Piling details - Reinforced Slab

[tscalfa]

Hi everyone, I'm after a bit of advice please. I'm having an attached garage built which due to the ground conditions and proximity to conifers means I've got to have a pile reinforced slab.

 

1. What is the standard practice for the damp proof membrane, should it go under both the slab and the ring beam as shown by the red line in diagram 1, or should it go between the slab and ring beam as per the red line diagram 2, or is there a third way it should be done? 
2. Does the ring beam need to be tied in to the suspended slab as per diagram 1 or is the slab just sandwiched in to the inner leaf in a similar way to a block and beam floor as per diagram 2?

[LSB]

I wouldn't think you want DPM under cellcore, the whole point of it is to crush if the ground heaves.

We have cellcore and in the recent weather we flooded and if the DPM had been under the cellcore then it would have become a swimming pool with no where for the water to go to.

In fact we have the DPM over the slab and Cellcore with the insulation and screed on top.

Where is your floor insulation going.

out cellcore sits inside the foundation with the suspended reinforced slab resting on the foundation.

 

But, ours is a whole house, maybe a garage is different.

 

[kandgmitchell]

Diagram 2 is going to be easier to damp proof. Diagram 1 would be a nightmare. Check with your engineer that the slab is designed to span off the inner leaf of blockwork and get the blockwork strength confirmed. The dpm should be under the slab because as mentioned the clayboard is crushable in case of heave.

[tscalfa]

On 14/11/2023 at 19:54, LSB said:

I wouldn't think you want DPM under cellcore, the whole point of it is to crush if the ground heaves.

We have cellcore and in the recent weather we flooded and if the DPM had been under the cellcore then it would have become a swimming pool with no where for the water to go to.

In fact we have the DPM over the slab and Cellcore with the insulation and screed on top.

Where is your floor insulation going.

out cellcore sits inside the foundation with the suspended reinforced slab resting on the foundation.

 

But, ours is a whole house, maybe a garage is different.

 

Yes that's a good point about the DPM needing to go above the cellcore. With it being a garage floor I'm not planning to have a layer of screed particularly as the floor will have point loads from trolley jacks etc. You can get cellcore with a layer of insulation though so I may still be able to have an insulated floor.

 

Is your slab tied in to the ring beam or does it just rest on top of it?

[tscalfa]

23 hours ago, kandgmitchell said:

Diagram 2 is going to be easier to damp proof. Diagram 1 would be a nightmare. Check with your engineer that the slab is designed to span off the inner leaf of blockwork and get the blockwork strength confirmed. The dpm should be under the slab because as mentioned the clayboard is crushable in case of heave.

I can see why the 2nd one would be easier but I've seen conflicting information on whether the slab needs to be tied in to the ring beam or not.

[LSB]

16 minutes ago, tscalfa said:

Yes that's a good point about the DPM needing to go above the cellcore. With it being a garage floor I'm not planning to have a layer of screed particularly as the floor will have point loads from trolley jacks etc. You can get cellcore with a layer of insulation though so I may still be able to have an insulated floor.

 

Is your slab tied in to the ring beam or does it just rest on top of it?

we have a reinforced 200mm concrete slab with 2 level of mesh.  The cellcore goes 'in the hole' sitting on top of packed hardcore and blinding sand. 

The mesh then sits on the foundation trench (all 4 sides) with castles to keep the layers apart and the concrete poured on top.  The bearing is 200mm with the cavity wall outside it, foundation is 700 wide.

We then have 200mm insulation with screed on top, so total floor depth is 550mm, so very deep.

We have a DPM above the slab and below the insulation with a VSL above the insulation below the screed (with UFH).

 

this was all specified by our structural engineer, over engineered according to our BCO for a single storey property, but as that's the spec he says we must have it.

 

so, it's not tied in as such, but when the slab is poured it will adher to the foundation.

 

 

Edited November 15, 2023 by LSB

[saveasteading]

Definitely not as 1).

