They sit on and are attached to the purlins and the wall plate and are usually formed from 4″ x 3″ timbers laid on the flat side.
Some people find they clutter up the frame, making it too busy and woody and want them covered up. Others (and I’m unashamedly in this camp) wouldn’t dream of not exposing them, so you feel you’re in the upturned hull of a galleon.
If you're considering an oak framed building (or larch, or douglas fir), let's talk. We'll gladly put together an outline quote (completely free, with no strings attached). And we need very little information from you to do so.
Equally, we're always here, at the end of the phone, to talk through your ideas.