to be very honest i think you really need to take the floor up....
you need to lay a barrier to prevent moisture rising (DPM) and you need to install proper structural joists (a suspended timber floor) and install adequate insulation between these joists. either that or lay the DPm, then insulation and then a solid concrete floor of about 125mm .....

Hi Dan,

No way am I an expert, but those joists look relatively new and seem to have been laid directly onto the floor. They don't look strong enough to support the floor if you dig the underfloor out. Looks like a bodge to me. Be v careful regarding putting it right.

Posted By: Paul in Montreal They look like they're 2x8s
Paul in Montreal.

Check the last photo. They seem to be <3" deep. Not strong enough.

Posted By: CWattersCheck the last photo. They seem to be <3" deep. Not strong enough.

You're right, I was reading the cm as inches!

Posted By: dan_aka_jackAs Dickster mentioned, the joists look fairly new

Actually, the wood looks old to me - it looks very similar to the studs I exposed in our 110 year old house - plus you have square cut nails in there. The dimensions seem to be "true" 3x4s - but they would not be strong enough if unsupported.

Paul in Montreal.

Posted By: CWatters
Technically I believe you would need Building Control Approval for this as you are changing an "insulating element".

Yup, I believe you are correct. Not only am I changing the insulating properties of the floor but it looks like I'm going to be changing the joists as well and I believe I need to get building control approval before I fit a whole new floor (even if I wasn't changing the thermal performance of the floor)... although I'm not sure if I also need to get a structural engineer involved to provide building control with the necessary calculations (the length the joist can go without requiring support etc). This could get expensive.

Posted By: CWatters
Think carefully about the ends of the joists and how they are fixed to/into the wall. Don't want the ends rotting.

Yup... what would you recommend? Should I glue some DPM onto the vertical face of the wall and then bolt a face-fixing joist hanger onto the wall? (i.e. the DPM would be sandwiched between the vertical face of the wall and the joist hanger).

We appear to be looking (with regard to this problem) at just one room so far, surely the sam problem is likely to occur in the other g/f rooms also (I guess they are probably the hall and kitchen?).
If you put in some honeycomb sleeper walls i.e traditional suspended g/f construction you can avoid the use of deep joists and even cutting in to surrounding walls, but you will need to sit the walls on strip foundations that go down below the frost line (notionally 450) or as otherwise agreed with you BCO. You will need to dig a small trial hole for him to look at. Sleeper wall could be as simple as one course of bricks, then dpm, then 100x50 timber plate, then (probably) 100x50 joists (insulation between, say CELOTEX). Do a little digging and see how helpful your BCO is. Check other rooms!

I weas going to say the joists look new. Every old floor I have lifted have the joists covered in grey muck, loose dust that has fallen down the cracks in the floor boards.
The joist needs to sat on something that does not transmit the moisture through it and can take the weight of the floor loading. Sleeper walls are entirely suitable but would require the whole floor to be lifted to gain enough working room, also the walls would need strip foundations, which could undermine the existing foundations of the house.
What I would do is to support the joists on a 9" square paving stone, every 18" along their length. Mark out with tape the centre of each stone on the floor boards (just a dab of tape would do). Lift two boards next to each other, one with the line of tape bits on. Dig out say 11"" square under one joist centre, giving a gap of about paving stone thickness + 1/2". Put down a batter of mortar, put a paving stone on it (leave a gap of 1/4"+ on top)and make sure its tight on the mortar (use a crow bar against the underside of the joist). Slip in a piece of DPC membrane and push mortar in the slab/DPC gap - make sure its tight. Repeat for the rest of the joists exposed. Avoid walking on that area for 3 days, but you can do various strips say at 4' centres. then when the cement has gone off, do some more. The theory is that you won't be disturbing the floor level or the walls too much.
Frank

If it aint broke don't fix it. Looks like dry soil, good timber joists and a nice old pine floor to me. Looks like it's been there for a century without problems. We don't do it like that now, but sometimes they did then!

Try a little bit or dig a hole outside to see how deep they are.

Wow, amazing timing... Just a few minutes ago I was just thinking about lifting the existing floor boards, removing the joists, digging out and fitting wet underfloor heating set into a concrete (or alternative?) floor. Excuse my ignorance but is it possible to fit wooden floor boards over a wet underfloor heating system?

Many thanks,
Jack

Great, thanks for the reply.

By the way, here's a photo of the end joist (nearest to the external wall) which isn't a happy joist:

http://xlk.org.uk/ext_images/house/DSC_0003.jpg

Re edge joist -- looks like little dampness as woodworm won't eat dry wood. Reduce g/l outside, check gutters etc.

Is the floor ventilated from outside?

dan_aka_jack hopefully here's a picture of dwarf walling for your info.

My joists were in as bad a state as yours so I replaced all ground floor.Your existing spans should be ok if not changed from original.Any deeper timber for joisting will mean a higher floor level or lower bearing,so bottom of joist deeper in ground.
You ideally need a clear space below joists for air movement & to stop moisture transfer.Joist should sit on timber plate on mortor bed on dpm on mortor on brick work.

Where is FGL in relation to FFL ? if the floor level is well above ground level then there is less of an undermining issue, if not it might prove a difficult void to ventilate and you'll need to investigate the foundations... engineers.... big hole in you living room for ages... etc. Are you sure suspended timber is the best option in this case...

J

Joist hangers aren't usually a feasable solution in older houses as the bricks are usually a soft composition & tend to crumble under loading.If you dig about under the soil you may find that it's loose debris on top of a lean mix oversite (concrete base)This can act as a surface to prop good joists from while you renew timber on bearing points,it might be easier(figuratively speaking)to put in extra strip footings & bearing points than faff about with deeper joists,cheaper timber costs too.Apologies for misspelling on mortar!