I'm building some raised vegetable beds with untreated oak sleepers, measuring 2.4m long, 1.2m wide, and 40cm high. I'm concerned about the heavy clay soil that gets boggy during winter. I thought about digging a 50mm trench and filling it with 20mm gravel to keep the sleepers relatively dry. However, I've read that MOT1 might be necessary under the gravel, but I'm worried it won't drain well. Could that cause water to accumulate and damage the sleepers? What should I really put underneath them?
5 Answers
The concern isn’t really about MOT1—it usually drains well. The issue lies with your clay soil. If your trench fills with water because of the clay, it could create the perfect conditions for the wood to rot over time. I recommend checking out some videos from The Restoration Couple on YouTube; they have good advice about drainage for raised beds.
I’ve had success with non-woven geo textile in similar situations! It drains effectively and helps with soil moisture. I used it in a clay-heavy area and it made a huge difference. You might find some good options if you look for merchants selling custom sizes, so you can avoid using unnecessary MOT1.
So, just to clarify, I should put the textile between the soil and the gravel instead of using MOT1?
Laying down a layer of gravel or coarse sand at the base of your raised bed will really help. It prevents water from pooling around the sleepers and will improve drainage.
What you choose to put underneath the sleepers isn’t super critical as long as there’s soil against them. Using permagard both underneath and on the sides can help extend the life of the wood.
Your plan sounds solid! As a precaution, consider applying a good oil-based wood treatment to the base of the sleepers. This will help protect them even more, though oak typically lasts quite a while compared to softwoods. If you’re using large construction screws, having a strong impact driver handy can make assembly much easier!
I totally agree about treating the ground-facing parts! If you go for a cement footing, that could also help stabilize the structure.
Thanks for that tip! I checked the channel out—it looks like they lay down a textile fabric before adding gravel, which seems to be a common approach here. No MOT1 needed!