Plastic sheet physical dpc insertion with a solid floor. In (A) the new dpc is tucked well down the wall, before a new concrete slab is laid with its own dpm lapping up over the wall dpc. In (B), we see a shorter downward lap, where there is an existing concrete slab, with its visqueen membrane lapped up over the new dpc, and a new finishing screed laid after. Sketches courtesy of Dampcoursing Ltd

Disadvantages Physical dpc insertion costs more than chemical injection per metre run of wall. But chemical injection dpcs become less competitive when both sides of a wall need to be plastered. There are potential structural risks, e.g. of wall settlement, as a mortar course is cut out in stages. Due to the staged insertion there will be numerous laps in the damp course. Cutting a mortar joint could damage services. Access is usually required to both sides of a wall. Additional means of damp coursing will be required if the property adjoins other property (i.e. there are party walls) and where vertical damp coursing is needed to proof below-ground sections of walling. There can be a lack of local specialist installers. Best suited to detached properties, or needs to be installed in conjunction with other less reliable damp coursing methods. Unsuited to some hard walling materials (e.g. flint or granite, which damage the chain cutters). Cannot be installed in extra thick walls or to irregular walls (e.g. rubble filled).

Figure 3: A dedicated power saw with tungsten carbide toothed chain cuts a chase to the third brickwork bed joint. Photo courtesy Dampcoursing Ltd

Figure 4: A low-density polythene damp course is carefully inserted into the bed joint following removal of debris by a clearing hand saw. The membrane, installed in lengths lapped 150mm, is soon wedged up using slate slips and subsequently pointed up. Photo courtesy Dampcoursing Ltd

Stages: physical damp course insertion Cut chase, usually in 1m lengths, by tungsten carbide chain saw. Clear out debris, etc. using a clearing hand saw. Cut membrane to size – commonly low density polythene. Insert damp course sheeting. Fill void above membrane with cement/soft sand 1:3 or 4. Plug chase on each side of wall using slate slips. Point up as far as 150mm from end of membrane section (to allow lapped joint). Proceed to work on next bay. Bays may be staggered in the same way as underpinning bays, to ensure the wall is at all times adequately and safely supported. Make good defective plasters after dpc insertion.

Note that cutting usually begins to the wall's outside face, so a neat joint can be cut and later pointed up – with less likelihood of damage from cutting from the inside. The cutting can negotiate rainwater pipes, etc. Skirting boards are usually taken off and refixed on completion.

Only salt-damaged plaster is hacked off as a matter of course. Some making good of plaster might be required once skirtings are refixed. It is sometimes possible to remove plinths to kitchen base units, allowing just enough access for internal making good, saving the need to refit the kitchen.

General comments on the physical dpc insertion method

An extremely effective barrier to upward movement of moisture is created if the dpc membrane is inserted carefully. The operation is not as expensive as many believe, although the actual damp course insertion is a fairly slow and labour-intensive method (i.e. around 10m per day for a 225mm thick wall by 2 operatives) and therefore can be expensive.

Only small patches of wall plaster are normally removed as there should be no need for an internal specialist plaster masking system, unless there is a significant lateral damp penetration to address. The physical dpc does its job reliably. If movement of moisture is stopped, transportation of salts stops once water already above the dpc has found its way out of the wall by natural evaporation and moisture transfer within the solid material. The drying out of the wall can be monitored using modern moisture measuring equipment. In some cases, salt at the plaster surface could simply be scraped/washed off, or the salts neutralised prior to redecoration.

Firms that undertake physical dpc insertion may also install chemical dpcs. This may be because some walls can be difficult to damp course physically (e.g. a party wall, where access is usually required to both sides of the wall during physical dpc insertion). There are ways to insert a physical damp course from 1 side only, but a contractor would often chemically inject a party wall (although remember that a party wall agreement may need to be obtained first).

Physical insertion may be a suitable technique for historic buildings. In a timber framed building, for example, a physical dpc can be inserted under a sole plate. This means a highly effective dpc will protect that timber member from further rotting from underneath. Internally, important historic fabric, such as wall plasters or timber panelling, can remain in situ where physical damp course insertion does not require any accompanying specialist plastering.

There are some great opportunities for dpm linkage with physical insertion, as the new dpc can be cut extra wide so that a substantial flap remains available for linking to; for example, a floor dpm. This option is not available with a chemical dpc insertion. If a wall is to be left unfinished, and certainly unplastered, chemical injection would be a risk! Without the second line of defence (i.e. the plaster) the chemical dpc may not be adequate on its own.