Raised Beds Detail

mark_baldy

Hi all, so I'm building some raised beds and would like your advise. The make up / detail of the beds will be as per the attached image, I am using pallets as the base so the water can drain away. However, as the make up goes (from bottom to top) earth - pallet - cardboard - sand - soil - compost. I am querying whether I should slit the cardboard at the bottom above the gaps in the pallet - to allow even better drainage. I usually would but I feel the sand will slow the run off down enough. What are your thoughts? Thanks

wild edges

The rats and mice will love living in those pallets if you don't block them up to prevent access. I'm not sure how you would do that though as they can tunnel down from above. I'd use a geotextile instead of cardboard or the card will just rot and everything will collapse into the pallets anyway. Or replace the pallets with a layer of rubble, level up with coarse sand and then use the membrane to keep the roots out of it.

mark_baldy

Thanks for that, I will be cladding it all later on which should help to prevent the rodents, plus I will be installing pest control as I also intend to get some chickens (which I hear are rat magnets) but I will take your advice on board and replace the cardboard with a geo-textile, preferably something rat and mice proof. I will update with more pictures once it's all clad and back-filled. 

Obelixx

Are the pallets there because you're building the beds on an impermeable surface such as concrete or tarmac?  If not, I'd just build the beds directly on the soil.

Otherwise, as @wild edges says, use a membrane that won't rot down but is porous to allow excess water to drain in wet spells.  Cardboard is good as amulch in winter to keep the soil free of weedlings and you can just top with compost and plant thru it so it will add to the soil fertility and quality and make lots of micro-organisms happy.

It's chicken food that attracts rats so just make sur eyou keep their grain bins sealed.

mark_baldy

Yeah pretty much, beneath the black anti weed membrane is compacted hardcore (as slabs were going to be installed in this area)
I've done a few vegetables in pots which have been well drained and they've done quite good, so I applied the same theory to the raised beds, hence the pallets. I'll keep my eye out for a robust membrane that won't rot yet allow the water to run off.
I'll make sure I keep the chicken food well sealed and secure.
Thanks for the advice, much appreciated

Hostafan1

Is the timber used for pallets of such a quality as to withstand , not only the weight, but the dampness?

Nollie

Looks like you are planning some great raised beds there, so I hate to rain on your parade...

For that height, the lightly battened structure would not be strong enough to withstand the significant outward forces of the soil, especially when wet. You really need sturdy, square-section posts of a minimum of around 3”/8cm at each corner and at intervals of around 1m, then clad in sturdy planks like scaffolding boards, firmly affixed to the outside of the posts, with internal corner brackets for extra strength.

I’m afraid I don’t think the pallets are a good idea either, unless you totally fill them with gravel. The pallets will rot and collapse over time, meaning the soil level will drop and you will have to top it up. If the beds are for shrubs and plants - anything other than annual veg - this will mean the plants will be at the wrong level, as you can’t raise the soil level around them without rotting the stems. It would have been easier without them, just putting a mix broken hardcore/gravel at the bottom. If you get a lot of rainfall and are concerned about drainage, the usual thing to do is drill ‘seep holes’ through the structure at the level of the hardcore/gravel base, so excess water can escape.

If you do use sand, make sure it is sharp sand and not builder’s sand, which is designed to clump and bond with cement (it does that equally well with clay soil, forming an impenetrable barrier). Consider using grit instead of sand, which creates a looser, better draining medium (imho) but sharp sand is fine if that’s what you’ve got.

For your growing medium on top of the hardcore/gravel, I would also mix the three materials together - sharp sand/grit, soil and compost. Creating separate layers of different materials is not a great idea, they may ‘set’ as different strata and will absorb water at different rates. You want the roots of your plants to thread their way through a consistent material, not suddenly encounter a different one.

I’m sorry of thats sounds very doom and gloom, but a sturdy structure with a well-mixed soil mix that drains well and lasts, will be much better than one that collapses after a couple of years.

Obelixx

@Nollie a,d @Hostafan1 are right about weigts and forces.  All to easy to think about soil and plants and not the engineering needed to hold it all together long term. 

Fortunately it's not too late for a re-jig - build a base using solid timbers and fill the bottom with rubble and grit for drainage then cover with a weed suppressant membrane and build sturdy sides from breeze blocks or scaffold boards or sleepers.  If using wood, it pays to lline the inside walls with plastic - sold by th emetre in good DIYs - t protect the wood from dampness in the soil.   Use a staple gun to pin it to the top layer witha  neat edge.

Fill wth soil soil, compost etc. and you can recycle the smaller timber from the current frame to make supports for insect netting and plastic cloching depending on the season and crops.

JennyJ

I don't think you need a base with a gap underneath. The hardcore should drain OK if you build the beds straight onto it. I agree with the others that you'll need much sturdier posts, and strong timber for the walls to hold the amount of soil that'll be in there.

Hostafan1

I remember once being told a rule of thumb that a cubic yard of soil weighs one ton.
No idea if it's correct, but gives a rough idea of the weights involved.

Obelixx

Little clue, in French a cubic metre is called a tonne. That's a lot of kilos and downward and outward pressure.