Solid Slab or Hollowcore floor slabs??

Brave Harvey

I would like people's opinion on their preference of slab for first floors. What are the benefits of either solution? Are concrete floor slabs a no no? What is the cheapest option? Can you structurally design precast slabs to be supported by the inner leaf blockwork wall, or would it be usual to build the inner leaf on the flat up to first floor for support? I am just getting a feel for it, this will be posed to the engineer but I would appreciate people's experience of the concrete floor slabs. Thanks.

topcatcbr

I think a conc 1st floor is a good idea. You can support on a 100mm inner leaf (not always) but your engineer will advise better.

Solid or hollow will depend on what suits your design best. Each has its advantage. I would also consider block & beam.

Brave Harvey

As far as airtightness goes what would be the best option, solid, hollowcore or block and beam??

hexosan

What are the pros and cons of each as I'm looking into this myself.

kieranhr

Concrete slab is very good on first floor, mostly for acoustic reasons - both for acoustic insulation between living room downstairs and bedroom upstairs, and for acoustic insulation between bedrooms and bathrooms upstairs, as it allows you to build blockwork partitions between these rooms. I would say that 85% of our clients go with concrete first floor. Most slab suppliers are fine with a 100 mm inner block leaf to carry the slab.

You'll get very few builders willing to set up formwork, fix steel and pour a solid reinforced concrete slab in-situ. Block-and-beam is fine, but you usually can't build blockwork on top of block-and-beam. It's mainly used for extensions and renovations, where there might not be access for a crane.

That leaves you with hollowcore or wideslab, and either is fine, so base it on price. Hollowcore is more popular. There is a challenge in sealing the wall-to-slab junction airtight, but it's to do with the gaps between the individual slabs as opposed to the voids within the slabs. It can be overcome by wrapping the end of the slab with an airtight membrane. It has to be a fairly robust membrane to survive the lifting of slabs into position intact.

sydthebeat

kieranhr wrote: »

Concrete slab is very good on first floor, mostly for acoustic reasons - both for acoustic insulation between living room downstairs and bedroom upstairs, and for acoustic insulation between bedrooms and bathrooms upstairs, as it allows you to build blockwork partitions between these rooms. I would say that 85% of our clients go with concrete first floor. Most slab suppliers are fine with a 100 mm inner block leaf to carry the slab.

You'll get very few builders willing to set up formwork, fix steel and pour a solid reinforced concrete slab in-situ. Block-and-beam is fine, but you usually can't build blockwork on top of block-and-beam. It's mainly used for extensions and renovations, where there might not be access for a crane.

That leaves you with hollowcore or wideslab, and either is fine, so base it on price. Hollowcore is more popular. There is a challenge in sealing the wall-to-slab junction airtight, but it's to do with the gaps between the individual slabs as opposed to the voids within the slabs. It can be overcome by wrapping the end of the slab with an airtight membrane. It has to be a fairly robust membrane to survive the lifting of slabs into position intact.

around the midlands wide slab is by far the most popular.. i suppose its down to what is supplied locally. Block and beam is hardly ever used.

Stone Deaf 4evr

Brave Harvey wrote: »

I would like people's opinion on their preference of slab for first floors. What are the benefits of either solution? Are concrete floor slabs a no no? What is the cheapest option? Can you structurally design precast slabs to be supported by the inner leaf blockwork wall, or would it be usual to build the inner leaf on the flat up to first floor for support? I am just getting a feel for it, this will be posed to the engineer but I would appreciate people's experience of the concrete floor slabs. Thanks.

1. Have them in my own house, I'd change many things about my house, but slabs are the one thing I'd insist on having - almost no noise transfer from above. It also lets you build your internal walls at first floor level out of solid block rather than stud partition - again better for noise.

2. The block on flat walls arise out of design issues, the width of your cavity wall, the distance the slabs are spanning, the layout of your slabs - without seeing your plans I couldn't say for sure.

I work with these every day, if you need any more info drop me a PM.

kieranhr

sydthebeat wrote: »

around the midlands wide slab is by far the most popular.. i suppose its down to what is supplied locally. Block and beam is hardly ever used.

