Dampness in building elements

Figure 1: Right flank – quite a narrow gap as you can appreciate between the subject building and a neighbour’s garage

Property details:

Detached building
Traditional load-bearing brick walls
Suspended timber floor
Built c.1900

Background

The client wished to use this former rent house as an art studio.

Many of the floorboards had already been taken up and the floor had appeared suspect in several places.

The floor rot was easy to discover, but the client had been confused by conflicting remedy advice:

• install a chemical injection dpc in conjunction with floor upgrade and timber treatment;
• install a new ground-bearing concrete floor;
• install a new suspended timber floor to a higher finished level; and
• lower external ground levels in conjunction with new flooring works.

The various remedial options suggested are discussed later, but let us not lose sight of the fact that the original floor had not really ‘failed’, but had performed quite satisfactorily for a century. It would be fairer to say that the existing floor had ‘reached the end of its useful life’. Need there then be a design re-think?

Detailed site investigation

The client had expressed a strong preference for a new suspended timber floor. The main task in the site investigation would then be to find out why parts of the floor had rotted so detailing could be improved in any floor replacement. The moisture condition of the perimeter walls at low level would also be assessed as we may need to apply remedial measures to reduce any significant dampness.

In figure 2 is the floor as you might see it stepping into the building for the first time.

Figure 2: Rotting timber floor, looking towards the front of the building. Now imagine you are looking back from the front, to appreciate the existing detail sketch

Seeing such a floor laid open in all its gory glory is a luxury for a surveyor. And alarmingly, what you see in that photograph could be lurking somewhere under many a nicely carpeted living room. So remember this photograph.

Often the clues for defects are quite subtle – perhaps a ‘sponginess’ of the timber floor, perhaps a tapered gap under a skirting board, where the corner of a floor has ‘dropped’ due to a rotted wallplate.

If you lift a floorboard, you may find slips of packing, typically of hardboard or strips of old linoleum, fitted to level the floorboarding. In such circumstances, be suspicious of the condition of the supporting floor structure.

Before entering the subject building, the lack of subfloor ventilation to each flank had been noted. Vents may have been covered by raised yard levels, or may never have been provided in the first place.

The timber suspended floor was pretty basic (as indeed most are): softwood wallplates running across the building at intervals, laid onto single course brick sleeper walls. Wallplates were laid on bituminous felt damp-proof coursing, but had still succumbed to woodworm infestation – mainly near the entrance door, where perhaps flooring had become dampened from unsatisfactory threshold detailing. The most serious damage was the rot to the end of wallplates where they extend to the right flank wall. The oversite comprised subsoil with a layer of lime on top. The subfloor void was quite clear of any debris build-up, and seemed quite dry.

Looking along the flank brickwork internally, you may just pick out on the photograph 1 or 2 dark patches of brickwork – dark cherry red rather than pink. The darker brickwork was clearly quite wet, and later tests pouring water outside on the yard concrete would lead us to make interesting conclusions concerning the cause.

The client had confirmed that the site was known locally to be dry, and not subject to any flooding in past memory. Remember this is a real floor. Surveying students might now open a standard construction technology textbook to compare best practice and construction. Even though this floor would not meet current Building Regulations, it is still useful to refer to them as a benchmark for good practice.

At this point, it is well worth preparing a proportional sketch – graph paper mounted on a clipboard may help, to assess the wall base detailing. Sketching helps the thinking process and facilitates further reflection on detailing.

Sketches are particularly useful where there are changes in external ground levels, or where you need to determine the difference in height between internal floor level and external ground level. You can often measure down each side of a window opening – unfortunately not in this case. But 2 other methods of checking could be used: a level pilot hole could be drilled through the subject wall, or alternatively levels transferred using lasers or traditional spirit levels. The author used what was available on site – a straight edge and small spirit level. Remember when transferring levels to reverse the straight edge, to eliminate errors caused by a non-true straight edge. Without a spirit level you might just follow a brick course around the building to determine site levels approximately.

