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Building up sub-floor

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Marineboy

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My son has a small utility room attached to the rear of his 1960's house. It is about 2m by 2.5m. The floor is springy and uneven. We lifted the flooring the other day and found that there is a concrete sub floor (which is bone dry) on which the previous owners placed timber 'joists' about 100mm deep on which the chipboard flooring sits. This to bring the floor level up to the kitchen. The reason for the springiness is not only some rot in some of the timber but also the fact that one corner of the subfloor is not level with the rest.

I am thinking that if we just replaced the joists we would still be left with the uneven sub floor and so need to use packers etc to level up the joists, which seems a bit of a faff. So, the thought occurred to dispense with the joists and simply bring the floor level up by mixing some concrete and pouring it over the subfloor. Would such a relatively small depth of fresh concrete (100mm) work? Is such an amount ( I work that out to be 0.5 cu metre) feasible to comfortably hand mix? And what should the mix be, eg should it incorporate aggregate or just sand/cement?
 

MikeG.

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Whether you can use concrete or not will depend on what the detail is at the junction of the walls and the floor, and whether there are services under the floor. If there are masonry walls with a DPC connected to a DPM, and the new floor won't be bridging the DPC, then you should be OK. If that is the case a normal concrete would do, or if you wanted certainty of levels and flatness, you could use a screed.

It's actually as simply as anything to pack up a timber floor to correct levels, and far less work than laying a new concrete floor. If the sub-floor is dry, then any rot in the timber is likely to be as a result of lack of ventilation of the void, and thus that should be the first thing you address if you decide to renovate the existing floor.
 

Marineboy

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Cheers Mike, there are no services under the floor as far as I can see, and I don't think any new concrete would breach the DPM.

However, your comment re the feasibility of packing up a new timber floor has made me rethink that option. There is no subfloor ventilation so I can easily knock a couple of air bricks in the wall.
 

MikeG.

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Bear in mind that if the joists rest on the concrete that there is no airflow underneath them, and therefore each little strip of void between joists is independent and isolated. Ideally, as they should each have some ventilation, there should either be an airbrick between each pair of joists, or there should be some holes cut into the timbers to allow air to move between the otherwise isolated gaps. Either way, the more ventilation the better.
 

Marineboy

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Thanks Mike. I don’t fancy knocking out an air brick for every inter joist space so I’ll go with the holes in the joists and just an airbrick at each end.
 

Rorschach

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Ventilation for what? The floor underneath is dry, there is no need to ventilate, that will only cause problems. People are obsessed with ventilating spaces that simply do not need it.
 

MikeG.

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You either ventilate it or you fill it. Un-heated and un-ventilated voids (against an external surface of the building) are classic places for condensation. Besides, this is a utility room. Therefore water will be spilled, and will find its way into the sub-floor void, and if it can get in, it needs some way of getting out again. Those timbers have rotted for one reason only: moisture.
 
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