Dampness in building elements

Figure 1: Not the kind of property you would expect to have wall base damp problems. Surely they knew how to install a dpc 30 years ago?

You will know about ‘bridging’ of cavity walls by wall ties that hold blobs of fallen mortar from careless bricklaying, and may have heard of or even seen examples of lintel problems, when metal cavity wall lintels collect water from rain penetration, and allow it to track sideways towards the lintel ends, to cause damp patches either side of a window head.

But the Billericay defect you are about to savour has not yet received the attention it deserves. Perhaps it is too often mistakenly labelled as ‘rising damp’, with the remedy an internal masking of dampness by waterproof plasters. Once masking of dampness is planned, there will be less effort to fully understand where the dampness came from and how it got where it did.

The aim of this case study is 3-fold:

1. to explain what symptoms low-level cavity wall bridging could present in a house survey;
2. to assess how the dampness got to where it did; and then the tricky bit ...
3. to devise steps to remedy the damp problem.

The wall base damp we describe in this case study was not flagged up by the homebuyer surveyor, who surely noticed the marginally high damp meter readings the author picked up. This was partly because ground floor rooms were nicely decorated, and the surveyor may have been reluctant to use the damp meter pins because the house owner was present. The surveyor may also have failed to realise the significance of marginally high damp meter readings. If you are not allowed to use damp meter pins as much as you’d wish, make sure you explain in any survey report the implications of not being allowed to do so. If damp meter readings are even just a little higher in one part of a building, or maybe higher up or lower down on a wall, you need to find out why – or recommend ‘further investigation’. The homebuyer surveyor paid the author's fee for the dampness investigation.

At Billericay, the damp problem first came to the attention of the owner some months after moving in, when skirting boarding was removed during alteration works. The owner was surprised to find rot and insect damage on the back of the skirting. Various remedial treatment companies came to quote for work. Remedies varied in scope, and the owner became confused. An independent investigation was needed to clarify the cause of the damp problem and how best to deal with it.

The author’s subsequent diagnosis was very much in line with 2 of the 3 remedial treatment companies - but the approaches to remedy diverged somewhat.

Contractors tend to look for substantiation to install the remedy they are geared up to provide. So never ask a damp-proofer to inspect a leaking gutter because you may receive a quotation to chemically inject it. Often the route is to mask dampness, rather than carry out external building work, hard landscaping, etc. for a longer-term cure. Masking dampness often involves a new plaster system that can quickly become dry and be decorated. This may, of course, be a suitable approach on some occasions – but not as a matter of course.

Symptoms presented inside and outside the building

Internally, the walls and joinery are visually perfect, so be on your guard. Even in a thorough dampness investigation, the author was finding it difficult to take as many damp meter readings on walls and skirtings as he would have liked: the new owner was watching closely, wincing as metal prongs pricked the pristine walls. The skirtings were chamfered and offered no useful concealed rebate to insert the pins.

EMM readings were noted on a floor plan to assess any patterns. Pin probe readings recorded to timber skirtings clustered in the mid-20% range – sometimes as high as 30% – along both external and some internal solid walls. Wall plasters recorded pin probe readings again of a maximum of 30R/R, but the readings tailed off to about 12–16 at a height of 300mm above floor level. This damp reading profile was consistent along the walls, but there was no evidence of plaster deterioration, e.g. salt damp. If salt dampness was present, moisture would be derived mainly from below ground, and we would expect to find much higher damp meter readings, often 100R/R and salty stains, blistering plasters, etc.

The readings were indicative of walls and skirtings just damp enough to deteriorate slowly over time. The flank was probably suffering the most dampness – it was along this wall that the skirting degradation was found by the home owner.

Figure 2: Where the wall base seemed most affected by lateral low-level damp penetration, along the north flank, a section of skirting was taken off and was clearly damaged by wood boring weevil. Much of the skirting in the house may well have been a little damp – but probably not quite damp enough to have succumbed to this particular insect’s tunnel boring. Typical weevil damage is a shredding effect of the timber along the grain

In the rear dining room, the 2 remedial treatment companies had carried out some destructive examination – a small section of masonry had been removed from the base of the internal cavity wall skin in 2 positions, and some floor screeding taken up, and interestingly a hole broken through the concrete floor slab. A thin dpm was found at the base of the floor inspection hole.

