Ridge beam

[rakuz66]

What are the role of rafters in a cathedral ceiling once a "ridge beam" is installed? Do they stiill carry any live loads or does the beam carry all the load?

[Richard A Hetzel]

Well, is it truly a ridge beam, or is it a ridge pole? One carries load, the other doesn't. If it is a single 2x member, it's a ridge pole, and its function is strictly as a nailer, and it carries no load at all. If it is a fairly deep beam, made up of multiple 2x's or multiple LVLs, or a single heavy laminated beam, it's a ridge beam, and it's carrying probably half the weight of the roof.

However, no matter which it is, the rafters still carry load in two directions in the case of a ridge beam...some to the wall, and some to the beam, or in one direction in the case of a ridge pole. In that case, it's as though the rafters are continuous spanning from wall to wall.

A vaulted ceiling is more likely to require an actual ridge beam, because it would have no means of resisting outward thrust at the bottoms of the rafters, When the rafters are carried by a ridge beam, there is no outward thrust.

[Rich]

The beam will carry a tributary load from the rafters.. essentially half the distance from beam to walls from each side of the beam. So if you have a 24' width on your building it will carry 12' of the roof load (6' from each side of the beam) and the walls will carry the other 12'. Rafters are still required to transfer the load to the beam and the walls.

[Rich]

I guess I took too long in formulating my answer :)

[rakuz66]

Thank you for the answers guys. I ask because I have half of a cathedral or vaulted ceiling. My building is 22 ft wide and half of that,( 11ft) is a cathedral ceiling. The opposite half is a regular ceiling with ceiling joist/rafters, and a purlin running up to the ridge. The vaulted area is 15 ft long. Anyway, there is no ridge board. I know the differnce between a ridge board and beam. The rafters are connected at the ridge by gusset plates and collar ties. My 2x8 rafters are overspanned/undersized about 18 inches in length for the snow load on the roof. I'm wondering if I put a ridge beam in place if this will help with all the snow loading in winter????? Will the ridge help reduce the snow load on my rafters???
Or I thought I could place some lvls at the midpoint of rafter span. Put them underneath the rafters and run the appropriate supporting posts to the foundation??? Would that help in reducing rafter span?

[Richard A Hetzel]

The ridge beam will have no effect on the rafters' ability to carry loads. The choices are another beam as you suggested, or sistering the rafters. A more exotic solution would be to glue and screw a 2x4 to the bottom of the rafters, making an upside-down T...that might add enough load-carrying abilty and only drop your ceiling 1 1/2 inches.