Dampness in building elements

Timber

Well-ventilated timber ground floors, built off dpcs under wall plates, usually give good service for many years, particularly if timbers are not built into masonry. A well-ventilated subfloor void not only keeps the floor timbers in good condition but also helps to control potential moisture problems at the base of walls. This sort of ventilation mechanism is termed a 'sink'; it helps any unwanted excess moisture to vent to the outside air. If the timber floors are in good condition and the walls incorporate effective dpcs, rising damp in the walls is not likely to be a problem.

• Use a moisture meter to check moisture content of the timber floor boards and joists, paying particular attention to the ends of joists near where they are built into masonry.
• The standard moisture meter probes will help you ascertain the timber moisture content of smaller sized timbers, but thicker timber sections need to be tested by other means, such as a hammer electrode (to assist in obtaining a deeper reading).
• Check that the floor is provided with suitable effective membranes to prevent dampness entering from the ground, and that floor damp-proofing is linked to wall damp-proofing.
• Measuring the relative humidity of the subfloor void may alert you to a dampness problem that needs further investigation.

Rot in floor timbers usually indicates poor subfloor ventilation, lack of dpcs or subfloor condensation problems. Ventilation may never have been sufficient; the subfloor ventilation may have been compromised by blocked or clogged floor vents, or a build-up of rubble in the void. Sometimes, ventilation openings are blocked when external ground levels are raised. Floor joists or wall plates may also have suffered from being built into masonry that is prone to significant dampness, and there may not be a dpc under the wall plates (see All above board? case study).

High water tables or plumbing leaks may create damp conditions in a ground floor construction. If the oversite appears to be wet or if the flooring timbers are in the 'at risk' range of moisture content, the walls are also likely to be significantly damp in places. This is because moisture-laden air in the floor void may rise and condense under the floorboards or behind skirtings, giving a false impression of a rising damp problem. This phenomenon is more accurately termed 'rising damp in the air'.

Upper timber floors are less likely to be ventilated, but they are also less likely to be subjected to moisture ingress, except where joists are built into walls or the floor is subjected to wetting from a WC, bathroom or kitchen.

Concrete replacement floors

Where all or part of an original suspended timber ground floor has suffered rot or beetle infestation it will usually need to be repaired and in some cases replaced. Contrary to prevailing practice, there is the option of replacing like for like. This has several advantages:

• It is easier to repair or adapt a timber floor, which is basically a nailed assembly of various timber members on brick sleeper walls.
• It is easier to supervise a suspended timber floor installation because its various component parts are more exposed and visible for inspection.
• A suspended timber floor will make the routing of service pipes and cables safer, more economical, efficient and straightforward to install.

Remedial works to existing timber ground floors are described in Suspended timber ground floor: remedying dampness due to inadequate ventilation.

If the new timber floor is replacing an existing floor, or part of an existing floor, it is not always necessary to incorporate a sub-base of concrete. But if the new suspended timber floor is to meet current Building Regulations, it will need a concrete sub-base on the ground underneath or sometimes fine aggregate on polythene on blinding. This can be an expensive option so, for reasons of economy, suspended timber floors are often either wholly or partially grubbed up and replaced by solid ground supported concrete slabs. (Another alternative would be to specify a suspended concrete floor using a beam-and-block system.)

Solid ground supported concrete replacement floors are not trouble-free (refer to figure 1 in Rising damp - myth or fact?). In most cases the replacement will seriously interfere with subfloor ventilation to any timber floor areas that are retained. This can be avoided if ducted ventilation pathways are provided through the new adjacent solid floor areas, and if correct detailing has been achieved at the abutment of new to existing flooring. Any hearth that bridges the damp courses should be removed to avoid possible problems of moisture transfer.

The new solid concrete floor may also cause problems for the perimeter wall base if it prevents the existing wall from breathing. Pipes embedded in screeds (on concrete slabs) may cause future damp problems if they leak, and any ducts may also create the conditions that enable rot to develop and flourish.

Surveyors have not always considered seriously enough how much damage the new concrete floor areas may be causing to walls at the floor perimeter. After all, a large previously ventilated void has been filled by a solid, dense and obstructive material.

