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I have a space between the studio concrete slab and the volcanic rock bed at widths between 370-400mm, since we thought 300mm will be sufficent. But building code calls for 460mm width, so I'll need to make do with what I have to at least make it look I gave my best to comply with it.
The longitudinal fall from one side of the building to the other should be 1:150 (to get any water that falls on the concrete pad away from the building) and the minimum height of the concrete foundations on one side of the building from the concrete pad to be laid is 100mm. This means that with 7845mm lenth of the concrete foundations, the height on the other side of the building needs to be approx. 155.23mm and luckily I've got around this height on the other side.
However, there is also a cross fall required from the building, and a 'channel' as per attached building code diagram similar to what you can see in some footpaths (I attached one footpath photo with a 'channel' for better understanding). I was wondering if it would be possible to make that 'channel' a bit narrower than what building code requires and have at least 300mm of concrete before the channel (I need that distance for the cladding and water tank). There may be few spots where we'd need to chisel off few centimetres of the rock to get that depth towards that uneven edge that was cut by concrete cutter. If 'channel' can't be done for whatever reason, then maybe we can just leave it flat with the required cross fall (1:18 or 3.08 degrees).
Because almost all of it is a volcanic rock, you will probably not need much GAP20/40 at all, only in few sections just to even out the surface that was previously cut out the volcanic rock by the concrete cutter, and we won't need boxing along the the length of the building, but we'll need it only on the sides.
Also, on the rear of the building, we will need to continue the concrete pad for another metre and since there is enough space here, we will need to comply exactly to the specification as per above diagram (450 width, 100mm channel, 1:18 cross fall and 1:150 fall). So the whole idea is that any water that ends up on the concrete pad flows from the front side of the building, along the width of the building and then turns for another meter where it will be taken to the soil underneath the future deck.
The 'channel' can probably be similar to what can be seen on the footpaths like in one of the attached photos.
In terms of the dimensions for the concrete pad, see attached sketch.
As I plan to put two 1tonne water tanks on this concrete (they are 26cm in width), the concrete pad needs to be around 100mm with SE62 ductile mesh reinforcement.
Also, since the concrete foundations were laid on top of the rock and concrete cutter disturbed some of the rock, there is one small section of the foundation where concrete was broken off and I need this to be patched with Sika Monotop 412n applied by trowel. Also the whole front of the building will need to be patched. My structural engineer didn't have any concerns, but because there is no footing on that side due to concrete on-ramp that was cut out, I need to make it appear smooth by moving some of the GAP 20/40 close to the foundation and then make this side tidy using also Sika Monotop 412n applied by trowel. The Sika product comes from my research rather than anyone specifying it, so if you think something else can/should be used, as long as it doesn't crack afterwards. I will lay the concrete around the slab on that side once the renovations are done, I am also happy to consider your suggestion.
See attached photos and we can discuss the details when you visit to quote.
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