TWO 2x6's vs.ONE 4x6'

Question

We are building a roof for our deck. The plans I gave the engineer called for the roof trusses to rest on 4x6 beams. However, when we got the plans back from him, he designated those 4x6's to be a set of two 2x6's instead. I don't get this?

Is he just using 2x6's to save money? Is there any real advantage to using them? As far as I can tell, they aren't as strong, or as a solid 4x6. So all I can see are cost savings. Why/or why not should I allow them to use the 2x6's?

Answer 1

Yes, he's trying to save money and time.  2X6's are common, easy to find, and pretty cheap.  4X6 beams are specialty items, and more expensive as a result.  Either way, you'll be the one paying for them in the end.  If he were to use construction adhesive between the sistered 2X6 beams, they'd essentially be as strong as the 4X6 system.

Answer 2

A 4x6 will measure 3 1/2" x 5 1/2" - perhaps a bit bigger, depending upon the mill it comes from.  Two 2 x 6's will measure 3" x 5 1/2".  If the wood was perfect -- no knots, perfectly straight grain, etc. -- the 4x6 would be slightly stronger because of the larger size.

Wood is not perfect.  You might buy a premium timber (for a premium price), but there will at the very least be some inconsistencies in the grain of the wood.  When you use two smaller pieces instead of one larger one, the deficiencies tend to occur at different places in the span.  One piece will be a bit stronger in places where the other may have some imperfection that makes it slightly weaker.  For this reason, in practice the two pieces will be better suited for a beam than one piece.

Answer 3

Two 2x6's will be a lot stronger with less chance of warping. You don't have to, but to make them even stronger, you could put construction adhesive between the two and clamp together overnight then put them in place.

Answer 4

They would be considered equal in structural strength and because they are two they may even provide a slight edge over the solid 4x6. Also they would be less expensive.

Answer 5

Well a 4x6 will be 3.5x5.5 two 2x's will be 3x5.5 so they will be slightly smaller. Plus it has a seam  that could allow water to get between and if the gap isn't sealed then you could have a problem when it freezes.

Answer 6

Actually, two 2X6's are stronger than one 4X6 if they are screwed or lagged together. Laminated beams are stronger than  solid for some strange reason. If it is imperative that you maintain the 3 5/8" width, use a 1/4" plywood filler between the boards. Since it's going to be outside, use treated lumber, then before it's assembled, coat the wood with Thompson's Advanced Brand water repellant. It is available in clear or colors (stains)

Answer 7

The 2 2x6 pieces together are stronger than one of the same size as one piece. This is standard in construction, you use 2 pieces for vertical support instead of one, it is stronger.  Glad you have gone over the plans, but follow the engineer/ architects specifications. That is why you hired them.