I just use the small square floors, and for roofs can't do much with upside down triangle gaps other than line it with beams.

At a guess they will "plug the gaps" as it were with new building shapes in the Home and Hearth update.

There's no easy way to do it, just need to rotate the regular square sections to fill the gaps. It doesn't always leave a smooth surface and might have some little gaps here and there but you can usually make it work well enough.

Hoping to see more options in the Hearth and Home update!

Floors are easy to overlap in most places, as for walls, hopefully that's something covered in H&H. It's one of the reasons I've paused on playing until it drops, as I very much enjoy building and am hoping for more options.

are you using vanilla game or valheim plus in valheim plus pressing f1 as you place an item it allows you to manipulate that piece, using ctrl alt also changes what you can do to pieces, like rotating the wall roof angle pieces in a horizontal or vertical movement.