Replacing Roof Support Beam with 1x boards?

[Norminn]

I hope I get terminology right - our condo is four buildings connected by continuous roof over atrium, two story. Finally, board has gotten around to repairing a long-rotted support beam and correcting this sagging portion of roof. I've commented before that the sag was held up for years by downspout tubing that someone installed between the ceiling and railing around upper deck. Duh! The old beam rotted entirely away along the end of a skylight that blew away last year :o) Rather than a solid beam, contractor is using 4 1x12's glued and nailed to each other. 2x4 along bottom of each side will hold the rafters - rafters have a corner cut out that the 2x4 fits into. The new beam spans about 15', but I haven't measured. The beam goes between two concrete? encased steel beams, one of which crosses the center of the atrium. The steel/concrete beam across the center gets support from a two story wood partition that isn't entirely healthy and takes a lot of stress in high wind. The span for the new wood beam from outer to center beam is about

22'. New skylight, about 5x15, will be installed, one end of which sits at about the center of the new beam.

[louie]

This laminated beam will probably be many times stronger than the original solid beam. It's actually becoming more common to see this sort of construction now.

[Wayne Whitney]

Why is this? The grain is running in the same direction in both cases. Is it simply that any defects are offset in the laminated beam?

Cheers, Wayne

[louie]

Partly that, but mostly a beam's strength comes from it's depth not width. The moment of inertia (directly related to load carrying ability) of a certain cross-sectional shape = width x height^cubed /

1. So a 3/4" x 12" beam has a moment of inertia of 108, which is equivalent to a 6x6 beam.

That said, you DO need SOME width to prevent twisting & buckling, but it's not the primary load characteristic of a beam. Also, many times, the beams are laminated with plywood or OSB, which does vary the grain direction.

[Goedjn]

Pretty much, yes. That and the fiber-stresses in a solid beam tend to all be uniform, and those is a laminated beam are averaged out, so a laminated beam will seldom twist with changing humidity.

[PipeDown]

He didn't describe an engineered laminated wood beam , he described a composite beam made up of four pieces of dimensional lumber. This would be a little stronger than a solid beam but not as strong as a truly laminated beam but will be dimensionally stable (less likely to twist or warp). This construction may be adequate if there is not a large load on that beam and would be much less expensive than a solid or laminated beam. If the contractor is good, he at least alternated the crown of the planks before gluing and nailing. (nails may be incidental until the glue cures depending on how much and what kind of glue)

It's probably adequate construction but likely does not exceed design requirements. The real thing is to find the source of the water that rotted the original beam and make sure that is resolved too.

[Norminn]

She :o)

The board/beam now has some big, fat bolts through it about every 3'. The rafters, which appear to be about 12" apart - didn't measure - each have those metal thingys that anchor them to the beam. The metal thingys are kind of a "U" as the rafter sits in them. It appears it might stay up a while :o) I've seen so much half-assed work it worried me, but this contractor is pretty cool. Even cleans up each day. Inspector comes today.