• Water may penetrate the floor edge where air vents from the original timber floor have not been bricked up so that water trickles in from the external yard area.
• In most cases, any dpm in the solid floor will not have been married to wall dpcs. (Marrying up a polythene membrane to an existing dpc is difficult.) For that reason the concrete floor membrane is nearly always simply tucked up at the wall abutments, but this could create a moisture pathway between wall and floor slab.

The new ground supported solid floor is usually, in effect, a collection of dry 'islands'. If you test the concrete slab, you may well find it to be dry, but what is happening to the bases of the walls? They are now sandwiched between dense concrete slabbing. Often the concrete mixer will have been wheeled outside and used to mix up huge volumes of concrete to raise external levels, so bridging the dpc. And in the absolutely worst case the mixer is then used again to produce volumes of render mix to be applied to the internal/external walls, completely sealing in the dampness.

Unfortunately the only long-term remedy (or cure) for the serious dampness problems that this would create is to grub-up concrete floors, replace them with suspended timber as original, and lower the yard back to the original level. You may also consider installing a suspended concrete floor. Removing external remedial renders would not usually be practical because the brickwork exposed by hacking off dense renders would be very difficult to reinstate, but it might be possible to hack off the internal waterproof render that might have been applied by the remedial treatment company. Any original dpc would then be able to function effectively as originally for at least another century.

Inspecting concrete floors

Inspecting a concrete floor poses a much greater challenge to the surveyor than a timber floor. The construction detail drawings or specification may help you to understand how the floor construction was designed, but not necessarily how it was built.

The structure of a concrete floor is usually a sandwich of hardcore, blinding, dpm, concrete, screed, possibly screed reinforcement or insulation, and a floor covering. The dpm is usually under the concrete and is therefore impossible to check once the floor is built without damaging the floor or membrane itself. The plastic sheeting for dpms is usually 4m wide. This means that, for most rooms, there will be laps or joins to sheets – a potential weakness in the membrane due to poor site work. Membranes are also easy to damage before the concrete is poured.

• If the concrete slab is wet within its thickness this may be caused by an ineffective dpm.
• Screeds are most often wetted by pipe leaks, overflow of sanitary appliances or leaks from kitchen appliances.
• A high site water table may create damp conditions in a ground floor construction and can, in some circumstances, cause a floor slab to 'lift'.
• Check that the solid floor been constructed correctly. The writer has found during the first of the serious floods of Carlisle in 2005 that some solid floors would not dry out. Not because of the flooding, but due to pre-existing defects in the solid floor construction. There were examples where there was an absence of sand blinding between the hardcore and dpm upon which the concrete slab had been poured and laid. Pressing down on the dpm, the hardcore angulations had pierced the dpm allowing ground water penetration.
• Other examples have found a myriad of negligence in floor construction to include:
  • laying the concrete floor directly onto earth;
  • laying the solid floor over the original and collapsed timber floor; and
  • overlaid solid floor over open drains, uncapped soil vent pipes and unremoved fire hearths or without any dpm.

Check that the floor is provided with suitable effective membranes to prevent dampness entering them from the ground. Check the linkage of floor damp-proofing to wall damp-proofing. It may be possible to remove a section of skirting board to find out whether a membrane has been taken up the wall at the floor's perimeter, but this is not much evidence to go on if you wish to make a meaningful report on floor condition. You would probably be able to identify the type and thickness of dpm used.

Consider using a masonry bolster to chisel up a section of screed so that you can establish the thickness of a screed and whether any insulation has been incorporated. But any invasive investigation of a concrete floor is risky because there could be concealed cables or pipes within the screed, or you could damage the dpm.

You could assess the moisture content of the floor by:

• using a humidity box, or a mechanical floor hygrometer; or
• placing a humidity sensor in a drilled void, , being careful not to pierce any existing dpm.

Again, these procedures can be risky (the categoryID=1155">table in case studies lists all the studies that use these techniques).

Figure 1: Dampness in solid wall caused by poor detailing of the solid floor physical dpm. The excavated edge of the solid floor reveals that the dpm is not interlinked with the dpc in the wall, allowing groundwater to penetrate into the habitable space