The likely design is that the pile is left high, then cut back with the reinforcement tying into the edge beam. Dpm would interfere with bonding.

No dpm is necessary as the concrete is (make sure it is)  very dense.

 

Neither option shows any insulation. Anyway, under the slab and tucked into the wall seems right to me.

[Gus Potter]

On 14/11/2023 at 18:11, tscalfa said:

Hi everyone, I'm after a bit of advice please. I'm having an attached garage built which due to the ground conditions and proximity to conifers means I've got to have a pile reinforced slab.

Do you also have a money tree growing in the garden?

 

Have you asked you SE about just how much movement can be expected from the adjacent trees?

 

What about building the garage on a 150mm thick or 180 - 190mm thick concrete flat structural slab with mesh that floats about and has a movement joint between the house and the garage structure? Yes I know the 150 or  180 - 190mm sounds like an odd number but there is a technical code compliance reason for this.

 

There are few technical details associated with this but it seems to me you might be using a sledgehammer to crack a nut and burning cash going for piles...

 

Is your site on a slope? Which way does it face.. as we look at prevailing winds and where the roots of the tree will grow.

 

Has your SE been paid enough to find the most economic solution? I'm not kidding you here.. with a bit of thought you could have a slightly thicker concrete slab that moves about but still strong enough to stop the walls from cracking.. it's worth asking the question.. imagine if you found out later you had chucked a load on money away on blindly following BC / standard guidance and put in piles.. all for a garage!

 

Now that is an odd question as a Client would expect that the SE fee would deliver the most economic solution.. but.. that is not the way it works.. as an SE will need to spend more time finding the more economic solution.. but that set against the cost of piling! Now I may seem to paint SE's in a bad light..

 

But there are folk on BH who think paying an SE or an Architect more than a grand is a waste of time as they know better.. well to those all I can say is.. the door swings both ways you smarty pants!

 

 

 

 

[tscalfa]

19 hours ago, saveasteading said:

Definitely not as 1).

The likely design is that the pile is left high, then cut back with the reinforcement tying into the edge beam. Dpm would interfere with bonding.

No dpm is necessary as the concrete is (make sure it is)  very dense.

 

Neither option shows any insulation. Anyway, under the slab and tucked into the wall seems right to me.

 

If I go for an insulated slab I'm considering Cellcore HX plus where the insulation is included with the as part of the void former.

 

17 hours ago, Gus Potter said:

Do you also have a money tree growing in the garden?

 

Have you asked you SE about just how much movement can be expected from the adjacent trees?

 

What about building the garage on a 150mm thick or 180 - 190mm thick concrete flat structural slab with mesh that floats about and has a movement joint between the house and the garage structure? Yes I know the 150 or  180 - 190mm sounds like an odd number but there is a technical code compliance reason for this.

 

There are few technical details associated with this but it seems to me you might be using a sledgehammer to crack a nut and burning cash going for piles...

 

Is your site on a slope? Which way does it face.. as we look at prevailing winds and where the roots of the tree will grow.

 

Has your SE been paid enough to find the most economic solution? I'm not kidding you here.. with a bit of thought you could have a slightly thicker concrete slab that moves about but still strong enough to stop the walls from cracking.. it's worth asking the question.. imagine if you found out later you had chucked a load on money away on blindly following BC / standard guidance and put in piles.. all for a garage!

 

Now that is an odd question as a Client would expect that the SE fee would deliver the most economic solution.. but.. that is not the way it works.. as an SE will need to spend more time finding the more economic solution.. but that set against the cost of piling! Now I may seem to paint SE's in a bad light..

 

But there are folk on BH who think paying an SE or an Architect more than a grand is a waste of time as they know better.. well to those all I can say is.. the door swings both ways you smarty pants!