Whereabouts in the midlands are you Syd? I'm in Birr myself, and it's mostly hollowcore around here.

sydthebeat

kieranhr wrote: »

Whereabouts in the midlands are you Syd? I'm in Birr myself, and it's mostly hollowcore around here.

Abbeyleix,

Have to admit, in 14 years working I think I recall only 1 hollow core build. Maybe its that the local suppliers don't do them?

topcatcbr

I am interested to see if anyone has done anything to help airtightness. It's not something I have paid special attention to and when it was mentioned I thought of it being a potential issue with new regs in mind.

Brave Harvey

Just from a bit of research, my findings are as follows, open for debate but this is what I came to: the hollowcore slab is recognized as the most suitable for a building a house. These come in thickness of 120mm up and are 1200mm wide, they can span as far as 16.5m! which seems very long. You can build first floor blockwork partitions on these if you wish. I think that they are less expensive than the wideslabs.

The wide slab option whilst leaving a superior soffit finish for ground floor ceiling are only used in cases where there are height restrictions and you would benefit from a narrower floor can go as low as a thickness of 85mm and come in widths of 2.4m. The length they can span usually end at around 7.5m depending on load. I think these are more expensive than the hollowcore? You may build first floor blockwork partitions if you wish.

Then the block and beam, which are used on tricky sites where its difficult to manoeuvre a crane. I thinks it is the least expensive option? I don't think you can build blockwork partitions off them. Would it also be the toughest to airtighten around each beam protrusion into the wall? Requires steel fixings to attach ceiling of ground floor. Anymore opinions would be great.

A fella called fish

If using block & beam for first floor construction then you will need to use polystyrene blocks for safety reasons. Most beam and block manufacturers provide these.

Solid concrete blocks on concrete planks are now only permitted for suspended ground floor slabs (i.e where a block falling during construction will not kill someone beneath!)

Personally I'd go with hollowcore or wideslab. No doubt the project's Structural Engineer will provide further guidance..

self_builder

Any thoughts on whether hollowcore should be insulated or not?

kieranhr

Brave Harvey wrote: »

Just from a bit of research, my findings are as follows, open for debate but this is what I came to: the hollowcore slab is recognized as the most suitable for a building a house. These come in thickness of 120mm up and are 1200mm wide, they can span as far as 16.5m! which seems very long. You can build first floor blockwork partitions on these if you wish. I think that they are less expensive than the wideslabs.

The wide slab option whilst leaving a superior soffit finish for ground floor ceiling are only used in cases where there are height restrictions and you would benefit from a narrower floor can go as low as a thickness of 85mm and come in widths of 2.4m. The length they can span usually end at around 7.5m depending on load. I think these are more expensive than the hollowcore? You may build first floor blockwork partitions if you wish.

Then the block and beam, which are used on tricky sites where its difficult to manoeuvre a crane. I thinks it is the least expensive option? I don't think you can build blockwork partitions off them. Would it also be the toughest to airtighten around each beam protrusion into the wall? Requires steel fixings to attach ceiling of ground floor. Anymore opinions would be great.

Pretty much. To fill out a few more details - most common hollowcore size for residential is 1200 x 150 thick, with levelling screed on top, which will span up to 5 metres comfortably with block partitions. Most common wideslab size is 100 mm thick, but usually needs 150 mm of structural screed, plus propping during construction. Basically your floor is going to end up around 250 mm deep either way. May as well go with the cheaper option, which depends hugely on your exact location, as myself and Syd just found out.

kieranhr

self_builder wrote: »

Any thoughts on whether hollowcore should be insulated or not?

I won't shoot it down, but I personally can't see any thermal or acoustic benefit in insulated hollowcore. Thermally it has a heated surface either side of it (i.e. ground and first floor of house) but is full of cold-bridges in any case, and acoustically the slab is more than adequate. I think the insulation is purely used as a void former during construction.

True enough, it stops airflow through the hollowcore voids, but a couple of hours work filling the ends with a can of expanding foam will do the same job.