Figure 3 is a vertical slice through the wall/floor junction, taken across the building, left to right on the line of a sleeper wall. It is easy to see why the wallplate end had rotted.

Figure 3: Existing cross section at floor/wall abutment, looking from front to rear

The original floor could have been better designed. The brick sleeper wall could have been continued towards the flank side so as to fully support the wallplate near its end. As it is, the end of the wallplate, rather than being supported on a sleeper wall, is bedded on the building’s projecting footings, and is offered some protection from damp by a strip of damp-coursing material slipped underneath.

If you think that the gas pipe had made it difficult to build a complete sleeper wall and the wallplate had been extended to be supported on the footings as a result of the pipe obstacle, like me you would be wrong. Careful inspection of the floor detailing on the left flank revealed exactly the same method of support for wallplate ends – and there was no gas pipe.

Homing in more closely, you would see that in the past there would have been a small air gap between the end of the wallplate and the flank brickwork. This would have helped isolate the plate from potentially wet masonry. There probably was a gap between timber and masonry for the first 50 years of the floor’s life. But in 1955 some replastering was carried out to the flanks, and the plasterers allowed cement mortar to drop down behind floorboards and build up as ‘bridging plugs’ (see Glossary) in the gaps between wallplate ends and low-level brickwork. The plasterers had scratched a date on the backing plaster.

So a bridge had been created by lazy plasterers, ready, available and waiting for the opportunity to help transfer moisture. All it now needed was for the brickwork to become wet – and the bridge would be an easy route for moisture from low level brickwork to timber.

The side alley was concreted, perhaps 10–20 years ago. This was the final piece in the jigsaw. Now the brickwork became wet from low-level penetrating dampness, and the bridge was there ready to be exploited by the continual supply of rainwater – the transfer of water from rain to path to brickwork to mortar droppings to wallplate began, and rot soon set in. A simple but inevitable chain reaction.

The concreting did not appear to be laid to falls from the subject building. Rainwater would no doubt collect in pools to trickle down between concrete and masonry, and soak persistently into the brickwork through open ‘perps’ or very porous bricks. The bridging plugs could rightly be described as true ‘bridges'.

Having now carefully checked the extent of the rot and prepared a proportional sketch of the wall base detail, it was now time to check out the right flank surface water drainage. The client was on hand to help, by pouring water onto the yard, and its downward flow was observed.

Figure 4: How water collects on flank concrete pathway: measure distance of pooling from frontage and check whether internal wetness lines up

The results of the water test pour were indeed revealing. Water collected in a large pool, and it was possible to see the water soaking against low-level masonry, and actually trickling into brickwork via open brick joints. Water was also noted seeping downwards between path and brickwork. The author could not easily replicate real driving rain, but later watched during a downpour and the same water pooling happened.

When you have watched water movement action in front of you, you realise how critical the detailing of perimeter pathways and low level masonry is. If you were able to crawl under a raised timber ground floor (actually termed a ‘crawl space’ in the USA), you would often find the oversite darker in colour towards its floor perimeters corresponding to the outside walls, where low-level damp penetration had occurred. You may pick up and feel with your fingers handfuls of oversite – to compare zones of varying wetness – helping you to home in on the source.

Surveyors looking at the height of the concrete side path in relation to the height of dpc might quite often describe what they see as a ‘bridge’. The assumption would be that the biggest threat is an upward movement of moisture from below to above the dpc. But look closely at the existing section. What has been described as a ‘ledge effect’ – where in this case an abutting hard surface allows water to collect near the base of the wall, to trickle downwards into the wall base – soaking brickwork that is too close for comfort to vulnerable flooring timbers. For the subject building, the author was able to watch this happening. The biggest damp threat is nearly always water soaking into low-level masonry from poor wall base detailing, rather than an upward moisture movement from ‘bridging’.

In addition to checks of floor condition and external detailing, the walls were checked for damp internally. Much of the walling had been finished with cementitious render coating. Such a finish is pretty waterproof, and the absence of significantly high moisture meter readings – using the pin probes from above skirting height – was not surprising.

