Subfloor Timber Decay

Assessing Subfloor Timber Decay

I recently attended a property to inspect a timber staircase where it was suggested by the client that the stair timbers were affected by timber decay. with a second suggestion that the sub floor beneath the stairwell may also be affected by timber decay. This just required a localised. albeit, highly invasive survey, so I attended with a mini circular saw and a multi=tool that would allow me to take up floorboards and access the hidden sub floor void.

Initial Observations

Initial visual inspection revealed no evidence of timber decay in the stair strings, treads or risers, however, I noted that a temporary ‘gallows bracket’ type timber support structure had been installed to support the staircase half landing, which appeared to have become detached from its wall fixings. Interestingly, I also noted that a most of the plaster had been removed from the walls in the understair cupboard, leaving exposed bare masonry.

Checking for Wet Masonry

It was important to check whether any of the stair timbers, in particular the timber strings, were in contact with wet masonry, so I carried out a calcium carbide check to assess the masonry moisture content. This returned a low reading of 0.5% TMC, so as far as the stair timbers were concerned, any risk associated with wet masonry could be discounted.

Assessing the Timber Sub Floor

With the floorboards still installed there was no evidence of timber decay to the floor, and timber moisture readings to the floorboards were all recorded at less than 20% R/R. However, when checking the subfloor humidity, this was recorded at a high 87% RH, which is particularly concerning when benchmarked against the ambient humidity in the property, which was recorded at a more reasonable 57% RH.

With this in mind it was time to take up some floorboards to inspect the subfloor joists. I removed a number of floorboards and noted that a number of joists were affected by severe timber decay, but in particular, the timber wall plate, which rested on a sleeper wall, running parallel with the front wall of the property, was severely decayed.

The cause of this decay was immediately obvious on closer inspection… I could see that the void between the sleeper wall and the front external wall of the property had become filled with muck and debris. and hypothesised that this occurred when the plaster was hacked from the walls in the understair cupboard, since there was were no skirtings installed, and a gap between the edge of the floorboards and the wall, which would allow this debris to fall into this void.

As expected, timber moisture content was extremely high at 37% and we noted a secondary issue, in that, this debris was also blocking the sub floor air brick, which provided ventilation to this section of the subfloor.

Recommended Repairs

In fact, this would be a fairly simple fix as there was no decay in the stair timbers, the fixings which attach the stair strings to the wall had failed and these needed replacing, after which, the temporary supporting gallows brackets could be removed.

With regards to the subfloor decay, we recommended that the decayed timber was cut out a minimum of 1m past the last sign of infection and replaced with new tanalised timbers. More importantly, the debris should be excavated from the floor void to remove the ‘moisture bridge’ which had formed. and further to unblock the subfloor vent and reinstate a robust flow of air to the subfloor.