Making Long Window Header Opinions

[ROANIN]

When making headers for windows etc, is it better to use Plywood or OSB between the 2 x's

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[RicodJour]

It doesn't make any real difference. Long headers are best made from LVLs - much stronger, stiffer and they don't shrink.

R

[Doug Miller]

Plywood is considerably stronger than OSB.

[Steve Barker]

what ever is the proper thickness for what you need to end up with. Sometimes i just use scrap blocks of sheetrock.

[ROANIN]

Does not the Plywood or OSB add a bit of structual strength to the header?

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[Steve Barker]

Not that i've seen or read. It's the 2 or 3 two by whatevers that provide the proper support. The spacers are just to make the whole thing the right thickness for the wall.

[Harry K]

But my buildign inspector required that I replace my double 2x6 porch support beam with double 2x8 with a 1/2" ply well glued.

Of course it is the old "what the inspector wants, is what he gets" :). I did talk him out of some of his other complaints. Built his way I could have parked a D6 dozer on it.

Harry K

[RicodJour]

[Steve Barker]

that's the problem with inspectors and inspections. They tend to do it "thier" way and not the book way. That's why i read the book and leave the inspectors, inspections, and permits out of the deal.

[jamesgangnc]

[Pat]

I like to use foam. If strength is a issue then I use steel.

[Roanin]

I agree. The ply is mostly just to make the two other boards conform to the 3 1/2" wall thickness. I've seen all sorts of scraps used in that application. If 2x8 was marginal then maybe the inspector was doing you a favor. What did the span tables call for? What's your span? Double 2x6 as beam is not a lot of beam. The ability to resist deflection goes up a lot as you increase the height of the beam. Not because of the increased strength but because of the increased distance between the top and the bottom. That's how they make I beams out of 2x4 and osb.

So from what I am reading, the glue and nails that hold the plywood or osb between the 2 x 'S do nothing to increase the strength of the header?

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[Steve Barker]

yes, that's been stated a few times now.

[RicodJour]

Any time you have a composite structural member the parts should be well attached. A simple window header doesn't really benefit from attaching the parts with more than the usual attention. Unless there is a great disparity in the strength/stiffness between the two 2x layers, the header parts will naturally work together.

There's an old saying - stiffness attracts load. The weaker layer will deflect more/sooner than the stronger layer. The stronger (read stiffer) 2x will take more of the load until it deflects to the point where the weaker layer starts picking up the load.

Bonding the layers together makes the composite header's layers work in unison immediately, which would tend to reduce the deflection by a little bit. But the header span is usually relatively short and the absolute deflection is small - much, much smaller than a floor joist would experience.

Window headers do move, but most of the movement - at least for solid sawn lumber headers - is shrinkage from drying. That's one of the reasons that there shouldn't be any shims between the header and the top of the window.

R

[jamesgangnc]