Walls were drilled at low level, and the drillings were tested by carbide meter (see Surveying equipment and tests). Brickwork just above the dpc and 2 courses below was confirmed to be very wet. Considering first the position of the wetness, it seemed likely to be the result of low-level damp penetration. Salts scraped from the plaster surface proved to be sulphates, with negative results for nitrate and chloride.

What we had here was lateral soaking and downward trickling rain damp.

Damp damage in old buildings is more often than not plaster, decoration or timber damage. At Wallington there was timber damage, more specifically timber floor damage. The internal hard render has kept internal wall décor dry. Being pragmatic, the emphasis of the remedy was on remedying floor damp.

Alternative remedy options

a) Install a chemical injection dpc in conjunction with floor upgrade and timber treatment

Close inspection confirmed the cause of wetness to low-level brickwork. The flank yard to the right side of the building had been concreted, and the surface was found to direct water towards the subject building, where it soaked into low-level brickwork. A dpc is not considered necessary in this case to stop or control dampness from below ground. The author cannot confirm whether a chemical dpc injection would even work to the subject building. The floor, after lifting boards, was found to be extensively rotted, requiring complete renewal. Structural floor timber will certainly need to be treated against beetle infestation, but by using factory treated timber.

b) Install a new ground bearing concrete floor

A concrete floor would probably help drive moisture to greater height up the external walls, as there would be no opportunity for moisture to evaporate out below floor level. It is difficult to reliably link a dpm in a retrofit concrete floor to existing dpcs. Running electric circuits and plumbing would be more difficult. (This option was in any case not favoured by the client.)

c) Install a new suspended timber floor to a higher finished level

The client wished to maintain existing ceiling height, so raising the height of the studio floor was not an option. Raising internal floor height would also have had implications for the amount of useable space in what is quite a modest floor plan. An internal step up would have been needed from the front and rear door entrances.

d) Lower external ground levels in conjunction with new flooring works

Could external ground levels be reduced? The ground had been raised around the building, compromising the effectiveness of the horizontal dpc – or had it? That is an easy statement to make, and quite a common one
in reports. But it is the author's opinion that a ledge effect has been created by the concrete pathway – where the hard and impervious surface, not laid to falls away from the building, has helped direct rainfall towards the low-level masonry. The zone of wetted masonry is in close proximity to vulnerable timber. Any horizontal dpc would not have prevented a lateral damp penetration from soaking rainwater through a solid wall, whatever its height. The key factors are as follows:

• the volume of rainwater impinging on the low- level masonry;
• the porosity of the masonry;
• the presence of holes and fissures in the masonry;
• the ability of the masonry to evaporate out as compared to take in moisture; and
• the proximity and vulnerability of timber and decorative finishes to the wet wall zone.

It has to be admitted that reduction of high ground levels around a building suffering from low-level damp penetration is often an ideal long-term solution. Lowering external ground pulls down the zone of wetness in the wall, helping to distance wet walling from vulnerable timber and internal finishes. But there were problems in reducing external ground levels around our building.

First, the concrete side path was laid unfortunately to knit in to the base panels of the prefabricated garage next door. So grubbing up the concrete could cause damage and instability to a neighbour’s property.

Second, the front pavement had also been built up over the last century – and even if the flank concrete was excavated just the minimum 150mm or so needed, the new finished pathway would be lower than the pavement at its bottom end. Obviously it would not be possible to lower one length of the public footway. It would be a considerable cost installing a dedicated drain for surface water to the right side of the building, involving running a below-ground drain under the building discharging to the existing drains to the left of the building.

Reduction of external ground levels is only liable to be feasible where surrounding ground is within the subject building’s curtilage, and there are the funds available to carry out extensive perimeter relandscaping/services alterations, etc. This is more likely to be an option for public housing, or larger houses set in large gardens. At Wallington, the building is bounded on the left by a private roadway, on the right by a neighbour’s garage in quite close proximity, and the front elevation is set directly off a public footway. Urban sites are so often quite restrictive and congested.

