They are fixed at an angle with one end connected to the purlin and the other on to a load bearing wall or a timber spread across ceiling joists.
Cânt eu rhoi'n sownd ar ongl, gydag un pen wedi'i gysylltu â'r dulath a'r pen arall wedi'i gysylltu â wal sy'n cynnal llwyth, neu bren sy'n ymestyn ar draws distiau'r nenfwd.