DIY Doctor

Replacement of rotten floor joists

Postby Nimrod » Thu Aug 16, 2007 10:04 pm

The floor joists in a 12' X 12' ground floor room in a terrace house have failed because of wet rot.

The joists are built into the exterior walls and the rot in the joists extends about four feet into the room. The joists have failed ie snapped about two feet into the room and are rotten for some distance beyond that point. It is therefore not a simple case of 'cut and replace' of the ends of the joists but rather a complete replacement of the joists (and floor boards).

The building is about 100 years old and it seems to me that it would be very difficult to introduce a cost effective and efficient damp proof course into the party wall into which the joists are built.

Given the circumstances it seems the most effective approach would be to ensure first that the air bricks are clear and secondly replace the joists with tanalised timber.

Is this feasible? Are there any attendant health risks in this approach?

Wrapping ends of joists in roofing felt seems to be a 'jury rig' solution.

Are there any other cost effective solutions or approaches to the replacement of the joists?

In carrying out the remedial works the floor boards will need to be replaced. In this day and age do you replace with T&G or is that too expensive? Or would it be chipboard and if so what thickness and how would you spec it?

Any help, comments, or directions would be very much appreciated.
Many thanks
Rank: Tradesman
Progress to next rank:
Posts: 28
Joined: Thu Aug 09, 2007 7:01 pm


Simply Build It

Postby jaggyboy » Fri Aug 17, 2007 10:34 am

my house is approx 10 yrs old and has been trated with a chemical DPC, where hols are drilled into the external massonry and a chemical is pumped into the stone under high pressure, this sorts out the dpc aspect of it.

I have also had parts of joists replaced and the firm that did this also replaced the wallboards that the joists sit on, inserting a fabric dpc around the wallboard and treated the last metre or so of the joist with a fungicidal paste to give the best possible protection against future rot. In addition to this where there has been no wallboard and no provision to insert one, the joist end was wrapped in a fabric dpc to protect this.

I would guess (but bow to others with superior knowledge) that whoever you get to replace your joist and floor will be able to also carry out these dpc works correctly - I used a building preservation company, rather than a joiner/builder for the work I had done due to the age and future needs of the property.

hope this helps in some way
Rank: Labourer
Progress to next rank:
Posts: 2
Joined: Wed Aug 08, 2007 3:26 pm

Postby Nimrod » Sun Aug 19, 2007 3:30 pm


Thanks for taking the time to respond and for your advice. The wrapping of joist ends in fabric dpc or roofing felt seems to be a popular response for built-in joists in old property.

The use of tanalised timber for joists would make the fungicidal paste or fabric DPM approach redundant.

I wonder if any readers have used 'tanalised' joists and if there are any health risks to occupants by doing so?
Rank: Tradesman
Progress to next rank:
Posts: 28
Joined: Thu Aug 09, 2007 7:01 pm

Display posts from previous
Sort by
Order by

  • DIY How to Project Guides
  • DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!

  • Related Topics