Replacing a floor

[richard.selwyn]

I've got to replace a floor that's been eaten by woodworm.  It is in the local lawyer's office and the secretary's chair just made a hole in the floor.  I've just got back from patching it.
From what I can see he current floor is nailed to joists about 100mm tall which are sitting on the ground.  If I replace the joists, what should I use for the joists and should I put a dampproof membrane under each joist.  There does not seem to be any ventilation and the client does not want to add any.  I had planned on 22mm t+g damproof particle board panels (CTBH here) as it will be covered by carpet foam backed carpet, screwed to the joists.

Any advice greatfully received.

(Please note I am in France.  The client has said that if my solution does not meet local building regulations he is happy to sign a release)

 

[chrisrosenb]

I am in the USA, so I do not know what materials you have available in France.
It is always a good idea to go by your local building regulations, or at the very least use them as a guide.

 

[Nigel]

Hi Richard,I suppose the question is whether it is damp under the boards?If not you should be OK with treated joists and the CTBH.I wouldn,t go less than 22mm though.You don,t mention the joist spacing?

 

[The Woodentop]

Tell him to call in a builder?

 

[richard.selwyn]

Hi Richard,I suppose the question is whether it is damp under the boards?If not you should be OK with treated joists and the CTBH.I wouldn,t go less than 22mm though.You don,t mention the joist spacing?

Seems fairly dry, though as the area is not ventilated and I'm in Normandy there must be a certain level of humidity.

The current joists spacing seems erratic from what I could see when I did the patch - varying from 400 to 700 mm on centre. I was planning 400mm spacing, retreating each joist before laying, placing each joist on a damp proof strip and possibly treating the floor/ground as well.

Thank you for your help Woodentop! (I was tempted to tell him to call in someone else as well, but if I turned down every challenge that came up I probably wouldn't have much of a business or a reputation!)

 

[Nigel]

When you say "the floor" is it wood?Above a "cave"."If this is the case I wouldn't worry about damp.I have a similar job soon in a cafe but I'll use 30mm CTBH at 400mm centres  as it's behind the bar and will see a lot of foot traffic.

 

[jakiiski]

current floor is nailed to joists about 100mm tall which are sitting on the ground. 

That sort of does not sound right? What is the "ground" made of? Sand, gravel, stone or mud? If it's not dry, there is really no material you can use to make it last except stone!
The proper fix would be to remove the "ground" and make sure the floor is really in the air, i.e. there is ventilation below - in Finland the minimum is 80 centimeters (mainly to fit a man crawling/fixing under the floor in case of service).  Any new wooden floor that would be installed there would rot in a few years - how long did the previous floor last?

 

[richard.selwyn]

Thanks for the further replies;  Nigel - my friendly architect also called Nigel agrees with you but suggests putting a damp proof membrane under the joists.

I agree with the Finnish idea of a crawl space, but floors here regularly don't have one - or any insulation  - which I assume in Finland is an essential feature.  The ground seems fairly dry- may be compacted rubble.  The existing floor is probably 50 - 75 years old, has not signs of rot, just the woodworm.

Any other tips?

 

[Nigel]

Ground floor wooden floor joists usually rest on bricks well above the ground in England.As Jaliski says the joists really need air around them.Seems strange to lay them directly on the ground.However as there's no rot in the existing floor......

 

[mattfc]

Is there no floor slab? In the UK I have seen either like another member suggested, a well ventilated crawl space (air brick on the outer walls) plus brick piers upon which the joists lay, or, a DPM is laid with a screed on top (150mm or so) and then joists screwed direct onto the screed, with insulation between the joists, and then ply or chipboard covering. You can also get end caps for the joists to prevent moisture getting in via condensation on the outer wall.

 

[jakiiski]

Also, before putting in any new wood - be sure to get rid of the damn critters - what type of woodworm was it? Please see for example this: http://www.safeguardeurope.com/applications/woodworm.php  - there is probably a french version of a similar document available somewhere but due to my non-existent skills in French I did not even try to locate it. Some woodworm (at least here in Finland) require a certain amount of moisture before they can actually consume the wood (i.e. dry wood is safer)  - so even if there was not rot, one contributing factor to the woodworm might have been the moisture.

 

[richard.selwyn]

Thanks guys - I'm going to put down a dpc I think and then lay new joists on top of that.

 

[The Woodentop]

Hi Richard,I suppose the question is whether it is damp under the boards?If not you should be OK with treated joists and the CTBH.I wouldn,t go less than 22mm though.You don,t mention the joist spacing?

Seems fairly dry, though as the area is not ventilated and I'm in Normandy there must be a certain level of humidity.

The current joists spacing seems erratic from what I could see when I did the patch - varying from 400 to 700 mm on centre. I was planning 400mm spacing, retreating each joist before laying, placing each joist on a damp proof strip and possibly treating the floor/ground as well.

Thank you for your help Woodentop! (I was tempted to tell him to call in someone else as well, but if I turned down every challenge that came up I probably wouldn't have much of a business or a reputation!)

Sorry but my point was if you don't know what to do with something so basic maybe a builder should be called in to protect your customer? I know you have to learn sometime but your question kinda tickled me! Are you new to this type of work?