Dampness in building elements

General notes for surveyors

General Principles

1. The ‘damp-proofing’ may not have been necessary anyway.
2. You will not know how damp the wall was prior to damp proofing, unless you had taken drilled samples to determine moisture content.
3. The injected damp-proof course will probably be controlling dampness behind a hard plaster shield, which would now be damaged by any taking of drilled samples.
4. You will more often than not be commissioned to survey cases where damp-proofing plasterwork appears to have failed.
5. Damp-proofing plaster is usually 1-1.2 metres high, but sometimes full height.
6. The extent of damp-proofing plastering and injecting as shown on a works quote may not reflect the actual extent of work carried out. But a building never lies.
7. The masonry behind the plaster may be dry, damp or wet. Remember that a chemical injection only controls damp.
8. The wetter the masonry behind the plasterwork, the higher the standard and detailing of plasterwork needed to mask the underlying damp.

Defects

Any darker areas of plaster/décor, staining or blemishes will usually mean there is a damp problem.

White fluffy depositions on the surface will usually be sulphates from drying, and can be cleaned off. Sulphates can be from within the plaster or brick. They are not usually a long-term problem, and can be cleaned off.

If the plaster has dried, you would not expect high damp meter readings in the skim. High capacitance meter readings could mean the wall is wet behind the plaster, but may not mean the plaster masking has failed.

If pin probe readings are low, and there are no damp stains, the masking plaster may be (at least for the time being) doing its job.

High pin probe readings will usually mean there is a damp problem - either the masking plaster has failed to mask damp from the masonry behind, or there is dampness sourced on the room side of the wall, for example a condensation problem.

Where there are ground salts collecting - i.e. salt damp - damp meter pin probe readings will usually be at 100R/R maximum and a plaster scrape will test positive for ground salt. The masking plasterwork will inevitably have failed, if subject to salt damp. If ‘salt damp’, expect to find discoloured decorations, plaster blisters, flaking paint. If the plaster is as yet undecorated, you can sometimes find a peppering of pink plaster skim on the tops of skirtings. This is gypsum skim that has been forced off when salts have crystallised near the plaster surface.

Condensation is usually announced by mould, which tends to colonise at room corners, behind furniture and at cold bridges. The Protimeter MMS has accessories to check for live condensation.

If you see wall stains/blemishes at the top line of a protective damp-proofing plaster, it is likely that dampness in the masonry is exiting at the junction of the new damp-proofing and original plaster systems. The damp-proofing plaster was probably not applied high enough up the wall to mask dampness behind.

Hard renders on both sides of a solid masonry wall encourages dampness to quite a height, quite often up to around 1.5-1.75m - and especially when there is no physical damp-proof course, hard landscaping and a badly drained plot. Much higher, and first-floor joists will be threatened.

If the masonry substrate was not reasonably flat before plastering, there may be positions of very thin or very thick plaster. Very thin masking plaster can allow dampness through to the decorative finish.

Look for damp patches near door frames. Ideally the masking plaster would have been carried around door openings with the door frames taken off, to achieve continuation. Also look for damp patches at low level above skirtings, where the skim plaster is soaking up damp off damp masonry or floors.

One of the disadvantages of only plastering to part height, is that the plasterer, in attempting to merge in new to old plaster and produce a very neat plaster junction, may often apply too thin a render coat, to match up to thin existing coats.

Sometimes joinery fixings or services entry positions provide a route through for dampness from within the masonry. In basements, where a wet masonry substrate is expected, joinery is often glued rather than mechanically fixed to walls.

Check the standard of finish achieved, using a plasterer’s metal straight edge (e.g. 2m) and a graduated wedge that can be slipped between the straight edge and plaster wherever there is a gap. The straight edge will rock on a humped zone of plaster.

Tap plasterwork too, and you may find off-key zones where plasterwork has probably not been carefully applied to a correctly prepared substrate. Scrape skim off and tap again to find out if the backing plaster is also off-key.

Cracks usually indicate a significant plaster failure. Are the cracks in the skim plaster, or more seriously following through into the backing renders?

Check all external plaster angles. You will often find rusting corner bead, where galvanised metal beads have been used instead of stainless steel. Often the slimmer plasterboard angle beads are used as render corner beads.

Plaster corner beads are often secured in place during the plastering by soft sticky bonding plasters, e.g. carlite bonding. This plaster is absorbent and will suck up dampness from wet masonry, causing corner damp patches.

A word of caution

If damp-proofing plaster is deemed to have significantly failed, the selected remedy is often to hack off and replaster. This will cause further damage to any fragile substrates, particularly at reveals and corners.

Before automatically hacking off and replastering to a similar specification, it might be worthwhile carrying out a thorough dampness investigation, to establish why the subject wall is still damp, and if anything else could be done to reduce this dampness.

If a general builder has executed plasterwork that has failed, you will not find it easy to convince the builder to hack off and replaster at his/her expense. The plasterwork will probably not have a valid guarantee. And everybody will blame everybody else. It’s always best to instruct the damp-proofer to execute the chemical injection and the replastering, plus fitting skirting boards and joinery – to create a single point of responsibility for any claims.

Picture examples of (mainly) damp-proofing plaster failure

Plaster failure, Example 1

Figure 1: Note the proliferation of hygroscopic salts puffing out just above skirting level. The wall has been damp-proofed, but the absorbent skim finish plaster had been taken down off the cementitious render onto damp brickwork. Dampness from the masonry soaks upwards via the soft skim, taking with it ground salts in solution. The skirting board is deteriorating.

