Using joist caps with renovation on josts at 130mm high

[Jeremy]

I'm pulling out a series of floor joists (63mm wide by 130mm deep) that run under about 40% of a sitting room. The joist ends are damaged by dry rot and there are signs of woodworm across the lengths. The floor is still sound, but I want to get this resolved before we move in. I've spoken with damp-proofing suppliers and the general advice seems to be to go for joist caps/sockets as this will prevent any ingress of moisture from the brickwork that they are laid into (on top of a sill plate).

 

This seems like a good idea to me, but here's my problem: the joist caps are made at standard sizing with a "max height" of 225mm (https://www.manthorpebp.co.uk/air-leakage/joist-seals-wide-range-sizes-and-permutations/joist-seals-0) and then they suggest you pack the remaining cavity with cut timber. This is an old victorian house without any sub-floor to speak of, just wood floorboards nailed to joists, so there's no where for me to put that extra 95mm above the joist in that cavity.

 

Has anyone worked with caps before and found a good hack / solution? I guess I could just cut off the top at 130mm and add damp-proof paint or membrane? The cellar is well ventilated, and will be fully tanked etc. so not worried about dampness in that way.

[tonyshouse]

Is the plate being removed? 

[Jeremy]

Yeah,  @tonyshouse seems like a good idea to take out the sill plate as well.

Edited April 26, 2021 by Jeremy

[George]

Wrapping joist ends with DPM is a common solution.

 

It is worth mentioning that all timber will rot in humid conditions - you must improve the subfloor ventilation (air bricks and gaps in sleeper walls).

[Jeremy]

Thanks @George for confirming.

 

Re: ventilation - I have this well in mind. There only seems to be rot on one side, which happens to be the front of the house, e.g. where cellar wall is soil all the way through and brick wall above, whereas on the other side where there is open venting to sub-floor of another room in the house, joist ends seem to be fine. I can't use air bricks there, as there isn't any air to be fetched, as soil level is above the joists so was thinking I might do a fan, or humidity extractor. Definitely open to ideas on this