It was possible to see a dark and damp mortar bed between the dpm and dpc of the inner cavity skin. See figures 3 and 4. The damp mortar bed later carbide tested at 6.5% AMC – very damp!

Figure 3: Billericay 1970s cavity wall – as found. 1972 Building Regulations stipulated that the cavity should extend to not less than 150mm below the level of the lower dpc. At Billericay the cavity wall infill extends upwards too far – creating the potential for accumulated debris in the cavity to create a damp bridge

Figure 4: Billericay cavity wall

The moisture route in:
A - rainwater collects on pavings near wall;
B - it makes contact with low-level brickwork or trickles between paving and masonry;
C - moisture soaks through outer skin brickwork or mortar bed;
D - moisture pools in cavity or soaks across build-up of mortar debris;
E - moisture soaks across sandy mortar bed above dpm, below dpc through thickness of inner skin, some moisture soaks through brickwork below dpm, probably mostly stopped by dpm except where dpm trimmed short;
F - moisture soaks up in sandy internal render, and into skirting board, as well as into screed edge.

The cavity was viewed via the 2 brick openings – and it was possible to actually see wet mortar debris at the base of the cavity wall.

On the morning of the survey it had rained, and the owner said that the mortar bed had definitely darkened after the rain. Low-level damp penetration was quite clearly at least one of the sources of moisture causing a damp problem. Was it the lead source?

The wall plaster had been originally applied down to the concrete slab – therefore the plastering had been carried out before floor screeding.

Externally, it was immediately obvious that the base of the cavity wall was subject to damp penetration. The dpc was a mere 60mm above paving level along one wall run, with the average perhaps 1 brick course. The wall base appeared darkened from direct damp penetration, with the flank wall showing also classic green staining up to 3 courses up the external cavity skin.

Horizontal damp courses can control or stop dampness passing them vertically, but cannot prevent dampness tracking through the wall laterally below or above them.

The fact that the cavity wall dpc was situated less than the Building Regulations minimum of 150mm above ground level, may not be significant in terms of the resistance of the wall base to low-level damp penetration, but could mean the wall base is subject to rainsplash above the dpc. It will also usually mean that there is an unsatisfactory relationship between level of external finished yard level and cavity wall detailing at low level. When yard levels are too high, dampness that does penetrate the wall base can cause more problems once in the cavity zone. Bricks splashed by rain often suffer more frost damage directly below the outer skin dpc, as water soaking up brickwork stops at the dpc level.

Pavings were levelled on ‘dabs’, so joints would be likely to be open and water would be able to collect underneath. This in effect creates an external perimeter reservoir of water that could feed into an unprotected wallbase.

Note how the cavity fill had been terminated only 1 course below the dpc level. Mortar droppings could therefore easily provide a bridge for wetness to soak upwards, and in this case directly to a soft and sandy mortar course between floor dpm and inner skin dpc.

Dampness could also soak through the brickwork just below the floor dpm, as the polythene dpm was trimmed short of the cavity face of the inner cavity skin.

While tamping concrete, probably with a long timber board, the plastic floor dpm would have probably been holed and chafed along the sharp brick arris at ‘X’ – so wetness in the brickwork could easily soak through to the soft sandy mortar bed above the dpm.

Imagine the outer skin dpc a course higher – would this affect lateral damp penetration? Probably not. But once inside the cavity, with dpcs only a course above the wetted-up cavity zone, moisture has but a short distance to soak up and cause a lot of problems.

Remedy

Efforts to isolate plasters, skirtings or screed from a wet mortar bed joint above the dpm are management.

Efforts to clear the cavity of debris, which would certainly help reduce or completely stop moisture tracking through to the internal plasters, screed and skirting – are again really management. The cavity debris has only become a route for moisture because of unsatisfactory paving installations that feed wetness into the wall. If we cleared the cavity, the wall base would still become wet within the cavity from low-level damp penetration. Low-level masonry would be wet from adjacent surface water, and masonry would be likely to suffer frost damage up to dpc height. So masking would allow some of the adverse damp effects to continue unabated.

