Precast vs Slab or other options on ground floor

briaineo

Hi

Just a question on the ground floor of a selfbuild.

At the costing stage and not sure what the best option is for the gound floor slab is: the area of the ground floor is 220m2

500mm of back fill comes in around 3k and the 150mm concrete for the slab comes in around 2k + vat so 5.5k in all.

I going with a precast 1st floor (structural screed not required). This comes in around 7.5k all in.

Thinking possibly a block and beam but was hoping to avoid a structural screed.

Any other methods that come to mind?

Thanks

Eoin

galwaytt

... I see a lot of Block & Beam in the UK, and imho it is a horrible system to put under a house. You can't but big loads on it, is a sieve for airtightness, and offers no fixity (e.g. for drilling into for columns etc). Imho it's only redeeming feature is it's low cost.

150mm slab to GF seem very light for carrying a precast slab to FF - have you had this checked by an engineer ??

Imho a structural slab (raft) to GF is the best foundation there is.

Mahogany Gaspipe

Why wouldn't a150mm ground floor slab suffice? It's thickness has no limitations to using precast concrete flooring on the first floor.

Calahonda52

any insulation?
34 euro/sq m for precast floor installed? u sure?

galwaytt

Mahogany Gaspipe wrote: »

Why wouldn't a150mm ground floor slab suffice? It's thickness has no limitations to using precast concrete flooring on the first floor.

150mm to support all GF walls, FF precast slab, FF walls, and roof ?

I'm no S.E. but I've never seen a load bearing strip that thin on GF - but I'm happy to be corrected.

oppenheimer1

Mahogany Gaspipe wrote: »

Why wouldn't a150mm ground floor slab suffice? It's thickness has no limitations to using precast concrete flooring on the first floor.

Are you planning on coming off the 150 slab with the rising walls? Are you thickening the edges?

Mahogany Gaspipe

The OP is presumably going with a strip foundation, I hope they are at least with that spec!

briaineo

Sorry maybe I wasn't clear yep going for the standard strip foundation probably 900mm x 400mm.

Nearly sure the quote for the precast case in at €34m2 plus vat installed but the upstairs is a smaller floor area. So will recheck that one.

It was only that someone mentioned to me that if you doing a large area sometimes te slab route works out more expensive due to the fill required.

Was just wondering if there were other options that's all

Thanks for the replies

Metric Tensor

You could do a suspended hollowcore floor - just like the hollowcore upstairs. Whether it is cheaper than back filling depends completely on the depth of fill you need. Also bear in mind that if you go this route your rising walls become retaining walls.

The are both advantages and disadvantages to having a void beneath your ground floor.

Haven't seen beam and block in any Irish building under 25 years old. I'd echo galwaytt's thoughts on it!

BryanF

Insulted Suspend timber floor

Not convinced it'd be any cheaper given the detailing out of thermal bridging/air-tightness and labour costs of all the layers but it would be lower co2 option.

briaineo

Hi

Leaving out the monetary aspect.

Could someone explain the advantages/disadvantages to a precast ground floor?

I am a bit unsure of the full process compared with the poured slab route.

I see that a radon barrier isn't required if the void is vented.

I have read that some people add some back fill others don't.

Is a vapour barrier required at ground level base?

I see it mentioned that the internal walls become retaining walls.

Is it possible to avoid this if the slabs were resting on an area other than the internal wall leaf.

Thanks

Metric Tensor

It was me that said they External walls would become retaining walls.

What I meant was that if you plan on having your footpath at or just below finished floor level then the rising walls that hold the slabs will be holding back the ground that is outside them. To prevent this happening you either need to fill inside them too or not fill outside. By not filling outside you will require a long ramp up to your front door.

briaineo

Thanks, the idea would to have a few steps up to the front and will put a ramp at rear up to the back door.

With the precast ground floor is it the same process as per 1st floor or would the internal leaf of blocks be usually on the flat?

Would I be correct in saying:

Internal leaf built up to desired height on flat or not?

Slabs rested on internal leaf, with a membrane wrapped around ends: seen a post where felt was used because of its durability.

Insulation layed on top of the slabs and a screed poured over this?

Void below vented to allow radon gas to escape.

Am I missing anything? Is there a damp membrane required at base of void?

Metric Tensor

Your ramp has to go to the front door.

Your engineer will tell you which blocks to put on edge and which ones to put on the flat.

Your build-up sounds like it's heading in the right direction but you need to be careful with all the details. I haven't seen felt used instead of membrane myself. What do you propose to do with the lower end of the membrane wrapped around the slabs? You can't leave it just flapping there.

I would always recommend a radon barrier regardless.

Nobody here knows your site or specific house details well enough to tell you where the DPM needs to go and how to form the void. Just ask your engineer to design it all when he/she is doing your foundations and give them some guidance on the way you'd like it put together.

Don't forget you'll need internal rising walls too.

hoodrats

briaineo wrote: »

Hi

Just a question on the ground floor of a selfbuild.

At the costing stage and not sure what the best option is for the gound floor slab is: the area of the ground floor is 220m2

500mm of back fill comes in around 3k and the 150mm concrete for the slab comes in around 2k + vat so 5.5k in all.

I going with a precast 1st floor (structural screed not required). This comes in around 7.5k all in.

Thinking possibly a block and beam but was hoping to avoid a structural screed.

Any other methods that come to mind?

Thanks

Eoin

why not just put in a simple raft foundation and be done with it ?
no wrapping felt or membranes or any of that kind of nonsense and might work out as cheap.