if I’m putting a wooden shelf above a radiator:
– do I need to protect the underside with some sort of heat shield?
– can I paint it with something to protect it?
– is there a minimum gap I should leave?
– will it ultimately dry the wood out and make it crack?

EDIT: why can’t I move this to the chat forum?

I wouldn’t worry too much, if the radiators in your house are hot enough to burn wood you’ve got bigger problems 🙂
If you use MDF its not going to crack.

that’s the thing, the radiators are hardly on anyway

I fashioned an mdf one with hidden brackets. Sits about 4-5″ above. No problems a few years on

Use well seasoned (kiln dried to about 9-10%) hardwood and you’ll be fine…..

i’d go for plywood rather than MDF if you’re storing heavy stuff on it

should be a good place for your seedlings in january 🙂