calculating load on a ridge

How do I calculate a load on a ridge with common rafters? I want to split the rafter load in half. Half going to the ridge, and half going to the plate. This would say that the ridge must hold twice the load as each of the two plates do. If this was a floor I would say this is true. A senter (or summer) beam would hold twice that of the outer beams suporting the joists. But in the roof (because of the angle)my epereance says the plate caries more of the potetial load. How should I size my ridge? The building in mind is 24' wide with a 9/12 pitch.

Hi Brendan,

Most traditional timber framed buildings do not employ a ridge piece. This device made its general appearance later on with debased forms of roof carpentry. There are a few exceptions including crucks and king post roofs where ridges were traditionally employed.

All that said your concerns about calculating and sizing a ridge piece for a common rafter roof are probably unwarranted. Virtually all of the loads from common rafters head down to the wall plate and where cross wind loads are applied then one rafter simply pushes on its coupled partner transferring virtually zero load to the ridge piece.

If a side purlin(s) is employed and the rafters are pegged to same then obviously the purlin will capture some of the rafter thrust and duct it away to the principal rafters. The exact ammount will be dependant on the size and stiffness of the purlin. On old buildings it is quite normal to see common rafters running over a side purlin but not touching same due to sag along the span of the purlin. Thus even with a side purlin roof fitted with common rafters you will still need to size the rafters to take full common rafter loads.

Regards

Ken Hume
http://www.kfhume.freeserve.co.uk

Thanks for your anser Ken,
Most all of the common Rafter frames I have built in the past have had queen posted purlins, where the wall plate and purlin shared the load. I have never Designed one with a ridge before. So in this case the wall plate should be sized to take all the load of the rafters on that side of the roof and no or min. load figured on the ridge beam, corect? Brendan

Brendan, You had it right the first time if you do not have any means available to keep the plates from spreading. On a 24' wide building, the ridge would be carrying 12' of roof, the eave plates would be carrying 6' plus the overhang. Generally the lack of a tying beam is seen where there is a kneewall that is too high to cantilever above the second floor and the client does not want any tie beams spanning from side to side. Traditional frames generally do not have these problems because if the eave plates were too high above the floor to cantilever, they put in tie beams even if they were noggin knockers.

I feel we need to define what this ridge will be in this case.
If this ridge is intended to hold up half the run of the rafters on each side then this is a "structural ridge" and itself has to be supported by a king post at each bent, or something like that. This would reduce the trust at the plate as the rafters would be "hanging" from this ridge.
If there is no support for this ridge then isn't Ken right that the weight of each rafter will be on the plate?
Jim Rogers

Yes, in order for the ridge to be structural it needs to be supported at each end by posts, trusses, etc.

This is intended to be a stuctural ridge that is suported by posts on all three bents, the bents are 15' appart. My question is, does the ridge take exactly half the load of each rafter as a joist would, or does it take less? my thought is the angle of the rafter may transfer more load to the plate, is this thought founded? and if so what proportion of the load is distributed to the ridge? Brendan

The tributary width of a ridge is determined the same way it would be for a joist or floor beam. In your case, half of the load on the building width would go to the ridge; exactly half of the load.