[cheshirecheese]

My daughter has just moved into a new house, and the small garden at the rear is just laid to lawn with a small patio.  She wanted a border down one side against the fence, but when we started to dig we discovered there were only a couple of inches of topsoil under the grass, and that had been laid on top of two or three inches of builders' rubble, underneath which it appears to be solid clay!  
   

So, I have three questions:  
1)  How deep does a border have to be for perennials to survive?
2)  If more than 4 inches, what's the best way of dealing with the clay?
3)  What's the best sort of soil to use?  I have a couple of bins of black Fen soil that I could mix with something else.

Thanks  

[arugula]

I would say it depends on which perennials you would be growing, how deep it needs to be. Builders rubble buried very shallow is a common problem in newer gardens. One way you could get round it is to build up - a deep bed.

[cheshirecheese]

Yes, I did think of that.  I'm presuming solid grey clay under the builders' rubble isn't good then?!!  If we do build a raised bed, what's the best mix of soil to fill it with?

[mumofstig]

If you lift the grass and put it to one side, you can use it for infill after you've dug the rubble out

Try to break up the clay a bit, by inserting a fork and wiggling - it's too much hard work to double dig
Put the turves back in grass side down and then get a couple of inches of bought in topsoil on top. You should be able to grow most things then  

If you need to dig into the clay to plant climbers or bushes, make sure you break up the bottom of the planting hole, and add some compost or manure to the hole, and for back filling.

[cheshirecheese]

Thanks ... once this rain lets up (if ever!) we'll give it a go!  

[Goosegirl]

While you are breaking up the clay, get a load of sharp sand / grit and mix that in as much as you can. Also, do the same when making planting holes - these will have to be about twice or three times the size of the plant ball or you will end up with a sump for water to collect there.  

[grinling]

I dug my clay by spade and mixed with manure. If grey it is prob water logged. Tedious job but well worth it afterwards. Might be useful putting in some drainage at the same time.

[cheshirecheese]

We've now decided to make this into a raised bed.  Would it be an idea to get some hardcore or something similar and lay, say, a couple of inches or so of that over the clay before adding topsoil?  I've bought some breathable membrane to put in between.  We're only just starting the bed now as the garden was like a quagmire after all the rain so we had to wait for it to dry out a bit to avoid damaging the grass by trampling all over it!

[grinling]

I would use gravel.

[Flatcat]

We hired a mini digger for this job in our garden, and removed all the rubbish clay and rubbly stuff and bought in a few tonnes of fresh topsoil