A cure is a more fundamental remedy, and in this case would address the poor relationship between external pavings and the cavity wall base detailing. If pavings were lowered sufficiently, the cavity wall would take in less surface water. Just removing pavings and creating flower borders would help, as rainwater would percolate down into the soil rather than collect, pool and splash against low level brickwork. Later we show how, instead of lowering pavings wholesale, we might just locally lower external yards by forming a drainage channel. Forming drain channels is not uncommon, but such remedies for damp have until now not been fully explained in published works.

A cure should be more successful over the longer term. It not only creates dry conditions at the decorative internal surface, but helps the building element return to a comfortably dry state through its thickness. Wet building elements can be thermally inefficient and deteriorate unseen within.

The downside of a cure can be that once you have stopped the moisture at source, time is usually needed for the cure to take effect and dry out the decorative plasters and decorations. Monitoring skills are needed to check the drying phase. Many clients find this inconvenient and so opt for the quicker fix – the masking option. Masking dampness and wetness is often the only option for a basement – when you simply can’t get to the outer face of the building elements because you are underground.

A, B, C and E are the author’s own remedy formulations – based on sound construction common sense. Remedy D is based on a quotation by a remedial damp coursing contractor.

Some remedial treatment contractors, faced with a dampish skirting and wall, would recommend standard chemical dpc injection plus specialist replastering to 1m height.

But there would be little point in installing an additional horizontal damp course if the existing damp course was working. In the author’s opinion, dampness at Billericay was simply tracking laterally via a bridged cavity and soft mortar joint.

Equally there was no sense hacking off sound plaster. There can be a case for hacking off badly salt-damaged plaster, but at Billericay we found just dampish plasters and no evidence whatsoever of salt damp or any physical damage to plasters at all. (Interestingly, one remedial treatment contractor quoting for a remedy for Billericay damp continued to refer to the damp in the wall as ‘structural damp’ – possibly in an attempt to make a stronger case for taking off the wall plaster.)

Not all skirtings would need to be taken off for renewal or refixing. Site inspections would be needed with an independent surveyor present, to lever off skirtings to verify which skirting runs have degraded from contact with damp walls. Skirtings were really only marginally damp, and may have been caught before rot had taken hold.

So far at Billericay, skirtings had only been found defective along the wet flank wall, especially at the rear flank corner, where the cavity was subject to high paving levels coupled with a cavity filled high with debris.

Remedy option A: management

Option A – skirting/plaster alteration

• The skirting has been taken off.
• Internal wall plaster is trimmed to 25mm above screed level.
• A new, larger skirting is fitted, protected behind by a plastic lining that tucks forward under the skirting.
  You could arguably get away with just fixing a skirting directly against the plaster, as without being fed from the damp mortar bed, the internal plasters should dry out relatively soon. Alternatively, skirtings could be left off for a few weeks and refixed when plasters have dried down sufficiently. It is a question of time, and most home owners want their property brought back to normal as soon as possible.
• The skirting is either glued to the wall or fixed using masonry plugs sealed with mastic.
• The mortar bed remains wet, but makes no contact with plaster or skirting.
• The second phase of remediation would be external alteration to pavings to eliminate damp penetration at source (cure) or clearing of the cavity to reduce damp bridging.

Verdict: The wall plaster should gradually dry out, and the skirting will be dry from fixing. This remedy involves the isolating of plasters and joinery from a wet mortar bed. It is really no more than a masking of a damp problem, but would allow internal refurbishment to continue until funds and time are available to make longer-term external alterations.

Likely cost: £3,000 basic, plus removal of fittings, cupboards, etc. for access, refitting of carpet and redecorations.

Figure: 5: Option A

Remedy option B: management

Option B – skirting/plaster/screed alteration

• The skirting has been taken off.
• Internal wall plaster is trimmed to 25mm above screed level.
• A new, larger skirting is fitted, protected behind by a plastic lining that tucks out under the skirting. The 75mm chamfered skirting can be replaced by a similar 100mm high moulding.
• The skirting is either glued to the wall or fixed using masonry plugs sealed with mastic.
• A mastic sealant would be applied after the soft wall plaster has been grubbed out. (Note that the soft sandy original wall plaster had been applied down to the concrete slab, prior to the screed being laid.)
• The mortar bed remains wet, but makes no contact with plaster, screed or skirting.
• The second phase of remediation would be external alteration to pavings to eliminate damp penetration at source (cure) or clearing of the cavity to reduce damp bridging.

