Foundations needed for brick walls?

[wayt43]

I am building some internal partition walls inside a bungalow, they will not be supporting anything apart from their own weight and anything hung on to them. I plan to construct these out of brick (preferred) or concrete blocks. The bungalow is a 1970s build and has a solid concrete floor slab (not insulated) about 150-200 mm thick.

Can these walls be safely built directly on top of the slab, or will some type of footing or other foundation be required?

There were some existing walls in the property that I have knocked down (all confirmed as non-structural) and these were constructed out of a mixture of bricks, cement blocks and some type of hollow terracotta blocks. I drilled some test holes vertically downwards directly over where some of these walls were, and from what I can tell the floor appears to be the same thickness there.

 

[motorbiking]

Why not build a timber stud wall? You could always double plasterboard it, if you think its too flimsy

 

[wayt43]

Better water resistance and being able to hold a coat of tanking slurry (one side will be a wet room), fire proofing, noise proofing, general solidness and so on.

Also none of the other partition walls in the building are stud (not that this means anything necessarily), they are either solid brick of made of this weird brick/block/terracotta mixture.

 

[Dereekoo]

If the floor slab is resting on solid ground beneath it there will be no issues, assuming the wall is 2.5m high the force on the concrete slab would be 0.05 N/mm2 ( the strength of the slab will conervatively be about 15N/mm2 and probably a lot more. The force on the underlying ground will be less than 0.01N/mm2, ground considered pretty carp is capable of taking a safe bearing pressure of 0.05N/mm2.

 

[RonnyRaygun]

Non load bearing walls don't need a foundation. It's not uncommon to see non load bearing block walls built off double joists (or even floorboards spanning between joists) in older properties.

 

[^woody^]

Non load bearing walls don't need a foundation. It's not uncommon to see non load bearing block walls built off double joists (or even floorboards spanning between joists) in older properties.

True they don't need a "foundation", but whatever it's supporting the wall needs to be able to take the load somehow!

Shear failure is a risk with any loaded slab, especially if the slab is unreinforced, and more so if the fill below it is poor.

 

[RonnyRaygun]

Well, maybe, but as the concrete is 150 to 200mm thick, it would easily span half a metre assuming it's tensile and shear strength are around 10% of its compressive strength (and that's assuming a weak C8/10 mix), so I wouldn't be too worried.

 

[Deleted member 174758]

Better water resistance and being able to hold a coat of tanking slurry (one side will be a wet room), fire proofing, noise proofing, general solidness and so on.

Stud wall with cement fibre board on one side (no need for tanking), mineral wool infill between the joists and 12mm soundblock PB on the other side. A lot lighter than masonry and meets your other requirements

 

[wayt43]

I'm back, picking this project up again after a bit. I am now thinking about using medium density concrete blocks which are roughly twice as dense as aerated blocks.

Because I am a bit OCD I did a bit more checking of the existing subfloor. The top 50 mm or so seems to be a relatively loose screed. In one area where it can be seen in cross section it has a grainy, sandy texture, which isn't obvious when you are walking around on top of it. I can also see the odd piece of embedded wood shavings or other rubbish like it was poured at a relatively late stage of construction. When drilling through, it is now quite obvious that the stuff below it is much denser and harder.

Would it be safe to build a block wall on top of this screed? Or, should I dig out a trench from it, deep enough to expose the strong concrete and say 200 mm wide, and fill with fresh concrete, building the wall on top of this?

 

[Mr Chibs]

Your screed should be sat on a concrete slab, 100mm+ (just seen 150-200).

Are you worried about sound proofing through the rooms, is that why lightweight blocks are out?

I suspect you'll be ok with medium blocks, in my previous house, I had a single storey brick wall built off a joist/T&G floor, so you'll be fine.

 

[wayt43]

Yes, underneath the screed there is at least 100mm of hard, dense concrete that takes a long time to drill through compared with the screed. What I'm wondering is should I peel off the top 50 mm of screed and replace it with harder concrete, before building the wall on top? Should be quite an easy job to peel off a strip of screed with angle grinder and SDS chisel, but I won't bother if there is no point.

Also Chibs as to your question yes my reason for going with medium density blocks instead of aerated blocks is mainly for sound proofing. IIRC a single skin of 100 mm medium density blocks meets the Part E requirements with or without render/plaster, aerated blocks won't. Others have made (perhaps valid) recommendations for stud walls but I have my reasons for not wanting to use these in this particular instance, partly preferential.

 

[Mr Chibs]

If you were going to take the screed up (I wouldn't) you could just build off the the slab, no need to put anything back.

That said, how long are these walls? There is quite a difference in weight between lightweight/medium blocks.

Is it the odd metre here and there on longer?

 

[wayt43]

The wall will be just under 2 m long and tied at both ends, one of which is a structural wall which surely must have its own footing. I will use a screw in wall starter on that end however, which probably doesn't transfer much load. The other end is a nonstructural wall that I will tooth in.

I originally thought about doing what you said (laying the blocks inside the trench formed by removing the screed) - the reason I then thought about infilling with concrete before paying the blocks was to ensure the slab was at least 150 deep at that point. That said - where I have removed walls in other locations the blocks are resting on the hard concrete, not tne screed.