 

 

 

 

 

Definitely haven't got a money tree unfortunately.... There is a slight slope to the site worst case around 1:6 gradient. I did have some initial consultations with a structural engineer and now I'm trying to understand the correct way to detail the job but I might ask if there's another way of doing it based on what you've suggested.

[Gus Potter]

On 16/11/2023 at 19:43, tscalfa said:

 

If I go for an insulated slab I'm considering Cellcore HX plus where the insulation is included with the as part of the void former.

 

 

Definitely haven't got a money tree unfortunately.... There is a slight slope to the site worst case around 1:6 gradient. I did have some initial consultations with a structural engineer and now I'm trying to understand the correct way to detail the job but I might ask if there's another way of doing it based on what you've suggested.

Ok no money tree.

 

What about taking this approach?

 

If you have piles and a ring beam then the concrete is probably going to be around a RC35 (the RC indicates that this is a reinforced concrete) and thus if you lay, compact and cure it properly it will be pretty resistant to moisture. Now we do this a lot on industrial buildings.. B & Q for example. Take a leap of faith and recognise that this is not a habitable space.. all you are doing is controlling moisture from the ground in most circumstances. This type of conrete is good enough to be pretty impervious.. but as it is a stronger concrete it will tend to crack. so we put in reinforcement to control the crack width.

 

In other words treat this as an industrial building and there are tens of thousands of these that are built this way. BC are ok with this appoarch. Just remember if you want to convert it later to a home office it may need a lot of extra work, not least as you have an unisulated slab.

 

If I was designing  this at concept stage this I would decouple the design into different elements. It will save you money on builder cost. I say concept as there are doors / threshholds etc that need to be considered and you may have varying ground levels externally.  I would pile it first. Now you have potential ground heave, hence the clayboard. I would put the clayboard under the ring beam so you don't generate uplift in the piles. I may want to put it up the sides of the ring beam as well to stop swelling soil pushing the piles sideways.

 

Once your ring beam is in place build your brickwork inner skin. Lay the clayboards in the middle and put the DPM on top of them.  Then do your reinforcement for the slab and cast the slab.

 

Now that sounds simple but there there are things you need to know about clay boards.. some need watered so they degrade and collapse.

 

To get more guidance tell us about what the size of the garage is (floor span and length is important, as it relates to a good economic slab depth  vs the amount of rebar you need to use and if you need joints in the slab) and why you need clay boards in the first place.. a soil report would be helpfull.

 

Some garages are small and don't need much thought.. but some are very big.. the size of a small house so you can lose a lot of cash if you don't grasp the basics of design. I think you need to go back and identify how the soils behaves, what the piles are doing, if you have trees close and thier type and size and your ground levels. Only then can you get the right position of the DPM and DPC's.

 

Best thing would be to post as much info you have as this will invite the best response from BH folk.

 

 

 

 

 

 

 

 

[tscalfa]

On 18/11/2023 at 22:22, Gus Potter said:

To get more guidance tell us about what the size of the garage is (floor span and length is important, as it relates to a good economic slab depth  vs the amount of rebar you need to use and if you need joints in the slab) and why you need clay boards in the first place.. a soil report would be helpfull.

 

Some garages are small and don't need much thought.. but some are very big.. the size of a small house so you can lose a lot of cash if you don't grasp the basics of design. I think you need to go back and identify how the soils behaves, what the piles are doing, if you have trees close and thier type and size and your ground levels. Only then can you get the right position of the DPM and DPC's.

 

Best thing would be to post as much info you have as this will invite the best response from BH folk.

 

 

 

 

 

 

 

 

Thanks Gus, I'll get some more information/diagrams together and then share them on here.

Edited November 22, 2023 by tscalfa

[tscalfa]

Here are a few more details, see the attachments for diagrams. A few points to note:

• The floor area is 6 x 6.5m
• The garage is 0.5m below the ground level at the house it is attached to due to the sloping drive.

Thanks again in advance for any advice.

PLAN FLOOR.pdf SIDE VIEW.pdf