In reply to MonkeyPuzzle:
Should be ok if it's a decent quality solid door.
Years ago, when living in the Lakes, I popped in to Lancaster to buy 3 sheets of plasterboard (...you can see this coming, can't you?). Being a callow youth, I lazily lashed side-to-side and got on my way. I'd not got past Hest Bank (at a time, I admit, when the absence of cameras etc enabled you to cane it quite impressively) before the front half of the load above departed...fortunately missing a moped rider (no helmet) and a Co-op delivery van running behind me. On braking the remainder slipped through my hasty anchorages, over the roof & bonnet and was deposited in a dusty pile just in front of me. True Brit stylee, I picked up the stuff, lobbed it in the boot, pretending it was all a carefully orchestrated event. I've subsequently always been more careful when carrying stuff up top.