Just to add to what Mandy's already said - the houses can be any shape at all. I've seen sheds, large dog kennels, we use an old metal oil tank in addition to our metal arc shaped ones. Lower roofs do help to keep them cosy. So long as the roof has a slight slope to let the rain run off and they've plenty of straw they'll be fine.
Wood has better insulation factors than metal, but has the disadvantage of being chewable
Remember to add ventiliation holes/hatches so that you get air flow
Our arks are not attached to the floors so when it comes time for a clean out we flip the top off and sweep out all the old stuff then replace the top and let the pigs arrange their new bedding themselves
If you search 'pig ark assembly' on the forum search you should see a step by step guide for making the curved ones.
HTH
Karen