Raised bed on an uneven base?

[Harpoon]

We have a small border outside the kitchen window and we'd like to make better use of it as a herb garden, moving existing plants out of pots (though the mint will certainly be staying in pots!). There's not a lot of soil depth and it's also narrow, so making a small raised bed looks a good idea. I've already got some 200mm (h) x 20mm (w) treated boards I can use.

My dilemma is that one side of the bed would be sitting on top of the retaining wall, which means an uneven base (picture below). My current thinking is to get the frame of the bed anchored / fixed into position and then use a mortar mix to fill in the gaps between the board and wall as neatly as possible. If I fill from the inside, I might be able to do some improvised shuttering to get a reasonably tidy finish on the outside. I can then line it and fill it.

I'm open to suggestions for a better way to deal with the gaps though - does anyone have better ideas?

[RC1807]

scribing the wood and cutting it so it follows the contours of the wall

[Harpoon]

Cheers but I think that's outside my woodworking capabilities. Over a couple of meters that's a lot of small detail to get right.

[worsy]

Could you line the bed?

[Harpoon]

I'd line it with some DPM or similar to protect the wood. I guess DPM or weed fabric could cover the gaps to stop the soil getting pushing out as it compacts over time. I'd need to anchor the membrane somehow to keep it tight at the bottom.

[omniflow]

Harpoon said:

Cheers but I think that's outside my woodworking capabilities. Over a couple of meters that's a lot of small detail to get right.

Scribing it wouldn't be that hard. You can get a simple tool that follows the contours and then has a pencil a little bit higher up to mark the wood. You wouldn't need to be super accurate on the cutting out either. Just make it a closer fit than you've got now and then either use mortar or a liner.

[paulwirral]

Harpoon said:

I'd line it with some DPM or similar to protect the wood. I guess DPM or weed fabric could cover the gaps to stop the soil getting pushing out as it compacts over time. I'd need to anchor the membrane somehow to keep it tight at the bottom.

Just lap it onto the inside of the raised bed 300mm or so , once it’s full of earth it’ll be fine , I did similar but with railway sleepers . and purposely raised them off the floor

[Harpoon]

omniflow said:

Scribing it wouldn't be that hard. You can get a simple tool that follows the contours and then has a pencil a little bit higher up to mark the wood. You wouldn't need to be super accurate on the cutting out either. Just make it a closer fit than you've got now and then either use mortar or a liner.

Do you mean a profile gauge?

https://www.screwfix.com/p/magnusson-polypropylene...

Or something else? If so, do you have a link?

Thanks!

[TA14]

paulwirral said:

Harpoon said:

I'd line it with some DPM or similar to protect the wood. I guess DPM or weed fabric could cover the gaps to stop the soil getting pushing out as it compacts over time. I'd need to anchor the membrane somehow to keep it tight at the bottom.

Just lap it onto the inside of the raised bed 300mm or so , once it’s full of earth it’ll be fine , I did similar but with railway sleepers . and purposely raised them off the floor

Yep. Dig down and attach 2x2 or 2x3 posts to the wall. Attach your timber plank to the posts. Put in black Visqueen dpc with a lot of drainage holes in the base. Throw in some pipe bedding/shingle if you have clayey soil. Backfill with top soil.

[Bill]

Is it needed? AIUI a lot of Mediterranean herbs don't need much in the way of nutrients or soil.

[4Q]

Harpoon said:

Do you mean a profile gauge?

https://www.screwfix.com/p/magnusson-polypropylene...

Or something else? If so, do you have a link?

Thanks!

You just need a pencil and a small block of wood. Tape the pencil to the top of the block with the tip poking off the end and run the block along the wall with the pencil marking your plank. Then use a jigsaw to cut along the marked pencil line.

Edit, similar to this https://youtu.be/9-cYp73ewEE?si=aDtTcz22enZfM_3x but on a wall that uneven you'll find it easier if the pencil is fixed to the block, alternatively drill a hole in the block and put your pencil in that. Make sure you use a big enough spacing on the block to span the largest gap.

Edited by 4Q on Friday 4th April 14:46