Hollowcore Floors + Block Work + Foundations

YourAverageJoe

Hi,

Digging foundations this week. Have been told by the floor people that 4" block on its edge is fine for holding the hollowcore floor but i am not convinced and think it shouild be a 4" on the flat. Any thoughts?

Also, should the foundations be bigger to support the extra weight.

Thanks

d o'c

really depends of the design.You should consult an engineer..


4" on the edge is common enough. block on flat where 2 slabs will meet.

I went overkill in some rooms with large spans, and build block on flat, where on the edge was enough. Makes door frames wider, but this doesn't bother me.

Foundation depths, again verify with an engineer.

If widening rather than going deeper, don't go too wide to the outside of the outer leaf.
This will make placing downpipes awkward, and allow at least extra "8 in width if you are building stonework after.

RKQ

Strip foundations are usually fine, subject to Engineer spec.

Conc hollow core slabs have other extra expenses too:-

• Steel lintols on all window and door opes (inner leaf) over a certain width.
• Steel lintols over certain internal door opes.
• Column or steel hangers around stair ope.
• 215mm wide wall in centre of structure, to support 2 slabs.

1200mm wide is the European standard width for 150mm thick hollowcore. 2400mm is available as a solid slab but this has a much heavier weight and therefore cost implications.

Airtighness at wall / junction may become an issue - how do you air seal the hollow voids? These voids are usually plastered but the face into the cavity, technically IMO air from the cavity could blow into the hollows.

IMO its best to seek Professional advice from your Engineer.

YourAverageJoe

1200 and 2400 are these the slab widths?

RKQ

Yes, they're slab widths. They usually make up narrower slabs to fit the dimensions of your floor.

On a current job I'm installing steel I beams over windows which is good as it turns out 4 narrow slabs will sit directly over the centre of external opes. IMO pre-stressed lintols wouldn't be safe.

Consider a 200mm thich hollowcore if considering ufh on 1st floor.

Best ensure your Certifier is fully aware of your plans and happy with all structural considerations.:)

d o'c

my slabs laid early this year
200mm hollowcore, these slabs are 1200mm wide, 150mm is available too.

There is only a few euro/m2 price difference between 150mm and 200mm.

I went for 200mm all round, there are areas where a 150mm would have been ok, but I wanted a uniform area.

where it doesn't work in for full width slabs, they are cut along their length prior to arrival on-site.

The longer the slab the more upward camber they have. (prestressed)


If on site when slabs are being laid, sweep the blockwork of 'crumbs' before the slabs are laid, they will sit more even.
and when being grouted, ( if going with a 'wet' fix)ensure that all 'drips' through to the rooms below, through the gaps are cleaned up.
usually the grout has a retarder added to slow curing, so you have a day or so to clean up.

My slabs were fitted with a remote controlled crane from the lorry, very accurate and safe.

some final adjustments can be made by sliding the slabs slightly with a crow bar.


I cut holes in the blocks from room to room to allow for running wiring thorough later, up at the very top of the wall. the slab bridges over these holes.
much easier then breaking through later.


I will be closing cavity facing open cores, to stop the pumped insulation entering later.

YourAverageJoe

Great info guys, thanks.