Do we need terram under my new veggie patch?

I don't mean to be rude, but why do you want a membrane? I can understand you don't want weeds growing, but you could use bagged compost and cover with wood chippings. To be honest, it does depend on what you're thinking of growing in the raised bed. Some plants like to be able to spread their roots, while others, like bedding plants, don't need too much depth. The membrane could stop roots going down into the soil. Covering with stones only makes sense if you're growing alpines or ornamental grasses. The stones could stop warmth from the sun getting thro' to the soil. I'm not an expert by any stretch of imagination, but I would be inclined not to use membrane.

Im with Crafnutter on this one , If you look at my blog u will see that I built raised beds last year and I put a membrane under them , I have now doubled some of them in height and have been ripping out the membrane if I am workin on the soil, although it wont hinder drainage it will stop root crops reaching full potential eg parsnips etc, I would fork over the ground if you feel energetic before installing the raised bed. good luck Mac