See figure 2. The minimum remedy would be taking off the skirting, re-skimming, and renewing the skirting board - ideally glue-fixing it.

Figure 2: Good and bad skirting detailing.

In figure 2, on the left side of the wall, the plaster skim continues down on to the brick substrate. The brickwork near the solid floor may often be very damp or wet whether the brickwork has been damp-proofed or not. Moisture tracks upwards in the gypsum plaster skim, and with it very often nitrate and chloride salts in solution. A ferrous fixing brad will usually deteriorate and fail because of the damp substrate. Many believe that as long as you trim the wall plaster just above the solid floor, all will be well, but this might not be so! The skim plaster is the big issue, the likely bridge. The lower courses of masonry may be very wet and be a real damp threat, as you can see from this detail - developed after dozens of inspections of real wall detailing.

On the right, the preferred option. The plaster skim terminates just below the top line of the skirting board, well up from the potentially wet brickwork. The skirting board has been fixed to the render by adhesives, isolating it from the wall and reducing potential contact of timber to damp masonry. The skirting has also been fixed a fraction above the solid floor - thereby also eliminating the risk of dampness from the solid floor. Some damp-proofers actually cut back the base of the skirting at 45 degrees. A solid floor, even if not damp from rising moisture, can be repeatedly wetted from the floor cleaning cycle. The right side skirting is fitted with a planted staff bead to improve appearance. The band of potentially damp masonry behind the skirting can be damp-proofed using a suitable coating, otherwise there could be a very humid zone behind the skirting. Some damp-proofers take the waterproof render down to the floor – not likely to be a problem if it’s a waterproof mix, but the skim must terminate well up from the floor.

Plaster failure Example 2

Figure 3: East Yorkshire farmhouse plasterwork.

New plasterwork was badly stained and damaged in an East Yorkshire farmhouse hallway (figure 3). The single power socket exhibits a burn mark, probably caused by an electrical short circuit due to moisture present. The metal back box behind the plastic faceplate is probably rusting away. Wet walls and electric fittings do not mix - electrical safety is compromised. The metal back box will have been first screwed to the masonry substrate, usually in a recessed cut out, then plastered around, so its position is a discontinuation for the plaster shield. If cables or pipes are fixed over the substrate, any solid plaster masking applied over them will have less thickness and therefore moisture resistance where it passes over them. So before application of damp proofing or waterproofing plasters, consider alternative routes for pipes or cables, chase them into the substrate, or surface run your services using glued clips.

The metal corner bead is rusting, probably held in position during plastering by soft gypsum bonding plaster. Hygroscopic ground salts are present at the plaster surface meaning the plaster has failed to mask wall damp. This plaster was applied by a general builder using unsuitable sands.

Plaster failure, Example 3

Figure 4: Plaster failure near a radiator.

In the same property as Example 2, near a radiator was another clear case of plaster failure (figure 4). It is very likely that at the epicentre of failure there is either an inadequate cover of protective plaster fronting a very damp wall, or even a poor plaster mix, where there could even be some larger aggregates present to compromise the effectiveness of the waterproof plaster. A scrape of plaster is tested and contains ground salts. At this position, for whatever reason, the protective masking plaster has failed.

Plaster failure, Example 4

Figure 5: Rising damp rears its ugly head!

Damp-proofing plaster here (figure 5) is only to a height of 600mm, and rather predictably a band of ground salts is confirmed just above. This plastering would probably need hacking off and re-executing to a greater height.

Plaster failure, Example 5

Figure 6: Unsuitable post-flood restoration work.

This Great Yarmouth house had suffered floodwater around 300mm deep over the whole of the ground floor. To dry the property, existing hard render was hacked off to around 1.2m height. The house was dried, then replastered, but using soft renovating plaster. Soon after reoccupation the plaster began to darken and develop moulds, as you see in figure 6. Nobody realised that before the flood, hard cementitious plasters were masking a long-term wallbase damp problem. So the long-standing damp re-emerges, to ruin the new absorbent plasterwork, which I advised should now be completely hacked off for replacement with either a solid waterproof render system or plastered membranes (see the How to outflank damp case study). Some years ago the original brickwork of this and neighbouring properties, old and without reliable physical damp-proof courses, was cement rendered over ‘ trapping dampness at the wallbases. A quickfix remedy with long-term repercussions.

Figure 7: Good and bad detailing of as-found newtonite lathing in a Charlton house. Probably 30–40 years of service so far, but now replaced.

In figure 7, on the right, wall plaster has been taken down onto a damp wall from the safe haven of a newtonite wall lining. Damp can now track from the damp wall upwards, to dampen the plaster above and also the skirting board. High pin probe damp readings were obtained for 500mm up on the plaster.

On the left, on another wall, where plaster on the newtonite stopped short of the bottom edge of the liner, as it should, plasterwork made no contact with the damp wall. Pin probe damp meter readings in this case were in the ‘dry’ range, on the skirting and low-level plaster.

Figure 8: A piece of the original newtonite lathing.

The newtonite lathing (figure 8) is a wonderful product, which, when tacked to a damp wall, creates an air gap behind because of the corrugations.

I actually stripped the old newtonite, and replaced it with its modern Newlath 2000 cousin, the new product solid plastered over with a cement/lime/sand mix and with skirtings glued on.