Patio above ground - Beam & block floor

[Paragraph]

Practical advice needed. I am designing a 'solid' patio to replace a timber decked area. The decking is about 1 metre above the ground, on suspended timber joists and sleeper walls. The new patio will need to be at the same level (1000 mm above ground) for a bit, then step down to an area about 550 mm above ground. I presume that raising the ground level by fill in both these instances would result in excessive settlement. I am therefore contemplating using a suspended concrete beam and block floor on sleeper walls. Has anybody out there done such a thing? What thickness of topping should I put on the floor before laying the slabs? Should I avoid screed in favour of concrete for the topping (frost action)? Do I need to ventilate the void under the floor? Should I design in some sort of movement joint where the suspended floor meets the ground bearing patio slabs, if this is in the middle of an area of slabs?
Advice will be gratefully received.

[Dave_L]

err???!! :rock:

[lutonlagerlout]

m8 you must have a lot of spare dollar if you want to do a block and beam floor for a patio
build the retaining walls, yes 225mm will be fine,then fill it 100mm at a time with type 1 MOT and wack each layer
20 tonne = 10m 3 approx of type 1 costs about £400
continue this up to your finished height
you could probably used crushed concrete for the first 500mm which is cheaper still
LLL

[Suggers]

A section drawing & photie would be handy - sounds like you need a properly engineered solution?

[Paragraph]

you could probably used crushed concrete for the first 500mm

I thought that any fill over say 500 was liable to settle. Building Control round here insist on suspended floors in houses where ground is this low, so why change this principle for an outside area?

[Tony McC]

You can have a suspended B&B floor, but as LLL says, prepare for a serious assault on the pockets.

Once the platform is in place, the usual spec is DPM then cementitious screed then flags. You have to take the drainage into account, which is why some designers are now using pedestals to carry the flags rather than a laying course, but as I don't know enough about your site, I can't say which is more suitable.

[lutonlagerlout]

suspended floors are used because they are fast and also in areas where heave is possible
we are working on some barns at the moment and the footings are 2m by .6 with clay boards and suspended floors===> all because the ground is really bad
IMO its overkill for a patio
if you wack the type 1 in 100mm layers settlement is very unlikely

LLL

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Hope this sketch helps. Upper level about 1000 mm above ground, middle level about 550 mm above ground.
I think the ground might be clay. The house is beam and block throughout, and I do know that a neighbouring house has had settlement problems in its (1970's) ground bearing floor slab. 20 mm vertical gap between carpet and skirting in one corner. So, does this point to beam and block for this patio then?
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[seanandruby]

Seems like you have your heart set on b&b, against all the advice about expense etc. May i suggest you do it your way, as we try to keep the budget as low as possible, because we do this for a living and hope to come out in profit.

[Tony McC]

I can't see a logical and sound argument for the use of B&B on that project but you should take the advice of an engineer that has seen the site and studied the ground structure report.

B&B does not guarantee no problems with ground movement. Good site prep does that!

[Paragraph]

I've spoken to a structural consultant. Having taken note of your comments, he's suggested lean mix concrete to fill up the 1 metre (next to the house), as whacking might put too much sideways pressure on the house wall. He also suggested 'pudding stoning' with conc blocks as the concrete goes in. Or Type 1 sub base mixed with a small amount of cement without too much whacking. Any comments out there?

[Mikey_C]

If you have advice from a local structural engineer who is familiar with site conditions and current building methods then he is much better placed than anybody who cannot see the site and has limited experience of the local ground conditions.

However, if you would like my humble diyer's opinion, a meter deep lump of lean mix, with concrete blocks thrown in still sounds overkill in cost and effort for a patio, regardless of ground conditions. There is very good chance that 1m deep type 1 or crushed concrete whacked in layers will not subside or suffer with clay heave, and if it should the worse outcome will be remedial works to part of the patio.

I also can not see the action of whacking the stone next to the house wall, having any effect on it.

Without wishing to question the advice that this structural engineer has given, I am sure he has considered the effect that the large mass of concrete would have of the house wall and the ground below?

[GB_Groundworks]

ok first of all your structural consultant seems like an idiot.

pudding stoning, no genuine contractor would contaminate concrete with possible weak points by throwing in a few concrete blocks.

type 1 with cement springled over it? did he ride a horse this guy. if he was talking about a cbr then maybe.

your block and beam is still going to have to sit on structural walls, so whats going to stop their footings subsiding and taking the patio with them.

looking at your sketch your going to have to tank the side wall of your house what ever you do to stop the damp.

depending on your ground a decent strip footing into good ground might end up being deep, or if its really bad i'd auger it and concrete pile it, top beam then come up with your walls.

block and beam it if you want, its overkill and expensive.

i'd use 7 newton block laid flat for all the walls and steps filled with type 1 compacted in 150mm layers, starting off with good firm ground



Edited By GB_Groundworks on 1252357743

[lutonlagerlout]

i give up on this one lads

LLL