Verdict: The wall plaster should gradually dry out, and the skirting will be dry from fixing. The existing screed would then be isolated from the wet mortar bed, and if damp, should now dry out. Again, this is no more than a masking of a damp problem but would allow internal refurbishment to continue until funds and time are available to make the longer term external alterations. There would be little internal dust and disruption.

Likely cost: £4,500 basic, plus removal of fittings, cupboards, etc. for access, refitting of carpet and redecorations.

Figure: 6: Option B

Remedy option C: management

Option C – skirting/plaster/screed alteration

• The skirting has been taken off.
• Internal wall plaster is trimmed to 25mm above screed level.
• A new, larger skirting is fitted, protected behind by a plastic lining that tucks out under the skirting.
• The skirting is either glued to the wall or fixed using masonry plugs sealed with mastic.
• The mortar bed remains wet, but makes no contact with plaster or skirting.
• A band of screed has been grubbed up, and new waterproofed screed laid, protected from the damp mortar bed by a simple strip of polythene, e.g. Visqueen 1200 gauge dpm material. Even without waterproofer, isolating the screed should suffice if the new screed is a strong sharp sand:cement mix.
• The second phase of remediation will be external alteration to pavings to eliminate damp penetration at source (cure) or clearing of the cavity to reduce damp bridging.

Verdict: The wall plaster should gradually dry out, and the skirting will be dry from fixing. Yet again, this is no more than a masking of a damp problem but would allow internal refurbishment to continue until funds and time are available to make the longer term external alterations.

Grubbing up and rescreeding would be a dusty disruptive job internally with questionable value in solving the damp problem.

Likely cost: £6,000 basic, plus removal of fittings, cupboards, etc. for access, refitting of carpet and redecorations.

Figure: 7: Option C

Remedy option D: management

Option D – skirting/plaster/screed alteration

• The skirting has been taken off.
• Internal wall plaster is trimmed to 25mm above screed level.
• A new, larger skirting is fitted, protected behind by a plastic lining that tucks out under the skirting.
• The skirting is either glued to the wall or fixed using masonry plugs sealed with mastic.
• The mortar bed remains wet, but makes no contact with plaster or skirting.
• A new physical plastic sheet dpc would be fitted to both cavity wall skins. The inner skin dpc is a wide sheet that extends under a 300mm band of new floor screeding.
• The dpc is fitted in 1m lengths, wedged in position with slate slips and pointed up. The operation has similarities with foundation under- pinning procedures.
• The cavity wall would be cleared.
• Remedy D was submitted by a damp-coursing contractor. The intention was to remedy the damp without carrying out any alteration to pavings externally. Lowering external pavings was mentioned as an ‘ideal’ but not mandatory work operation.
• The remedial treatment company also offered to insert a new higher dpc to the external cavity wall skin – so the correct minimum height of 150mm from pavings to dpc would be achieved – but 2 new dpcs seems like overkill as even 1 new dpc surely isn’t needed

Verdict: The wall plaster should gradually dry out, and the skirting will be dry from fixing. This is fundamentally a masking of dampness, although clearing the cavity of debris goes some way to achieving a ‘cure’. However, any surveyor inspecting the house would still be faced with poor detailing to note around the building perimeter, even if there are no significant damp readings to record internally.

This would be rather a dirty and disruptive remedy option, involving wet trades and much hacking up of floor screeds internally.

In the longer term interests of the building work would be needed to resolve the high pavings around the building.

Likely cost: £11,000 basic, plus removal of fittings, cupboards, etc. for access, refitting of carpet and redecorations.

A second remedial treatment contractor also quoted for a remedy but did not produce any reproducible sketch details.

Figure: 8: Option D

Remedy option E: cure (or probably as close to a ‘cure’ as possible)

It would be impossible to produce the correct relationship between the top of the below ground cavity fill and the dpc heights. However, it would be possible to lower paving levels or produce a drained channel detail around the building linked to the underground surface water drainage system.

