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Groundwork & Foundations

Best course of action for rotten joists where there isn't proper ventilation?

I'm thinking of buying an old terraced house where the two front rooms, hallway, and bathroom on the ground floor are on suspended timber floors. But the kitchen (at the back of the house) and a cloak room of sorts are both concrete. The floor over the timber is sagging in several places, and after a peek under the floor, it looks like there's a problem with wet rot.

I suspect this is due to poor ventilation. There's ventilation bricks at the front of the house which are all clogged with dirt and barely above ground level on the outside, under ground level on the inside. At the back of the house there's no ventilation bricks at all, and the ground level is about the same as the flooring.

The two main options people have been telling me is either replace all the flooring with concrete, or rip out the concrete and use suspended timber throughout, and of course adding proper ventilation in the second case. I'm not sure exactly how that would be done considering the ground level issues, but I've heard that adding the concrete throughout would be even bigger trouble.

Which is the the better course of action?

EDIT: In response to KO Carpentry&Joinery : I had heard that laying concrete may not solve the damp - that it might move up into the walls? Don't know how reputable that person was, though.

Thanks for your input.

5 Answers from MyBuilder Groundworkers

Best Answer

A lot in favour of the concrete option, which I have to agree with. But, how deep is the void under the floor? If it is 2-3 feet deep, or deeper, then it will take a lot of hardcore to bring up the level to a useful depth to allow a sand blinding, damp proof membrane and 25mm polystyrene insulation to the base and perimeter prior to pouring a concrete slab. This will help prevent cold bridging in the sub-floor. We would normally then install more insulation, Kingspan or Celotex up to 100mm deep then apply a sand/cement screed between 50 and 75mm in depth. This way your pipes and cables can also be run in the screed as long as they are protected.

Alternatively, you could introduce ducting into the existing concrete floors by cutting channels in them, roughly 150-200mm deep and installing pvcu pipes running from the floor void to the rear of the property? Air bricks can then be installed in the outer walls with the ducting running up to the back of the air brick. This allows the free flow of air through the sub-floor. Although this depends on the depth of the existing concrete.

As for damp in the walls, you may have another issue entirely as the ground level outside could be bridging a damp proof course, which sounds likely if the air bricks at the front are just above ground level. Or you may be getting splashback if the damp proof course is lower than 150mm above ground level?

Really need to see your property before embarking on a definitive course of action for solving your issues, however, you have a lot of good pointers here.


Answered 23rd Jun 2012

Hi, if it was me i would lift the floor boards to the rooms with a problem, remove any rotten timbers clean off sleeper walls and make good. Also make sure there is nice gaps for air to circulate between rooms, then i would place dpc on sleeper walls replace floor joists, floorboards. Also make sure airbricks are clean and add some more if need be good luck coin.


Answered 8th Jun 2012

The quickest and simplest solution is most likely to concrete it. Cost is obviously dependant on area and depth of void but it's likely that concreting will also be cheaper than timber flooring. You need to check how your pipework and electric cables are run. If they're under the floor you'll have to pay for a rewire and for the pipes to be lifted and rerun.
What problems have you heard of with concreting? You shouldn't have problems so long as a suitable damp proof membrane is installed and the work is carried out by a reputable builder.


Answered 9th Jun 2012

Im with KO on this.You already have some solid floors that are ok,so wheres the concern? If you are going to get damp problems in the walls,hows a timber floor going to prevent it? I definetly would not be pulling up a solid floor to replace it with timber:) Why not wait to see what the survey throws up and take it from there.Proper advice will tell you if you about to purchase a moneypit:)


Answered 10th Jun 2012

I'm with KO. Concrete all the way, like he said, depends on depth. Surely timber is easier though if you clear the air bricks plus add some more?


Answered 11th Jun 2012

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