concrete slab under ventilated wood beam floor DPM ?
- Thread starter 46philh
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Are you referring to an oversite concrete cover, then a ventilated subfloor space/crawl space, and then suspended floor joists.
If the answer to above is yes: then no, you dont have to lay a membrane, and then pour the oversite concrete over the membrane.
Its often best practice, or a remedial requirement, to lay a membrane (aka a moisture barrier) over the site concrete.
If the answer to above is yes: then no, you dont have to lay a membrane, and then pour the oversite concrete over the membrane.
Its often best practice, or a remedial requirement, to lay a membrane (aka a moisture barrier) over the site concrete.
I would definitely lay a moisture barrier if there was only a 6" space. I think 6" is the Bldg Regs min. clearance for the above situation.
Solid floor kitchens are vulnerable due to lack of cross ventilation - typically, where the adjoining floor has been converted to solid.
Solid floor kitchens are vulnerable due to lack of cross ventilation - typically, where the adjoining floor has been converted to solid.
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Key point with a concrete oversite is that it should be higher than the highest point of the external ground level, or if not have a drainage system.
If not you could get ponding, whether or not there a membrane in place.
If not you could get ponding, whether or not there a membrane in place.
For what its worth, I think Bldg Regs still calls for the oversite soil level to be higher than the external ground level. But that was for wood suspended floors.
It seems to have been a rarely followed Regulation.
I have seen moisture barriers floating on water ponded over the oversite. But, because they were lapped at joins and sealed to concrete foundations, they served their purpose of protecting the flooring above.
I've also seen wet oversites with non-sealed moisture barriers that still protected the flooring. Far better than nothing.
It seems to have been a rarely followed Regulation.
I have seen moisture barriers floating on water ponded over the oversite. But, because they were lapped at joins and sealed to concrete foundations, they served their purpose of protecting the flooring above.
I've also seen wet oversites with non-sealed moisture barriers that still protected the flooring. Far better than nothing.
Yes, Building Regs today expect the site concrete to be higher than the adjacent ground level, otherwise some form of tanking or drainage is required.
But many older houses (mine included) have site concrete a good foot or so below adjacent ground level. And if the ground is reasonably dry and free-draining, and there are sufficient air bricks, there's usually no problem.
Perhaps the relatively large volume of air helps dispalce any vapour rising through the concrete?
Personally, I'd be a little doubtful about putting any impervious membrane on top of a concrete oversite slab in that situation. When people put vinyl and lino coverings on concrete cellar floors, water tends to pool under it and starts to smell.
But many older houses (mine included) have site concrete a good foot or so below adjacent ground level. And if the ground is reasonably dry and free-draining, and there are sufficient air bricks, there's usually no problem.
Perhaps the relatively large volume of air helps dispalce any vapour rising through the concrete?
Personally, I'd be a little doubtful about putting any impervious membrane on top of a concrete oversite slab in that situation. When people put vinyl and lino coverings on concrete cellar floors, water tends to pool under it and starts to smell.
I never worked on a site in the UK where a DPM was put over the top of a concrete oversite. If it was lower than ground level, water would come through the foundation brickwork or blockwork and on top of the oversite anyway. As long as it's higher than the external ground level water won't pond on it.
tony1851,
Exactly, ventilation is the factor that will disperse the moisture vapours. And your situation is extremely common in my limited experience.
stuart45,
i have worked on jobs where remedial moisture barriers were applied across the oversite - soil or concrete.
It was often scheduled as one item in a Damp & Timber remedial solution along with added air bricks etc.
Your claim that "water would come through" etc. isn't the case in practice. Surprising, but true. If it wasn't the case the majority of suspended floor oversites would be flooded most all winter.
And, FWIW, forums like this would be flooded with posts asking how to deal with the issue.
Exactly, ventilation is the factor that will disperse the moisture vapours. And your situation is extremely common in my limited experience.
stuart45,
i have worked on jobs where remedial moisture barriers were applied across the oversite - soil or concrete.
It was often scheduled as one item in a Damp & Timber remedial solution along with added air bricks etc.
Your claim that "water would come through" etc. isn't the case in practice. Surprising, but true. If it wasn't the case the majority of suspended floor oversites would be flooded most all winter.
And, FWIW, forums like this would be flooded with posts asking how to deal with the issue.
Water does come through the foundation walls when the water table is high enough. I worked in the trade since 65 and have seen plenty of cases where there was flood water below the floor, coming through the walls. Most people don't realise what is below the floor. If the oversite level is below ground level regs call for a drainage system, not a DPM.
In the UK we have masonry walls for foundations, not concrete like in the USA.
In the UK we have masonry walls for foundations, not concrete like in the USA.
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