Dampness in building elements

As you can see in figure 1, tarmac roadway levels rise towards the front of the right flank. The outside ground is around 225mm above internal floor level at the front house corner visible here. This means that there may need to be some ‘waterproofing’ of the walls. Mere ‘damp-proofing’ may not be appropriate for a below-ground situation. It would not be possible to lower the side roadway, as it is owned and used by others.

Figure 1: This Charlton detached house is on a pronounced slope.

Inside the property, it was possible to measure and establish that the front corner of the house was a little below ground level. For the remainder of the right flank wall, internal floor level was at, or above, external roadway tarmac. A surveyor might think the timber floor to be free of any damp threat, where the inside floor level is comfortably above external ground – and might not realise the significance of the diagonal cracking seen here (figure 2), to the front room partitioning on the right flank side.

Figure 2: The inside front corner of the house.

Figure 2 shows a diagonal crack, at point ‘X’ in figure 3.

As the ground floor plan shows, the floor joists span from front to back, supported by intermediate piers and timber beams, rather than the standard honeycomb sleeper walls we are more familiar with. At X is the end bearing position of the main central timber beam, which supports the floor joists at mid span and also the main central loadbearing partition. I needed to take up some floorboarding to assess the condition of the beam end. Could a failure at this position be the cause of the diagonal crack?

Figure 3: Charlton floor plan.

Looking at the plan, note how the ends of joists are supported off inner piers and support masonry, so they do not extend into potentially wet brickwork.

In figure 4, the large cross-beam sits on a brick attached pier on a slate dpc, but this would not prevent its end from being wet from contact with a wet wall. The beam is deep, and careful measurement led me to understand the detailing of this wallbase, so a sketch was drawn up (figure 5). Diagnosis of the structural issue now completed!

Figure 4: The large cross-beam is built into the flank wall, has become very damp, and has suffered rot to its end.

Figure 5: Charlton flank beam section.

Even though the outside ground is more than 300mm below inside floor level, the floor timberwork is very vulnerable to damp penetration, as you can appreciate. Damp damaged plaster tested negative for ground salt. But the brickwork near the beam end was found to be very saturated from lateral damp penetration.

Key remedy features

1. New softwood beams bolted to each side of the existing built-up support beam, using through bolts and toothed timber connectors.
2. The new sister beams supported off metal joist hangers.
3. Walls replastered, using a modern cavity drain membrane – Newlath 2000.
4. New skirtings glue-fixed.
5. Localised treatment of live woodworm.
6. Wallplates each side of chimney breasts would be renewed using treated timber off new dpcs, with the new wallplates distanced from masonry.
7. New airbricks would also be installed to improve ventilation along the flank wall. The building is deep, and had no airbricks at all along the very long flank.

The Newlath membrane meets the existing plasters at around 15 brick courses above floor level. Existing plasters were approximately 25mm thick, so it was possible to achieve a neat meeting of plaster systems, with the plaster join reinforced with a hessian scrim. The air gap should help keep dampness lower down in the wall, as ventilation will encourage evaporation.

Figure 6: See how the Newlath membrane extends down below the floorboard level. You can just pick out the air gap between membrane and brickwork. This section is at the front flank corner of the property, where the external ground is higher than the internal floor level.

The existing slate damp-proof course is still well down in the ground (level with the top of the floor wallpate), but it is not possible to reduce the roadway height.

The external cement plinth is too high, at nine brick courses, and really ought to be reduced to say four courses (300mm) so it can stop rain splash and any running water from unduly wetting the wallbase, but not be too high so as to lock in dampness in the lower part of the wall. We could consider installing a perimeter drain channel (such as devised at Billericay).

I’m not sure how effective such a chemical damp-proof course actually is, but this is only a support measure. The membraning system extends anyway a metre into the building at corners and the cross-wall intersection.

Figure 7: An injection cream damp-proofing product is gunned into a series of vertical holes at the junction of the main flank wall and the brick cross-wall.

Figure 8: The plastic dimpled sheeting is offered up to the brick substrate, temporarily held in position by a few lengths of floorboard. The membrane can bend quite easily for a snug fit at an internal angle. The face of the sheet has a welded mesh, over which a wet plaster coat can be trowelled.

Figure 9: Mastic is gunned into the fixing drill holes.

Figure 10: Plastic plugs are hammered home to support the membrane. The drill holes need to be exactly the correct diameter for a tight fit; the manufacturers of the membrane supply the correct HDS masonry bit for the purpose. This membrane is fairly transparent, so you can see a suitable place to drill through the membrane.

Figure 11: The membrane ready for plastering, tucked down behind the floorboarding. Some boards may need trimming back. This is a good thing to do - so often, boards are fitted too close to perimeter masonry, and rot at their ends as a result.

Figure 12: Sand, lime and cement are mixed together in a plasterer’s bath to produce a suitable plaster mix for the membrane. The mix was approved by the manufacturer of the membrane.

Figure 13: The author applies the plaster mix – the first coat steel-trowelled on.

Figure 14: The rotten end of the main cross-beam is first trimmed back from the main wall, and then the end is given a liberal dose of gunned-on paste preservative. The metal joist hanger is installed ready to receive one of the two new side-beams.

Figure 15: The back of the new skirting board is given a gunning of contact adhesive.

The new plaster is given a skim of a lime-based finish coat, which merges in beautifully with the existing lime-based skim coat. A hessian reinforcement scrim eliminated any cracking at the perfect junction of new to old plaster. Once lined and emulsioned, you would only be aware of a join of plaster systems when tapping the walls.