Under house retaining wall

smiatke · Jan 04, 2024 7:19 pm

Hello,

We live on a very steep block and our house is built on concrete/steel stumps. We have a space under our house which I would like to "box-in" and convert into a study. I am wanting to build a retaining wall to retain the earth as shown in the images below, then build a timber framed wall in front of the steel stumps/retaining wall which would then be insulated and clad with plasterboard.

I have two main uncertainties I would like advice on before moving forward.

1. I would like to use the steel stumps as the vertical posts for the retaining wall - potentially by welding or bolting C-channel to them which would house sleepers. Is this concept feasible? My concern is that if the earth moves down hill over time it would be pushing against a wall which is being held upright by the same posts that are holding up the house, which could compromise the stability of the house?

2. There are two areas under the house where moisture develops. Typically it is just damp dirt, but after significant rainfall some pooling does occur. Diverting this water from further uphill is not feasible as a trench would have to be very deep and the only suitable location is not practical. Also it is not clear where exactly the water source is. I do know that neither is from a pipe leak as this has been investigated.
We have decent airflow under the house. I am wondering if backing the retaining wall with plastic sheet and draining captured water to outside would be sufficient to prevent mould growth on the plasterboard and whatever flooring is used in the room. If so, how deep under ground would the plastic sheet need to be buried? I would be conscious of removing too much soil surrounding the stumps.

Any advice on these points or in general would be appreciated. Happy to provide more information/pictures if required.

Thanks,
Sam

Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅

Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅

Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅

Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅

Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅

supersleuth · Jan 05, 2024 10:20 am

I did a similar DIY project several years ago to build a mancave.

I would recommend against using those existing house posts for your retaining wall. I dug new retaining wall post holes well away from existing footings. Then left about 300mm gap between retaining wall and new mancave stud wall.

I didnt have any standing water issues but for your situation you should install drainage to take that water away.

smiatke · Jan 08, 2024 9:12 pm

Hi supersleuth, thanks for your reply.

Is your reasoning for not using the steel stumps for the retaining wall the same as the concerns I had, or something else?

Can I ask what you did for flooring in your man cave?

Any other findings/lessons learnt from your project would be appreciated!

Thanks again, Sam

building-expert · Jan 09, 2024 2:27 pm

1 Get it properly designed and get a building permit.
2 Check if subfloor has been sprayed with Aldrin for termite control, if so you may be sitting in a study that will give you cancer.

supersleuth · Feb 20, 2024 12:37 pm

smiatke

Hi supersleuth, thanks for your reply.

Is your reasoning for not using the steel stumps for the retaining wall the same as the concerns I had, or something else?

Can I ask what you did for flooring in your man cave?

Any other findings/lessons learnt from your project would be appreciated!

Thanks again, Sam

Yeah my reasoning is just that those house posts are designed for vertical load not lateral. I just didnt want to risk interfering with the existing house engineering.

I hand mixed and poured a concrete slab for the floor - given I didnt want to add height with a deck plus the ground was very rocky.. Doing it now I would get a concrete truck and pump, or maybe even just pay someone to pour it rather than trying to save money.

It all turned out pretty good considering it was DIY and I'm an amateur. No building permits, engineering or anything. I ended up buying/taking a couple of old kitchens from gumtree/friends, and used all the cupboards/benchtops etc for storage which worked out well. I plastered it all internally and even had downlights so inside it was nice and cozy. Used ply for one of the walls/splashback to hang tools and work near rather than plaster which also worked well. Even had a bar fridge down there, was a good little setup.

It was built under an exposed deck, so I had to design and build a colorbond roof that attached under the deck and kept water out of the mancave. It never leaked once so I did something right.

good luck.

Dont know what(if any) value it added to the property when we sold but I'm sure it had some value.