How to build raised garden bed along fence?

Question

Good evening all!

I am fresh off my first DIY paving project and now want to keep the momentum going and construct a raised garden bed that will line the backyard perimeter fence. I have freshly cleared the area of assorted trees and getting close to figuring out how i will put in a concrete strip footing then select what type of brick/ concrete block i will use. As I am pretty green to concreting and brick laying i have a lot of questions on how i should attack this project.

The Idea:

Build a raised garden bed/ plantar from brick or concrete blocks (then render if i go down this path) onto a concrete strip footing along existing color bond fence leaving a 100mm gap between the fence and the new wall. Additionally planning to paint the color bond fence steel monument. I would like to include conduit for reticulation and power and there will be seating incorporated into the wall using timber sleepers. I plan to do this by narrowing the width of the plantar and having the seat inside the boundaries of the plantar, if that makes any sense. I am aiming to have the plantar 0.5m-ish high and 1m wide. The plantar will stretch 5m one wall, 19m the next wall and 5m the following wall in a U shape. I plan to use mesh rebar throughout the footing and plastic sheeting in the trench. For drainage i was planning on leaving weep holes in the brick work and agi pipe at the bottom of the plantar covered with blue steel for additional drainage. The plantar would then have olive trees and assorted infill plants installed. Lighting to be placed under seating and near olive trees. Reticulation to be installed also.

Initial Questions:

* What width and depth would you make the strip footing for a 0.5m high wall? I was thinking of going 300mm wide and maybe 150mm deep but im being told this may not be adequate? 
* I have 2cube of road base i wanted to get rid of, is it a bad idea to mix this with the cement to create the strip footing or am i better off using the road base as a sub base to the footing?
* I am pretty confused on which brick or concrete block to use, some people i speak to say that if i use brick i would need to run a double wall with concrete infill to support the bed if going down the brick route. Others tell me 1 layer of besser sized 390 x 190 x 190 also would not be adequate, would be good to hear suggestions on what block to use. As i mentioned if i go down the brick facing route id probably leave it bare brick, if i went down the besser block route i would be inclined to render it white.

Would be great to solve the above questions so i can make an attempt at getting the strip footing down before returning to work or be informed of anything i am overlooking or havent thought about. I have attached some pics of the backyard area and a design i made using chat GPT, the design isnt exactly what im after but just a ball park sort of look. I would like to have curbed corners for a bit of a sleeker look.

Thanks everyone!

Jacob

Dave-1 · Answer

Good Afternoon @Jacob93

You have done a decent amount of research and apart from @EricL's extra suggestions I cant come up with too much. :)

One thing tho is allowing for posts in the garden beds, lots of people end up wanting a trellis or something similar, maybe wire for a passion fruit vine and as you are installing the beds from scratch it might be an idea to incorporate some posts, either by adding the concrete foot a little wider so you can bolt some stands to it or internally in the bed.

I like @EricL's suggestion for using the left over road base material for the base of the concrete, why waste some good material :)

With bench seating, id suggest to have a play around with benches you find about the place. Take a tape measure and measure the rise and depth of the seating part that feels most comfortable. Id stick close to the ratio that you find as having a too short depth means people wont be as willing to sit and the seat being to high or short will stop it being used.

It sounds like a fun project :)

Dave

EricL · Answer

Hello @Jacob93

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your concrete block garden project.

I think your overall plan is very achievable, and the proposed 100mm gap between the planter and the Colorbond fence is a good idea. It will keep the planter independent of the fence and give you some separation for drainage and future maintenance. With around 29 metres of planter at approximately 500mm high, though, I suggest treating it more like a small retaining wall rather than simply a decorative garden bed.

For the strip footing, I would be a little cautious about going with 300mm wide × 150mm deep without having it checked first. The appropriate footing size will depend on your soil conditions, reinforcement and the type of wall you eventually build. Given the overall length, I recommend having a structural engineer or suitably qualified professional nominate the footing size, reinforcement, starter bars and control-joint locations. It shouldn't be a particularly complicated detail for them to provide, and it will give you a solid starting point for the rest of the project.

I don't recommend mixing your leftover road base into the concrete. A much better use for it would be as a compacted sub-base beneath the footing, provided it is suitable for that purpose.

For the wall itself, I recommend using the 190mm hollow concrete masonry or Besser-style blocks, particularly if you're planning on rendering everything white like your concept image. I suggest using them as a properly reinforced and core-filled wall, with starter bars tied into the footing and the required block cores grout-filled.

I recommend waterproofing the soil side of the masonry, protecting the waterproofing from the backfill and installing a socked slotted ag pipe along the bottom surrounded by drainage aggregate. Geotextile can help prevent soil from migrating into the drainage material, and weep holes can provide additional drainage where appropriate. Make sure that the agi pipe has a suitable discharge point. I don't recommend lining the entire footing trench with plastic, as you don't want to unintentionally create an area where water becomes trapped. Please refer to your engineer regarding this option.

I suggest keeping the timber out of direct contact with the soil and, if possible, making the seat sections removable. A good tip is to use the space underneath them as accessible areas for your lighting and irrigation services rather than permanently enclosing everything. Please note that any mains electrical work will need to be carried out by a licensed electrician.

Finally, I'd paint the Colorbond fence Monument before starting the planter. Trying to paint behind a finished wall with only a 100mm gap won't be much fun. Keep that gap open and free-draining, and avoid attaching the planter or seating structure to the Colorbond fence or its posts.

Once you've had the footing and reinforcement details confirmed, I think you'll have a very good foundation for the project. The combination of the rendered planter, recessed timber seating, olive trees, lower planting and integrated lighting should work particularly well with the look you're aiming for.

Let me call on our experienced members @Codey, @Dave-1, @Noyade, @ProjectPete, @sannear, @scottlockley, @Nailbag and @DIY4Knuckleheds for their recommendations.

I also suggest having a look at these discussions - DIY paved firepit area with built-in seating by @Codey, Circular fire pit area using brick and Ironstone by @scottlockley and Poolside timber cabana with concrete bench by @ProjectPete.

If you have any other questions we can help with, please let us know.

Eric