Replacing damp concrete floor with wood.

[InsomniaCalls]

Hi there,

I'm new to this so go easy

The survey on my new house reported damp in the concrete kitchen & bathroom floor and suggested digging up, laying damp proof membrane then re-laying concrete.

Rather than re-lay concrete, I'm going to put a wood floor down to make these rooms consistent with the rest of the house and allow me to run the plumbing for these two rooms under the floor (rather than around the skirting as is the case currently).

I've had a little go at the concrete already and it looks like a fairly thin layer of self-levelling compound then about 3 inches of concrete on top of rubble (it came apart with a few hits of a club hammer).

Assuming I use 2"x6" joists, what is the minimum depth I need to dig the floor to? Once the floor is up do I still need to lay a damp proof membrane before I install the wooden floor?
I'm planning on putting a few air bricks in for ventilation in case that makes a difference.

Thanks for any help in advance.

[stuart45]

It's usually the other way round for kitchens and bathrooms, replacing timber with concrete. However you will still need to concrete an oversite under the timber floor, and then have a minimum clearance of 75mm to the bottom of the plate and 150mm to the bottom of the floorboards. If you build some sleeper walls you can reduce the size of the joists.

[leebwk]

Hi,

Basically it's what stuart has said you still need to hardcore dpm and oversite but obviously deeper down, personally i would bite the bullet and get the floors sorted with a dpm and new oversite, otherwise this will come back to haunt you

[InsomniaCalls]

Thanks guys.

So I'd still like to have a wooden floor but I hear what you're saying about risking damp returning.

If I dig deeper and lay concrete below the flooring how thick should each layer be?

Hardcore then DPM - how much concrete?
Then presumably if I leave 75mm of space between the concrete and the bottom of the joists I should be OK?

Thanks again.

[stuart45]

The oversite needs to be 100mm and above ground level unless laid to falls to a drainage outlet.
You will also need to insulate the floor between the joists.
The air bricks need to be correctly positioned to be effective.
Are you 100% sure the survey was accurate. Damp meters can give false readings on concrete, and also in a kitchen and bathroom the floors are often damp through other reasons than rising damp. Surveyors doing an HSV usually only briefly check the damp and don't really have time to fully investigate it's cause.

[InsomniaCalls]

Pretty sure - the house was empty for a while before the survey so it's unlikely that any moisture would be to do with the kitchen/bathroom being used.

The floor definitely needs to come up (if for no other reason than it's not level and is crumbling in lots of areas) and I'm not keen on the lack of flexibility concrete gives me with regards running pipes etc. so I'd really prefer to put wood down.

[stuart45]

If the concrete floor is shot, and you really want a timber one, then just follow the guidelines for a modern suspended floor. It's important to get it right as dry rot is a common problem with damp suspended timber floors.
Bathrooms and kitchens are especially at risk.

[InsomniaCalls]

Great, cheers.

[stuart45]

Then presumably if I leave 75mm of space between the concrete and the bottom of the joists I should be OK?

Thanks again.

It's 75mm min to the bottom of the PLATE. 150mm min to the bottom of the JOISTS. I made a mistake in the post saying 150mm to the bottom of the floorboards.

[InsomniaCalls]

Apologies (I'm dense) - can you just confirm that the plate is the thing I'm fixing the joists to?

Also, I'm looking at this guide for laying a concrete floor. I assume I can just follow this guide but at a greater depth (and excluding the screed)?

[stuart45]

Apologies (I'm dense) - can you just confirm that the plate is the thing I'm fixing the joists to?

Don't worry, I'm worse saying 150mm to the bottom of floorboards.
The plate is a piece of 4x2 or 4x3 that sits on top of the sleeper walls. These are honeycomb brick walls built close to the external walls off the oversite. The joists must be fixed with a clearance from the external walls, so that they are not in contact with damp brickwork. You should also put a DPC under the plate, which is bedded on with mortar.

[stuart45]

Don't put any insulation in either, as any heat getting through the timber floor and it's insulation will be lost because of the ventilation.

[InsomniaCalls]

Excellent, thanks for all the help.

[stuart45]

No problem. Remember to make sure that the top of the oversite will be above the highest ground level. Water always finds its own level, and you don't want ponding on top of the oversite, which will create a damp atmosphere.