What timber to use for retaining wall?

[ososbg]

Hi all,

First time poster here and sorry if it's a naive question.

I want to build a retaining wall and also to be a raised garden bed at the same time. I've two sides in my backyard I want to use both of them, one side is 15m and the other side is 14m, the retaining wall height is about 500-600mm.

My question is, what timber I should be using for this:

1.8m H4 Treated Pine Sleeper 200 x 75mm.

2.4m H4 Treated Pine Sleeper 200 x 75mm

1.8m H4 Treated Pine Sleeper 200 x 50mm

2.4m H4 Treated Pine Sleeper 200 x 50mm

Thanks in advance

[ososbg]

Hi @MitchellMc , thanks for the warm welcome and the links, the video is very informative, thanks for sharing, I've got a few questions for you:

1- What's the drill and the drill bits used, I would appreciate if you can recommend something for me for DIY. I've got big rocks in my backyard and looking for something that can help drill through these rocks.

2- What's the diameter for the holes for the posts for the 50mm timber sleepers.

3- What posts do you recommend for me based on the measurements I've in my main post, shall I go with the timber posts as in the video or with the steel ones ?

[MitchellMc]

Hi @ososbg,

For the construction of the retaining wall and screwing the horizontal sleepers to the posts, you'd only require a drill driver and landscape screws. The screw box should include the star drive bit needed. For drilling into rock, you'd require a hammer drill (preferably a mains-powered version) and masonry bits.

The rough rule of thumb for retaining wall posts is half in the ground, half out of the ground. For a 200mm sleeper, you'd need around a 400mm diametre hole.

The choice is yours on whether you'd like to go with steel or timber. The steel does have the advantage that no drilling is required as the sleeper slips into the post. Here's a great guide: How to build a raised garden bed for privacy.

Please let me know if you have questions.

Mitchell

[ososbg]

Thanks so much @MitchellMc , too much to digest in the weekend, will spend sometime today to learn and will circle back for any questions. Thanks again.

[ososbg]

Hi @MitchellMc , this is me again

I've progressed my project but I did 2 big mistakes and I need some guidance from you:

• I bought the 75mm timber sleepers, but unfortunately I didn't pay attention and I got CCA instead of ACQ or MicroPro treated timber and I finished half of the job now. 😑
• I used the timber posts inwards instead of doing it outwards, so, the load now will be on the nails instead of the post if I understand this correctly (as in the attached photo).

I did some research and found out the risk of using CCA can be mitigated if I use builder's sheet or plastic sheet before I put the soil, any guidance on if this is a good idea or there's a better option, bonus point if you can post links for some products you can recommend (sheets or anything else).

Also, will the posts position will be able to handle the load from the soil, the max height I had for the retaining wall is 60cm.

Thanks in advance.

[Jason]

Hi @ososbg,

Just a heads-up that @MitchellMc won't be back on the site until tomorrow. Let me see if @EricL can assist in the meantime.

Jason

[robchin]

Hi @ososbg 

Good luck with this project. Sure it will turn out great. 

I work in the Risk and Compliance team at Bunnings and wanted to make a note about using treated pine. May not be relevant to this example but I would put out a general warning if you are building raised garden beds for vegetables and herbs then be careful with your timber selection and don't use timber treated with copper chrome arsenate (CCA) - they're the ones that look a little green. There are alternatively treated timber sleepers available. 

Just a general word of caution. 

[ososbg]

Hi @robchin Thank you for the note, as I mentioned it was me being naive and not paying a lot of attention, and hence I got the CCA and already done most of the job, so I don't think there's a way to rollback, all what i'm asking now is how to help reduce the risk of CCA, I did some research and I learnt that I can use a builder's sheet (i don't know what this is) between the timber and the soil, or alternatively to stain the timber with water based stain, again this is my very first experiment so I'm not an expert at all. Hence i'm looking for some guidance if you or someone from the team can provide some help with links to some of your products, that would be appreciated.

[EricL]

Hello @ososbg 

Can you please tell me what screws you used to anchor the sleepers to the post? Although the raised garden is not too high its always best to have full support when you start filling it. What post size did you use? It's difficult to tell from the photo, is it 90x90 posts?

Allow me to double check the plastic to be used for the garden bed. As soon as I have more information, I'll get back to you.

Thank you for your patience.

Eric

[ososbg]

Hi @EricL , thank you for your reply.

I've used 125mm timber construction screws, and the posts are 90x90, the sleepers are 75mm.

Let me know if you need more information from me.

Thanks

[EricL]

Hello @ososbg 

Thanks for sharing those extra details of your build. Because you've placed your posts on the inside, I would like to suggest using Buildex 16-8 x 125mm Climacoat Type 17 Timber Construction Screw, it's thickness and head size will be able to hold a fair bit of weight.

In regards to the plastic cover, I've conferred with Mitchell and I propose using Builder's Plastic 200um in thickness. My best advice is to make sure to provide adequate overhang at the top and bottom of the sleeper wall.    

If you need further assistance, please let us know.

Eric