The compromise of forming a drained channel along the flank side of the building would also be problematic, as there is no convenient drain outlet opportunity near the front corner of the building. Water collecting in a perimeter drain or channel needs to exit somewhere, ideally by designed ‘falls’, otherwise a moat is created to help soak water into the subject wall – exactly the opposite of what you are trying to achieve. A channel also reduces the effective width of a pathway.

The remedy proposed

The chosen remedy, to install a new suspended timber floor, independent of the perimeter walls, and in conjunction with skirting ventilation, had not previously been suggested to the client, who agreed to this innovative remedial option after being shown the existing and proposed sketches and given a full explanation of how the proposed strategy should perform.

The author has not yet come across a lay client who could not understand carefully explained principles of construction. A client who understands the key principles and the pros and cons of alternative remedies is better able to make the final decision on remedial approach, with an understanding of the relative risks of the available options.

Advantages of a suspended timber floor:

• A timber floor would be in keeping with the traditional architectural style of the building.
• Although now partly rotted, the original floor had performed well, lasting nearly a century.
• Installing a timber floor would be a relatively clean and efficient method of construction.
• It would be a simple matter to lay pine boarding over the floor joists, and so produce an attractive floor finish.
• It would be easy to run cables and pipework under the floor.
• A timber floor of traditional design would not be too costly.
• A timber floor would help ventilate, and hence keep drier, the base of perimeter walls.
• The installation would be non-invasive.
• The construction is accessible and repairable.
• Such a floor installation is ‘reversible’; that is, easily removed or changed without risk of damage to the existing building fabric.
• Thermal insulation could be incorporated if requested.

Factory treated timber would be used for the floor structure, to reduce the risk of insect damage in the floor.

A compromise solution was needed, as ideally we would either reduce external levels or raise the floor height to deal with the low-level damp and floor ventilation conundrum. But neither option was available.

Finding reducing the flank pathway problematic, it was necessary to devise a way of ventilating the subfloor. It is possible to use mechanical ventilation systems, but natural ventilation pathways are better where possible – they are reliable and virtually maintenance-free.

The ventilation solution devised, shown in figure 5, was not an entirely new approach. If we refer to chapter 5 of The repair and maintenance of houses by Melville and Gordon, in Figure 62F we see a similar method to ventilate a subfloor subject to high external levels using ‘skirting ventilators’ – with routes for ventilation provided by ducts. Those authors also suggest that the ventilation may be tested by ‘holding a lighted match against the air vent’ – probably now considered a health and safety risk, but fun all the same.

Figure 5: Proposed detail RH

The detailing devised isolates the timber floor from potentially wet perimeter masonry – a key requirement. Ventilation is provided by 3 terracotta air vents to each flank, as well as existing ventilation below the front step and to the rear end of the floor internally.

The need to provide adequate ventilation cannot be over emphasised, and is flagged up by the BRE, who consider it to be a prime requirement for durability of roof timber. The BRE states also that:

‘the measured moisture content of timber must be maintained below about 22%, and preferably below 20%’

to reduce the risk of dry or wet rot development. But woodworm can be a threat to builder’s softwood at even lower moisture contents. Woodworm prefer to inhabit damp timber, and larvae have been able to survive in timber with a moisture content as low as 8–10%, although optimum conditions may be within the moisture content range 26–50%. Using pretreated timber, just a little more expensive than the untreated would reduce the threat of woodworm attack. For health and safety reasons the author would always advise the use of pretreated timber, rather than the application of preservatives on site – although cut ends of softwood should be liberally brushed with a suitable general purpose preservative. Wallplates are to be protected by damp courses.

