What are the best house stumps?

[amal]

hi we are in the planning stage of constructing a bathroom extension to a cabin. the design will be a stump subfloor approx 5m long x 3m wide. the site is sloped and the finished floor height will vary from 200mm -600mm from ground level. i understand we need to consult an engineer to get specific advice but looking here for a starting point to think about how to approach the build. 

1. can you please advise with stumps will hold out better in the long term. threaded rod concrete stumps or steel stumps? some have the opinion that the threaded rod in the concrete stumps rusts out and causes issues and others say that steel stumps also have issues of rust. would like to rule out using timber stumps as termites and dry rot can be an issue here. 

2. in general what depth do stumps need to be laid ? (again not looking for specific measurements just a general guide to work from). we are in the subtropics with a high rainfall and heavy clay shifting soils. the position of the extension would be southerly facing. damp and dark 

3. what size bearers and joists are standard ? there will be a lightweight acrylic bathtub. will this need an extra line of bearers directly underneath it or would extra joists suffice? 

thanking you kindly for any direction here. 

amal 

[EricL · Accepted solution]

Hi @amal

Welcome to the Bunnings Workshop community. Thank you so much for joining us and sharing your question about which house stumps are the best. 

If you are worried about rust and dry rot on your post, you'll have no choice but to use concrete stumps to guarantee longevity. How deep should the stumps be buried? Factors such as soil density, the weight of the structure, and the number of posts you will be putting in will dictate how far it should be buried. For planning purposes you could for example provide a depth of 800mm and a width of 500mm for the post foundation. It may have to go even deeper if the soil is too soft, your engineer will determine the final depth when they see your plan.

The common size used for bearers and joists is Treated Pine Outdoor Timber Framing 140 x 45mm. Again, your engineer will recommend what timber thickness should be used for the build. Sounds like a fantastic project. Any updates you can provide would be much appreciated.

Let me tag our experienced members @tom_builds, @TedBear, and @r23on for their recommendations on what post is best for your situation.

If you need further assistance, please let us know.

Eric

[r23on]

As @EricL has suggested the soil will be the key factor I would advise checking out the local council reg’s on soil conditions depth for stumps and clearances between ground levels and flooring. As for which product is best will depend on how much water flows under the top soil

Have you had a soil test done?  

[tom_builds]

Hey @amal and thanks for the tag @EricL,

There are a heap of factors that go into the sizing of bearers and joists, with material, span and desired rigidity all affecting the design (if termites are a major consideration you could opt for a steel sub-floor frame). Typically your bearers will be larger then your joists, but the engineer or draftsperson you use will be best placed to design this for you. 

As for the bathtub, if the design allows you will want to try and align the long edges of the bath along the length of the nearest floor joists, and also double up the width of those particular joists by nail laminating them. 

Best of luck,

Tom

[amal]

thankyou for your reply eric. hope to post with an update soon

[amal]

hi @r23on, thankyou for your reply. we are still awaiting a soil test.

we have a few other buildings on the property. one is a besser block, judging from the cracks that have formed over time, there is substantial movement of water and clay in this area. 

ive tried to upload some photos of these buildings, unfortunately its not working.  

[MitchellMc]

Hi @amal,

If you've had structural issues in previous buildings, your best bet is to leave the footer requirements up to the professionals. They'll tailor a solution that will work with your soil conditions.

To upload photos, use the little camera icon above your text in the reply box. If that's not working for you, feel free to send them through to workshop@bunnings.com.au, and I'll post them for you.

Mitchell