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Building New Raised Bed

I have some scaffold boards that I will cut up to create a raised bed 6" x 6" by 18" high
and I will use black membrane to form a barrier at the bottom to stop weeds
? What should I fill it with and why and to what height please
Everyone is just trying to be Happy.....So lets help Them.
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Hi what do you want to grow? Flowers or veg or both. Are you preparing the bed for next year? Cheers
Sounds like a very narrow planter or a very tall window box for a very small window NewBoy. Are you sure those are the measurements?
I live in west central Scotland - not where that photo is...
I'm guessing 6 foot each way. You could fill the bottom 6 to 9 inches with rubble or poor quality soil and complete with top soil. If you use rubble be prepared to have to add more soil once the initial fill has had a chance to settle.
Just re read my original posting and it should be 6 foot.x 6 foot by 18 inches high
The scaf boards are 11 foot 9 " long so hence the dims
Its just for veg
I built one last season and the carrots and garlic were brilliant but I wanted advice and input from the GW Experts as its going to cost a bit to fill it and I can then adjust the current one
For a start I'd be inclined to make it less than six feet wide. If you halve it to three feet you can easily reach the middle from the sides so you never have to walk on the soil. (Four feet is better still as it gives more space if your arms are long enough but three will fit the boards better). This is a good thing as it avoids compacting it, forcing out air and damaging earthworms' burrows and the rest of the soil structure. Then you never have to dig! You could, if you have room and fancy it, make it/them longer. 18" is a good height.
Filling it - if you have a stables nearby, you could use some of their manure, which will probably be free if you can collect it. That should halve the quantity of topsoil you need. If you're going to line the bottom with polythene (I wouldn't) it might be worth digging out some of the topsoil from underneath before you start, further reducing the quantity you have to buy. But I'd leave the bottom open for drainage and deep rooting. Cover it over the winter and plant spuds in there in the spring. They love muck.
You could also take the topsoil from where you're laying a patio/path etc.
I have just created 5 raised beds each one being 1.2 meters wide and 2.4 meters long. We have had to do this as out soil is full of slate and is also on a slope. Luckily we have a good supply of farmyard manure from the local farmers and as I will not be growing in them until the spring I have also added rotted grass cuttings, a good amount of shredded paper and cardboard and I also plan to add leaves when they have fallen. The worms are already doing their job of composting and mixing it all.
Good Luck with the beds
Why do you want a raised bed? What is it on. If on soil, why not just have enough height round the edges to retain compost. I have 4" high boards around 3 foot wide beds, 20 foot long. The boards do no more than retain 2" of compost put on each year. The soil is hard clay. Plants grow well. Very well.