# Forum Home Renovation Sub Flooring  Footings for Deck - Trying not to over engineer

## jasonv

Trying to determine footing depth for a deck is starting to drive me crazy  :Confused: . I know how to determine footing size it's just depth for braced and unbraced posts that I don't understand. 
Basically to teach myself how to build a timber deck to go around a planned above ground pool I bought a copy Allan Staines Decks book and got a free copy of AS 1684.2 and 1684.4 from work. I'm now confident on all parts of the deck construction other than the footing depths. 
I've already designed the deck and have approval from the council, it's a Complying Development so footing details are not required for approval but are inspected as part of the certification process. 
My deck design (no roof) is freestanding 40m2 with a total of 28 posts (<1m high), a bearer span 2400 and a FLW of 1200. 
My problem is in Staines' book the footing size tables aren't suitable, it only gives 4 possible footing depths based on area supported per post, the smallest size shown being a supported area per post of 5m2, given mine is less than 1.5m2 per post the footings proposed are way too big.  
If I refer to the 1684.4 that's just too simplistic, Sect 8 just says that cross bracing is required for posts embedment at min of 450mm for a max post height of 1.8m. How can I use information to determine the correct post embedment as my post height is way less than 1.8m (it's only 1m at the heighest point)? 
I know if I go to Sect 8 of 1684.2 then it gives me the tables for every footing depth and post length but it only tells you the bracing capacity in each case nothing else. The problem is I have no idea what the required bracing capacity is.  
Obviously the Staines' book & 1684.4 must have assumed a bracing capacity (worst case) to determine the footing depths they each provide but they don't mention anywhere what that bracing capacity is. It's the only piece of information I'm missing. Does anyone know what the bracing capacity they assume is? That way I can then look up the full tables in 1684.2 to determine how deep I need to embed the posts without wasting a heap of money on extra concrete and dirt removal. 
Btw, thought I should mention that I can't just go 600mm+ deep to be safe as I hit rock before that around 500mm. I have no issue with bracing the deck either. 
Hope someone out there can help.

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## Terrian

just go 500mm to be safe  :Smilie:  
na, I am sure one of the decking experts will raise their head in here shortly.

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## Schumi

Hi Mate,   Don't stress too much on the calculations, I am currently building a deck with posts through to the roof, my highest floor point is 1600mm of the ground, I had an engineer/draftsman draw it all up for me to submit to council, he specified footings of 500x500x900 deep for my steel posts. So I sat there digging them out, only to have the certifier from council to come and inspect them and laugh and state” are you going to hold up the harbour bridge" He told me to fill them in to save on concrete.   I also hit rock on some before I got to 900mm depth, As long as you have a solid surface under the post it won't be a problem.  I find it interesting that your plans are approved by council with no footings details yet they want to inspect them prior to pouring concrete.   I questioned a detail on my drawings with council and the reply I got back from them was "Your plans are stamped approved so what is on the paper will pass"  Whilst building the structure I have come to notice how much detail was left out of the drawings, so I am ready for the argument the day the final inspection is due should they pick on anything because my reply will be "I have built it to the drawings and they are stamped as approved"  Anthony

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## ringtail

Mostly depends on the bearing capacity of the soil. If you have solid rock, in one respect you are lucky. Up here there is a lot of highly reactive clay. The holes I drilled today are 1300 x 450. My certifier wont accept anything less than 1200 x 450 - a lot of concrete been pumped in the morning.

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## jasonv

The certifier will need to come out first to inspect the pool base, so I'll be able to ask him about the deck footings at the time. 
I'll explain the problem and see if he's prepared to accept a 400m-500m depth.

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## Bloss

400-500 is more than enough for a deck less than 1m high with no roof or pergola - virtually no uplift forces. Easiest is to go to 400 and see what the certifier says - easy enough to go another 50 or 100mm if they say too than go deeper when un-needed. 
Permanent bracing of posts on desks under 1m is rarely needed either although sometimes temporary bracing while under construction can be useful. In any case adding bracing later is usually not a problem.

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