• Note: There is a case for infilling the screed/wall junction, as the groove could collect debris to form a ‘bridge’.
• A – Plastic pre-formed drain channel runs along most of the house perimeter to falls, connected to a water trapped gully and then to the underground drain system.
• The channel is bedded on concrete – you can use wooden pegs to level down to, as well as to a tight bricklayer’s line.
• To improve appearance, consider laying a bed of rounded pebbles (not pea shingle). See how far down water collects below the vulnerable sandy mortar bed.
• B – Make sure the end of cut or relocated paving is solid bedded, with a chamfer of mortar or fine concrete to direct water into the channel.
• C – Apply a waterproof cement plinth chamfered at the top edge. The gap between channel and house wall would be solid filled with mortar packing. We have created an L-shaped wall base protection system (see Glossary).
• D – Refix skirtings after taking off and trimming plaster from the screed. Renew the skirting to most damp-affected walls, or even install a mastic joint at the screed edge, so drying out of plaster and screed can begin at once.
• E – Targeted cavity clearing. Identify cavity debris using a borescope. The debris will probably no longer be able to even act as a bridge once the channel is in place to efficiently direct surface water downwards and quickly away to the drainage system.

Likely cost: £8,000 basic, plus removal of fittings, cupboards, etc. for access, refitting of carpet and redecorations.

Figure: 9: Option E – new external channel drain proposal

Verdict

In the author’s opinion, option E was the best practical option for the client and building.

The house would look perfectly satisfactory externally and internally when surveyed.

We could speed up the drying using targeted professional drying techniques (e.g. forcing dry warm air into the lower cavity zone).

The cavity should acceptably dry down naturally, as surface water cannot impinge on it significantly from paving run-off or splash. The author would expect the plasters internally to take 6–9 months to dry satisfactorily on their own. Drying could also be accelerated if skirting boards were fitted some time after the external works had been completed, as the wall plaster can then evaporate out moisture where it is not painted. And again, air movers could speed up drying if needed.

It is not critical whether the pavings slope towards or away from the channel, but away from the channel is preferred. The new channel would be there mainly to protect the wall base from seeping water and rainsplash, rather than as a means of collecting surface water from the whole patio or paved zone.

Lowering pavings wholesale (perhaps the best technical option for the building, over the longer term) would have involved works completely beyond the client’s budget, but forming the channel was viable. And remember, the option to install new physical damp courses required funds in excess of £10,000.

By dealing with the damp mainly externally, and with no wet trades undertaken internally, the client could be saved considerable unnecessary disruption to living areas.

Lessons learned

• Flag up damp meter readings that are a little higher in 1 zone than another – even if you cannot see deterioration of walls, floors, timber, etc. directly. Protimeter rightly advise that further investigation is advised when damp readings are in their risk (amber) zone.
• Be methodical in taking moisture meter readings. Always take a reading away from a damp zone – a ‘control’ reading.
• Note high yard levels around buildings as a likely scenario for low level dampness. The least you should do is compare yard level to dpc level, and compare yard level externally to internal floor level.
• If you are dissuaded from using your moisture meter pins, note this in your survey report, and state the risk and the implications of not carrying out a full survey. If you have not been allowed to use the damp meter consider advising ‘further investigation’.
• Look carefully at the construction detailing. Draw up a proportional sketch from site dimensions taken. Work out what is wrong with it, assess the moisture pathways.
• If you can, check out moisture content of building finishes and materials at depth.
• If external levels are too high, you can either spend money internally or externally to remedy any resulting dampness. In the longer term, work externally will help the building most, and of course can often reduce internal disruption from dusty masking plasterwork.
• Just because there is a damp problem does not mean we need to employ a specialist. One of the basic functions of a building is to cope with damp and water ingress, and the building was built by a builder. Most of the damp remedy work illustrated in options A to E is basic builder’s work.
• Consider carefully the complete pathway of moisture movement from outside to inside, and how the moisture movement chain can be broken.
• If asked to advise on a damp remedy, consider producing for your client 3 alternative remedy options, explaining how each meets key criteria. Weigh up the pros and cons of what each offers.
• Keep it in mind that the damp damage could be caused by more than 1 moisture source.
• The channel could of course be bedded in a little deeper in the ground – so increasing the height between the pebbles and the wall dpc. This may be necessary for very exposed elevations where rainwater could potentially bounce up and soak the wall above the dpc.

Compare what you find to good practice, building regulations of the day, etc. Consider basic building repairs that can be undertaken by a general building contractor.