To prevent the tie beam of a couple-close roof, when supporting a ceiling, from sagging or bending with the weight of the ceiling,1 it may be supported in the centre by an iron king rod (KR), suspended from the ridge r.

1 From Wray's Application of Theory to the Practice of Construction.

For spans of from 14 to 18 feet a collar may be introduced half-way up the rafters to stiffen them.

We have now arrived at a form of roof in which the feet of the rafters cannot spread outwards, and whose tie beam is prevented from sagging or drooping in the centre.