# Forum Home Renovation Concreting  Minimum requirements for double garage concrete slab

## blak

I am seeking documentation which specifies the minimum requirements for a double garage concrete slab. 
Are they in the BCA? 
Can someone provide me a URL please? 
I have requested this information from council and they told me to consult an engineer.
I live in Newcastle, NSW. 
I have an existing slab that I am proposing to build the garage on. I am unsure if it is up to spec. If it does not meet the minimum requirements, then I need to find out what to do to bring it up to spec. e.g. levelling, waterproofing, piering etc etc 
Any help would be most appreciated.

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## dazzler

Hi 
cant help with a link unfortunately but i imagine that unless you can demonstrate that the footings have been done in accordance with them you may be up the creek. 
At the very least I would have thought that the strip footings around the perimeter would need to be 450mm x 450mm with reo in it. 
best of luck

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## chuth77

Have you had any site soil tests done. There is an Australian Standard, AS2870 which gives you all the details of the footings, reo and slab thickness for each soil type. 
I can give you some details out of the standard depending on what type of slab your chasing? Waffle, Raft, Suspended? etc. 
By following these details in the standards you don't actually need an engineers approval because these standard have been written very prescriptively and by following them you comply with the BCA requirements, without the need for calculations etc

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## ausdesign

as chuth said, AS2870 covers concrete footings etc. I would think you will be lucky to be able to use the existing slab if your thinking of using it to support loadbearing walls.
Unless your on sand - Class 'A' or 'S' type footing then there will need to be internal beams [ max. 4 meters in ] running in both directions which would have to be exposed somehow to prove they're there.
There would need to be SL72 mesh through the slab which you would also have to prove is there in entirity.
If your doing a perimeter footing and using the slab as an 'infill' then that's a different storey.

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## blak

> Have you had any site soil tests done.

  No but the sounds of it Im going to need to arrange one. I know the soil is quite sandy here, but in terms of exact classification I have no idea. What would I expect to pay for soil testing?    

> I can give you some details out of the standard depending on what type of slab your chasing? Waffle, Raft, Suspended? etc.

  Im really not chasing any type of slab. I am just trying to understand what I need to do to the existing carport slab to bring it up to spec for a double garage. The facts as they stand; 
1. Existing slab is 40m sq. It is 125mm thick and fully reinforced with steel reo.
2. Proposed structure is a double garage (6.6x6.0), timber frame, weathertex cladding, gal roof. 
3. The western wall of the garage will be shared with side wall of the house. (e.g. this garage will not free standing)
4. Existing slab is NOT waterproofed.
5. Existing slab is flat but is NOT level. Slight fall running from left to right.
6. Existing slab is not piered. 
Various builders have made suggestions like cutting 450mmX450mm holes in the existing slab, digging down 500mm and pouring the footings in the load bearing areas.
Also other suggestions are to pour a new slab, on top of the existing slab, and this will overcome the levelling and waterproofing problem. 
You can see where I am proposing the garage in the attached picture -  
What do you guys think?

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## fubar

sounds quicker and cheaper to start from scratch if you can breakup and remove old slab yourself
goodluck retrofitting to bring existing slab upto specs soil testing will be necessary $200-$300 last time got one done

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## ausdesign

I would look at cutting through the slab [ if you can't get access from the side ] & putting in 3 load bearing posts with a side pitching beam to support the roof structure.
Infill framing between the posts to carry the cladding.
Soil test for founding depth is a good idea but many councils will accept the depth of the pad as per the existing house footing.
The size of the pads would need to be designed taking into account uplift area of the roof.
Another factor to look at would be any fire rating requirements, being attached to the house, if you are within 1 meter of the boundary.
In Vic there is a max average height of 3 meters for the boundary wall - if it's within a meter of the boundary & a max. total length that you can build along the boundary line. Not sure of NSW regs.

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## chuth77

> What would I expect to pay for soil testing?

  Not sure down there, but up here around $500...    

> 1. Existing slab is 40m sq. It is 125mm thick and fully reinforced with steel reo.
> 2. Proposed structure is a double garage (6.6x6.0), timber frame, weathertex cladding, gal roof. 
> 3. The western wall of the garage will be shared with side wall of the house. (e.g. this garage will not free standing)
> 4. Existing slab is NOT waterproofed.
> 5. Existing slab is flat but is NOT level. Slight fall running from left to right.
> 6. Existing slab is not piered. 
> Various builders have made suggestions like cutting 450mmX450mm holes in the existing slab, digging down 500mm and pouring the footings in the load bearing areas.
> Also other suggestions are to pour a new slab, on top of the existing slab, and this will overcome the levelling and waterproofing problem. 
> What do you guys think?

  1. Good solid slab? Any edge beams or footings?
2. Good light design. Doesn't need a huge slab, but your loading points need to be considered.. 3 posts or entire wall?
3. By the sounds of it'll be a load bearing wall?
4. What do you mean waterproofed? Sub slab sealed?
5. now your problems start.... How much fall? Can you live with it? Can you use a self levelling compound?
6. Not piered,but is it beamed? 
One thing to think about, what finished height will you want relative to the exisiting slab? 
The hardest thing about putting in new piers or footings will be tying it to the existing slab sufficiently... A couple of good quality dowel joints would suffice...

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## Cementer

Hi Blak, From the picture it looks like Mayfield or Tighes Hill area? My opinion would be to cut the existing slab to pour new perimeter footings and a topping to level the existing slab. I serioulsy doubt there is any sort of footing with the existing carport. If anything there will be thickened pads where the posts are. Use the old slab as a base, the topping only has to be 50mm at the house side and make level to the fence side. Reinforce it with steel. Use a 25mpa/10mm stone concrete mix.A local engineer will draw up a plan to submit to council. This will include footing sizes. Call if you need help with this. 
Neil

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## blak

By a stroke of luck I have actually been able to speak with a guy who is building surveyor and used to work for the council. He advises that I am thinking far to hard about this. All I really need to do is cut 450x450 squares into the existing slab at the propsed garage load bearing points and pour a pier. He said soil testing is overkill for a small project like this. 
Neil: Im in Broadmeadow. Will be in contact with you through the week. Will definetly be pursuing the levelling slab. With the way the fall is on the existing slab from left to right, a closed garage door would look very shonkey. 
My design so far is below. Please be gentle, I drew these myself ...

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## blak

Some more pics of the existing slab ...     
Have some prep work to do. e.g. Remove bedroom window, move air-conditioner, move electricity meter box.
Front part of the house has been repeired abt 6 years ago, so Im pretty confident they will be up to the task of supporting the western wall of the garage.

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