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I want to replace a 20" W window ( 20 5/8" rough) with a 36" (rough) window. I want it to be as close to the ceiling as possible. It is a single story house with 2x4 studs on 16" centers. It has a double top wall plate.

enter image description here  photo DSCN3771.jpg

Full size image here.

What is the smallest size header I can use. The new window is 36" W x 12" H. (rough).

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it depends on a lot of things:

1) dead load above 2) live load (do you get lots of snow, rain, etc.) 3) wall finish (for practical reasons you may have difficulty changing the header on a stucco wall, which is what it looks like from the cement stains on the interior of the sheathing boards) 4) material. steel is much stronger than wood. spruce is stronger than pine.

in a 2 x 4 wall, you can do a double header construction with a 5/8" ply liner between them as a spacer. given this, the fact that you are increasing the span from 20" to 36", then the span will be almost twice what was there before, but its still only 36". generally, you need a 2x6 double header minimum for just about any scenario, so if you are worried about it, go to a double 2x8. here in ontario, lintels under 2x8 are not permitted (table a-12), but your span is so small it doesn't even fit the minimum requirements. just use a doubled 2x8. its overkill, but thats the best way to build anything.

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  • The existing header is made from two 2x 6's doubled up (no 1/2" spacer in between). The wall sheathing that is visible in the photo is 3/4" tongue and groove. The tongue and groove wall sheathing was likely used for the foundation forms and then used for the walls thus the cement stains. The roof is very low pitch--a little more than 1:12. I saw charts that showed 4" (2x4 with 1/2" spacer) as being good for up to 4' spans, but I don't know where the charts came from. Thanks for the response. – C Johnson Feb 19 '16 at 19:41
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Without knowing more about the construction of the house, I'd replace it with the same size header. I don't recall seeing header span tables that go lower than 4' spans, but the sizing depends on the supported load. You can find a set of span tables at southernpine.com, but most loading scenarios for that span are going to call for a doubled 2x8 at minimum. If that's a doubled 2x10 there now, I'd assume that you'd need a doubled 2x10 for the new window.

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  • Right. If you had a 2x6 wall you'd have more options. – isherwood Feb 18 '16 at 19:07
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If this is holding up a floor and a roof those I would not go less than what is there now but if it just holding up a roof it would depend on the span of the roof and the species of wood in your header. A double 2 x 6 header should be safe but check with your local inspectors with a phone call.

I you want the window as high as possible throw a piece of steel angle as a lintel with double 2 x 4s, be sure the angle is over the jack stud for full bearing.

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