Sub-Floor Ventilation

[Phil Richards]

Hi all

This won't be a DIY project we'll undertake ourselves, more for general advice.

We have had an offer accepted on a 3 bed mid-terraced house about 100 years old. As is typical, the house is suspended on wooden joists close to ground level. It is L shaped with a ground floor bathroom built on as an extension to the kitchen at the back. This is on a concrete foundation probably many years ago as the house was never originally built with a bathroom. In addition, to the right of the kitchen/rear of the dining room a conservatory/lean-to has been constructed and the floor has been concreted up to the height of the floor in the house.

We had a full buildings survey carried out which we have read through. One "serious" defect raised is lack of sub floor ventilation to the rear as a result of no air brick vents. We suspect they did exist but were effectively blocked off/removed whenever the bathroom extension was built and the lean-to floor concreted over. There is one air brick located underneath the front door which again the surveyor has pointed out is too small.

The result of this could mean stagnant damp and dry rot to the under floor timbers. Of course they surveyor could not prove presence of dry rot under the floor especially as the kitchen floor is tiled and the hall/living rooms etc, are carpeted.

Our doubts are whether the back part of the house has not had proper sub-floor ventilation for many years (since the bathroom extension was build possibly many years ago?) and yet could be perfectly OK to this day. However it may not.

Various recommendations have been suggested which basically mean more air bricks to be fitted. Suggestions are to insert them by channelling into the lean-to floor underneath the door from the dining room again which leads out to the lean-to. Also one to be inserted into timber floor where it abuts the solid floor in the doorway from the kitchen to the bathroom extension. This is suggested with the "disclaimer" it will cause draughts and must not be covered.

One fairly drastic suggestion is that a channel is laid from the back of the house underneath the bathroom and kitchen as far as the hall.

Any thoughts would be appreciated.

Thanks.

[The Natural Philosopher]

fill the subfloor in with concrete, DPM, insulation and screed, and put the floor boards back over it.

Enough ventilation to stop rot is enough to cause bloody draughts.

[RubberBiker]

Did the survey actually find evidence of rot in the floor timbers? I've seen similar situations as you describe - with blocked or non- existent provision for underfloor ventilation - and yet the underfloor space and ground underneath is bone dry.

Probably more important than ventilating the airspace is good drainage around the building - no adjacent soakaways, or leaking drains, and the surrounding ground naturally draining away from the building.

Other factors will be gaps between floorboards, room ventilation, floor-coverings and position of furniture - all contributing to the underfloor space being ventilated by other routes.

Were there any signs that the floorboards had patched repairs - particularly along on side next to an outer wall?

If there hasn't been problems so far, and you keep the building dry and well ventilated - things are likely to continue the same way.

However, once the house is yours, pull up boards in a couple of places and check what's going on. You may even find the space is ventilated by a route the surveyor was unable to see.

[Phil Richards]

No as all the ground floor is carpeted or tiled over.

Noted. Also no sign of rising damp suggesting the ground below is not particularly wet?

The kitchen is fully tiled, the rest of the ground floor reception room & hall is carpeted. We plan to leave it that way.

Not seen as carpeted or tiled over. This is always a disclaimer surveyors write in as they never lift up floor coverings. I doubt for one minute a vendor would agree for us to do it.

[Phil L]

You can put vents in the walls above ground level in your conservatory, but they will be staggered with the corresponding vents underneath your internal wooden floors, nevertheless, it will create a cross-flow of ventilation, which is what you are aiming for.

Obviously you will need more vents at the front, but these should present no problems, but again, if the outside levels have been raised so that you can't get them underneath the DPC (internal floor level), then you can use the method described above

[Hugo Nebula]

On Mon, 08 Jun 2009 22:43:52 +0100, a certain chimpanzee, Phil Richards randomly hit a keyboard and produced:

Whoever built the extensions should have run 4" pipes under the slabs wherever there had been an air brick to vents coming up through the cavity to air bricks on the external elevation below DPC. I say, "should"; it doesn't always happen, and happens even less with conservatories (which are exempt from Building Control inspections).

The suggestion, which if I read it right, to ventilate the sub-floor void INTO the building sounds bizarre, and would almost certainly cause more problems by providing an easy path for warm, moist air from inside the building into the cold sub-floor void. As suggested elsewhere, if there is no cross ventilation, it may be better to replace the joisted floor with a concrete slab.

Give the floors a good stamping, especially close to the walls to see if there is any movement in the joists. If there is rot, the joist ends and the boards may feel a bit mushy.