How to prevent leaking water through timber sleeper retaining wall?

[dave9]

I am considering building a garage over a space that used to house a carport.  One wall of the garage would sit on top of a timber sleeper retaining wall replacing the fence that is currently there.  The frame of the garage would be attached to the existing concrete slab. I have circled a sample of the retaining wall in the photo below.  It runs the entire length of the slab and is about 1 metre high.

My problem is that I would like the garage to be watertight, but sometimes during heavy rain, water leaks through timber sleeper retaining wall and runs over the slab.  I have been racking my mind on how to prevent this without excavating on the other side of the retaining wall as that is an established garden bed on common property.

Is there some kind of treatment or structure I could put in front of it to prevent the leak?  Or would a cutting a channel in the slab to manage the water flow be the best bet?

All ideas welcome!!!  Thanks in advance.

[Jewelleryrescue]

Hi @dave9 

I like timber sleepers but they will last at best 30 years.

To  be approved  by  councils you will need an engineering plan as it is larger than a certain cubic meterage no doubts.

I dislike the Council word but if you go to sell the house one day  you may have to demolish an un approved  garage.

regardless a   garage  needs long term support especially where water is an occasional  issue.

Might I suggest a  proper besser block  retaining wall  complete  with ag line and gravel drainage as a  guide line if an engineer thinks the same.

[dave9]

Thanks for the reply.  Unfortunately I don't think I can replace the retaining wall.  We are part of a strata group and the retaining wall is common property.  The fence that I want to replace is also common property, but I'm hoping that I can get the strata committee's approval to replace it.  I don't want to push my luck with the retaining wall though.  The retaining wall was built in the mid-90s so is already about 30 years old.  But it is still doing its job.

[EricL]

Hello @dave9 

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your retaining wall.

In order to stop water from flowing out of your retaining wall the soil must be dug out and the retaining wall sealed from the other side. But if this is not possible your idea of cutting a channel in the slab would be the only viable solution. Putting anything in front of the retaining wall is not going to be practical as the entire retaining wall will need to be covered up.

Perhaps when you build your garage you could create a sort of masonry wall that is waterproof on the other side, but the issue will be where the water will flow out to. 

Let me call on our experienced members @Dave-1 and @Nailbag for their recommendations.

If you need further assistance, please let us know.

Eric

[Nailbag]

Hi @dave9 

My suggestion is to get a draftsperson out as you will need a decent set of working plans to obtain the building permit. They will be able to advise what may be possible against the shared wall area. At a guess, I doubt you will be able to build directly upon the existing retaining wall as its structural specification is unknown. As @Jewelleryrescue mentioned, if you were to go-ahead without council approval and get caught, they can order demolition.

Nailbag 

[Dave-1]

Morning @dave9 

I was trying to figure various ways around teh issue for you when @Jewelleryrescue and @Nailbag 's words finally sank in. You are want a sealed garage. Straight up that means building approvals and also draftsperson to draw up plans.

My first step would be to contact council and find out what their rules are (They will be published on your local council website most probarly)

Second step see what the strata group says.

Then contact a draftperson and or builder and price it.

😕 Far better to inverst the time sorting the above questions out as it will eithe be a yah or nah type decision path. The drafsperson will factor in the solution to that retaining wall if its ok to go ahead with a structure.

Dave

[Jewelleryrescue]

Hi @dave9 

If you plan for the garage being inside the fence line this may reduce red tape complications  with  the strata group and make the process easier.

Also many  Metal  garages are already engineer approved  (Double check  this) and may simplify council approval.

That  Retaining doing a  very good job and the wall may look sound but I would estimate within 10 years it will need replacing with more visible signs of aging appearing so if your garage sitting on top if it is asking for dramas down the track, I dont want that for you.

[dave9]

Thanks @Nailbag , @Jewelleryrescue , @Dave-1 , @EricL , these have all been very helpful comments.

This is a bit of a chicken and egg situation.  It is hard to seek approval from the council and strata committee until you can show them exactly what you plan to do, but a draftsperson will not know what to design without some guidance from the council/strata committee :-).

Some of the larger garage fabrication companies can quickly produce plans at no charge and I was hoping these will be sufficient to take to the council and strata to get their first reactions - so it has not cost any money by that point.

Just to clarify for those who have been helping - I was not planning on building any load-bearing structure on top of the retaining wall - the steel posts would be fixed to the concrete slab on my side, and the wall of the garage would just "hang" over the top of the retaining wall via a side-wall girt.  I would use a channel of some sort to keep water out from between the top of the retaining wall and the bottom of the steel sheet garage wall.

My thinking at the moment it to not touch the retaining wall, and instead build a dummy wall in front of it with a channel in between to deal with the water.  I hope to speak to a builder to get an idea of the practicalities of this.

Again, thanks for your help and suggestions.

Dave