Expert: Daniel Humphrey - 12/6/2008

Question
QUESTION: I have built an addition to my home. I was only able to achieve a 90" (7'-6") height. The header in question is for a 6'- 0" sliding glass door built in a 2x6 ext. wall 16" o.c., that sits on a doulbled 2x6 rim joist. I also have a concrete footing with a 5" post directly under center. If I only have 7" for a header would doubled trimmers and a doubled micro-lam header (1-3/4 x 2) be sufficient to carry the load of a second story sun deck?

ANSWER: Dear Dayne,

If you put a double trimmer over micro-lam header, you should be fine. You can increase the bearing capacity of that by sheathing over the header and trimmer, joining them together in sheer, creating essentially a 12-1/2" deep header, with approximately four times the stiffness of those two just stacked.

You mentioned the bearing post directly beneath the center of the door--make sure you have enough support there, beneath the door jambs.

Good luck,

Daniel

---------- FOLLOW-UP ----------

QUESTION: Hi Daniel, I believe you meant a double trimmer under the micro-lam header which I have done. I only have room for a 7" header in height. This is a 2x6 ext. wall. The maximum width of the header can only be 5-1/2". It spans 78". I'm now thinking a triple micro-lam header should be sufficient. Your thoughts again please.  Dayne

Answer
Dayne,

I was thinking of a doubled rim joist above the header, as part of the deck support. Sorry, I typed "trimmer" instead. I also misunderstood and thought your proposed micro-lam header was 5-1/2 deep by 1-1/2 or 2 inches wide! No--your proposed 2" deep header is not stiff enough, even with doubled trimmers.

It's really difficult to imagine the structure you have up above your header without a picture or drawing. And it is that weight and structure bearing down upon the header that determines both the strength needed and the possibilities of adding to that strength with the floor structure itself. Lacking that, I gave you ways to maximize the strength of the header by tying into the rim joist above, if there is one.

To help you figure out the strongest arrangement, stiffness is a factor of width x depth x depth x depth.

If you add header pieces side by side, they are additive; three headers side by side are three times as stiff. And if you just stack the pieces, the increased stiffness is again only additive. But if you add depth and tie it together so that it acts as one piece, the stiffness is increased by the additional depth cubed.  

I should point out that trimmers just nailed flat to upright header pieces do not increase the stiffness as much as if those header pieces were deeper. In other words, a 5-1/2" header with two flat trimmers is only 55% of a 8-1/2" header! But if you can secure those trimmers to the header with glue and sheathing, you get all that strength of a deeper header. In fact, if you build up a triple header 8-1/2" deep and 5-1/2" wide, using a couple rips of 1/2" plywood in between, you get the same strength as a double 2x10 header with nailed double trimmers--which is the standard for a patio door header. If you can do the same thing with your full 7", you will have a header 72% as stiff as the standard, which is somewhat overbuilt.

I've guessed at the actual floor load and figured that a full 5.5" width and 7" depth will be sufficient, even out of standard lumber. Here is my reasoning:

The load of that section of floor is determined by dividing the total weight on the floor by how many square feet of that the header supports. You can assume a live load on a second floor to be about 100 pounds per square foot, and a dead load averaged out at about 5 pounds. I was guessing your floor is 16 feet wide, and that the header will be supporting 8 feet of that floor; multiplying by the length of the header, I'm estimating the header supports about 48 square feet and its 5,040 pounds. To make sure the header will not deflect, we assume all that weight will be positioned about midspan (though it is not really). To support 5,040 lb without significant deflection (1/360th the span), the minimum header of Douglas fir should be at least 5.5 inches wide and 5.5 inches deep.

That's why I'm saying a triple header with plywood sandwiched to fill out the 5-1/2" should be fine, even if your floor is a bit larger, as long as you make an effort to tie all the pieces together above your header to include their depth as well.

Hope that is more clear. Good luck with your plan review.


Daniel