As discussed earlier, low-level masonry seemed to have become very damp, due to a low-level penetration, rather than dampness from a below ground source. So the cementitious plinth shown on the sketch proposal should help to reduce this, by providing a barrier at the base of the wall and sealing the gaps between brickwork and concrete path – where water was seen trickling through. The skirting board was spiked onto treated softwood battens, which should last a good many years, and could be quite easily replaced anyway. You may wish to isolate the pre-treated batten from low-level brickwork using a dpm. The skirting is easily accessible and would be relatively inexpensive to replace in say 40 years’ time if needs be.

Skirting boards are a key feature in the control of low level dampness. In many traditional buildings, high skirtings spiked to timber grounds offer an efficient damp control solution. Air pathways behind the skirting help evaporate out any developing dampness, and the plasters, commencing at some height up from the floor are less likely to be affected by low level damp.

Maybe this time round, with an improved specification, the floor could even last for 2 centuries.

Teething trouble?

Soon after floor completion, distortion to floorboarding to the rear part of the floor was discovered. Could leaks be finding their way into the subfloor and creating high humidity to distort floor boards?

A close scrutiny of the boards confirmed ‘crowning’. Crowning occurs when a floorboard has expanded to its upper surface – as opposed to ‘cupping’ where boards would appear to have turned upwards towards their edges.
The author's mind was put at rest. The client explained that just after the skirtings had been installed, the fixed light at the rear end of the room had been replaced, and during the work some rain had got in. The flooring had become wet from above. The boards should return to their original shape once normally ‘dry’.

Site works Having cleared out the old sleeper walls, new sleeper walls were built incorporating numerous quarter brick sized openings between bricks for cross-ventilation, with a new dpc bedded on top after filling of the brick frogs with mortar. The floor rests on the sleeper walls sheerly by its weight. Joists are spiked to wallplates, carefully cut back approximately 25mm from the brickwork. Textbooks nearly always show an angled cut to the joist end, which might reduce the chance of debris lodging between joist and wall. After building the sleeper walls and bedding in the wallplates the author gave the client some advice on fixing the floorboards, explaining the importance of using a pair of flooring cramps to squeeze the boards tightly together, to reduce the risk of unsightly gaps developing between boards from shrinking after fitting. The floorboards were nailed down using floor brads, to finish a fraction below the board surface using a nail punch. Always drill small pilot holes before driving in the 2 nails to each end of a board, to prevent splitting. One or 2 boards were left un-nailed in order to access the subfloor void (where plastic sleeves had been fitted in low-level brickwork) to monitor the moisture condition of brickwork and structural timber after the remediation work. Three airbricks were installed to each flank. In addition to the flank vents, some opportunity existed for installing floor ventilation along the building frontage. The more traditional terracotta brick offers rather restricted ventilation openings, but the author refuses ever to fit a plastic vent into an old building as they look out of place. You may sometimes need to incorporate support to the ventilation opening cut through the brickwork – traditionally this was done by means of a strong slate bedded in over the air brick. A cement plinth was run along the flank side of the building to help reduce lateral water penetration.

Evaluation and monitoring

The client was extremely pleased with the finished result. The floorboards have been varnished and the finish is attractive and appropriate for an art studio. Because of its modest size, the studio is easily heated by a single electric radiator.

At the time of publication monitoring was still in progress. This is required:

• to assess whether the structural floor timber remains satisfactorily dry, to reduce any risk of rot or beetle infestation;
• to evaluate the wetness in the brickwork below floor level, where the new plinth should hopefully reduce wetting from collecting rainwater.

Step by step guide: diagnosis – remediation – monitoring

Step 1 - Assess floor condition.
Step 2 - Transfer levels using a straight edge and spirit level.
Step 3 - Pour water to check surface water drainage and likely entry points for water.
Step 4 - Draw up a scaled sketch of the wall base to aid diagnosis and remedy formulation.
Step 5 - Formulate a remedy and identify key site supervision issues.
Step 6 - Install the new timber floor.
Step 7 - Install support measures – air vents and cement plinth.
Step 7 - Install ventilated skirting, leaving key sections available for easy removal for monitoring visits.
Step 8 - Monitoring.