Cathedral Ceiling Ridgebeam support

[Inland_Empire_CA]

I am in southern california where most houses are slab on grade, no floor joists and subfloor. I want to build a large shed on slab floor. I think it will be on about 600-900 sq feet of concrete slab. I may go two-story (600 sq feet slab) or just one story (900 sq feet slab).

I just read a book that says only that both ends of the much larger beam (that is the ridgeboard) in a cathedral ceiling roof have to be supported by the end (rake) walls. It says nothing about the usual column support for beams. In my case the length of the beam will be about 32 feet.

I also know that collar ties only 2-3 feet long (which is the horizontal line of the letter A) across opposing rafters add to the portion of weight born by the walls.

Is it true that columns in the middle may not be needed for a catheral ceiling beam, the ridgebeam? Only the two ends need to be supported?

Also, I think the ridgeboard beam can be made from 2x's, say five to six 2x4 face-nailed together, staggered. Is this true?

face-nailing five 2x4's together is actually only 7.5 inches thick, but is norminally a 4x10. This is the code interpretation?

[Don_P]

OK but I'm staying in the yard, there's a few dangerous thoughts going on.

1st, you cannot make a beam of any type by face nailing horizontal plies. The allowable design value for this will be one 2x4 flatwise. You will not find 32' lumber easily so there are unsupported splices also, no go, this is not a beam... ever, anywhere. The only way you can field build up a beam,header or girder is to vertically nail laminate lumber, for instance 3- 2x12's standing on edge face nailed, the bending force is in the timber not in a joint.

It is going to take quite a beam to support half the roof by 32' long. I don't have enough information to size it for you nor can I, this is an engineer's territory. To give you a rough idea the beam here would be an LVL built up or a glulam in the neighborhood of a 6x18x32'. Your mileage will vary considerably depending on local loads on the roof.

Ties should be in the lower third of the rafter span to be effective.The moment the ties are raised above the wall plate they begin to weaken the rafters and reduce the allowable span for a given rafter size. Raised up near to the top they are effective as gussets in holding the rafters to the ridge in a wind or earthquake but they are too high to help with rafter thrust.

Each end wall will contain a column that will transfer the load to an appropriately sized footing capable of transferring the load to the soil. In this scenario I would push for either a center column (which needs to be sized large enough for its height that it isn't in danger of buckling due to slenderness) or check into scissor trusses ;)

[Inland_Empire_CA]

"The only way you can field build up a beam,header or girder is to vertically nail laminate lumber, for instance 3- 2x12's standing on edge face nailed, the bending force is in the timber not in a joint."

Thank you very much

Makes sense, the book doesn't say.

Is it a common practice to sandwich a piece of 1/2 inch plywood between two 2x8's so that the beam is the same dimension as a solid piece of 4x8? I know that this is done for headers, but is it for a beam for the ridge of a cathedral ceiling?

I ask because the way the rafters are cut (bird-mouth) to fit the ridge beam differs depending on the width of the ridge beam. The half-inch changes the angle of the bird-mouth a bit. For cathedral ceiling, the rafters have two bird-mouth cuts (and a tail cut).

Thanks

PS

However, the cap plate and top plate of a weight bearing wall are face-nailed together horizontally. The top plate and cap plate nailed together horizontally in a middle of a structure also functions as part of a beam. Since I will have a long weight bearing wall that supports the ridge beam, so, in essence, the cap and top plates of the weight bearing wall is a part of the ridgebeam. So the ridgebeam will have mix nailing, face nailing vertically and horizintally. So the top part of the beam that is vertically nailed should have the same width as the lower part, which is 3.5 inches wide. Also, I need 4x8 or 4x10 ridgebeam, so I need only to nail two 2x4 or 2x6 vertically to get a beam that is 4x8 or 4x10 in total, counting the top and cap plates of the supporting wall.