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hi community,
I want to build a garden house with a frame construction. One-storey, flat roof with ~ 30° angle.
The garden terrain is rather uneven so the idea is to put the construction on brick or concrete poles to level the floor.
Now my question is, how do I calculate the number of poles needed respectively their distances from each other and
their dimensions. And what should be the dimensions of the joists to bear the load of floor, frame construction and roof
and how should they be connecting the poles.
The 1st image shows the frame construction, the 2nd image is an example of how the poles could be distributed but it is not based on any calculations or legal regulations so probably insufficient/wrong.
The project is located in NSW.
Thanks in advance
Stefan
Welcome to the Bunnings Workshop community @stefanglatt. It's wonderful to have you join us, and many thanks for your question about building a foundation.
A build like this has many technical aspects that need to be taken into consideration and is far more complicated than, say, a deck of this size. The first thing to do is contact your local council to determine whether approval is required for the structure. Given its size, I suspect it will be, and you'll need to engage a draftsperson or engineer to produce plans for council submission. During this design stage, your engineer will use available data from your local area to determine adequate footing sizes and types. From there, they will start designing the structure according to building codes. These building codes set out requirements for your build. The number of footers and supports will be determined by the calibre of the bearers you'll be using and soil type. The size of the joists will be determined by how far they need to span between the bearers. The load applied to these bearers and joists will also need to be taken into account; this is where span tables come into play.
Unfortunately, there is not just one answer to any of the questions about individual elements, they all need to be taken into account together.
As much as I'd like to provide specific timbers and advice on suitable connection techniques, an engineer needs to look at that. I suggest you start there and have some plans drawn up. Once you have them and the specifications for the required timber, we'd be more than happy to assist with the selection and any questions you have.
Please let me know if you have any questions.
Mitchell
hi Mitchel,
thank you for your comprehensive reply.
I already feared/suspected it would be that complicated. 
Will have to re-think the whole approach.
Stefan
We are definitely here to support you @stefanglatt. I'd love to attempt to design the whole lot for you, but unfortunately that could all be for naught if your council requires an engineer to sign off on it. I'm pretty good at D.I.Y. but the council might need a bit more convincing that the design is structurally sound.
Mitchell
... I do understand that. Never mess with council
Thanks for your offer anyway, I will keep that in mind but i need to discuss the whole thing with my partner.
