No, its not a good idea to peice together any header because it is going to be bearing full time weight and over time it will sag or even fail.
I hope this helps you project and thanks for the chance to help.
HiCustomer There seems to be some confusion regarding headers here.
Assuming you have a 2x4 wall, your header should consist of 2- 2x10x10ft cut to 9ft. They sandwich a 1/2 plywood filler. The purpose of the filler is to make the header 3.5 inches thick.
In a 2x6 wall the filler would be 2 3/8 to widen the header to 5 3/8. -- 2x4 top and bottom, ripped to 2 3/8, would be used. If the load required 3- 2x10, in the 2x6 wall, the filler would be 7/8. The 2x10s do the load carrying, plywood fillers add a little strength, but are not for carrying load. Since available jobsite plywood is usually 4x8, anything over 8' will have to be pieced in. Do not confuse this with splicing the header(2x10). The ideal location for your plywood fill is centered, with 6 inch pieces on the ends. But a single 10 inch piece is fine. You nailing schedule is every 12 inches- 1.5 inches in from the top and bottom, AND staggered from those, down the center, every 12" as well. The nails should be 3.5 and angled so they do not penetrate through the the entire thickness.
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