Whether it is the Gloucester or this Cheltenham Dollhouse, you must use stucco or some kind of other cover for the roof bottom, which will turn into your third floor ceiling. This roof will have a lot of gaps around the flat inner base so painting it only, will not look good. You need something that will fill in those gaps or hide them. You can certainly use foam core for a better fit, rather than the flat base it came with. You can also cover the foam core with a wallpaper frieze for added detail to your vaulted ceiling. You can cover up the gaps with crown moulding as well.
Once you have finished the roof, you need to install it to the dollhouse and though it is an easy install, it is very difficult to make it look right. The roof goes on the house in a very tight and perfect fit and when you don't have leeway on a dollhouse, perfection falls short. When it does, gaps appear. I tried to sit my roof as perfectly level to all the dollhouse walls as possible but there were still minute gaps all around. I used spackle to hide them. You can use more stucco or you can use crown moulding to hide them as well.
I suggest that you sit your roof on the dollhouse and acquire the perfect fit. Then go all around with tacky glue and glue it in place. Do not try to add the glue before the roof is sitting on the dollhouse because getting the perfect fit takes time and you do not want your glue to drip into your finished upstairs rooms.
The joint where the roof and dollhouse walls meet is not the only area with gaps. You will have gaps along the top part of your third floor partition. This gap will be too large for you to hide with stucco or spackle so you will have to use trim up there.
Do not worry about the exterior gap between the roof and walls because that will be hidden with shingles.
