Sorry Charlotte but what is a 'troft'?

a troft is a long rectangle box that you fill up with soil and put plants in it

Ah a trough! Why would you want one in the greenhouse? Troughs are normally used outside to display small plants and create an effect. Made of wood they will be heavy when full of soil and yes they will rot. The Scottish Rock Garden Club has turned what is a waste product into an art form and the members create troughs from polystyrene fish boxes which would otherwise end up in landfill. Do remember to let the box sit out in the weather for a week or to so it doesn't smell. Instructions to make are at this website
http://www.srgc.org.uk/feature/fishbox/troughs.html

We've quite a few of the fish box troughs in our garden and they look very effective.

I think Charlotte wants to build a raised veg bed in the greenhouse? The timber you choose will have to be treated to stop it from rotting, but even so I think it will still rot wihtin a few years because of the moisture in the soil. I've got a raised veg plot outdoors and it's edged used old elm floorboards (seems a waste of good timber, but it did have woodworm). It's been there 3 years this year and no sign of rot yet. I do beleive that some preservatives can be harmful, so you might like to post a new re-phrased question to find out what to avoid. Good luck!