Header Span and Wood Framed Rough Openings 5

[Engineeringisthebest]

The majority of my career has been in wood design. When I was a young EIT I was told to not worry about using the rough opening as a span and I never really looked back, even though there is often an extra 3". I had an EIT I was training ask the other day if they needed to include the full rough opening and I told them no, they asked and the best answer I had was, "Well we never did it before", which to me is a lack luster answer.

I'm curious if others are similar or if most folks are taking the rough opening as their span. Also, if anyone has any documentation from say AWC, WoodWorks, or ASCE, or whoever else that may discuss this topic.

 

[XR250]

I usually use RO + 3" although, in reality, it ain't gonna make any difference

 

[RPGs]

I do the same as XR250 for the same simple reason.

 

[phamENG]

If you really want to split hairs, design span is clear span (RO) + half of the required bearing length at each end. (NDS chapter 3). But for headers adding 3" is a good and expedient practice.

 

[RontheRedneck]

I was taught to design wood hearers/beams from centerline to centerline of the bearing surfaces.

Not all rough openings have 2 cripples at each end.

 

[Engineeringisthebest]

XR250 and PhamENG, just to be clear, you both mentioned RO + 3", what are you taking as your RO? I know in door openings they are framed wider than the door itself to accommodate the jamb and shims, so for example, are you saying for a 3068 door you would do 36"+3"+3" = 42" span from center of required bearing to center of required bearing (where required bearing is approximated at 3").

RontheRedneck, I think we are discussing two different things, rough opening refers to how wide the opening is to accommodate the door or windo fixture, which is typically oversized 1 to 3 inch so they can fit the jamb and shims (in the case of doors) or just have flexibility in the case of windows, so the rough opening is not affected by the number of trimmers (I'm assuming when you said cripples you are referring to trimmer/jack studs, in my area cripples are the shorter studs above the header, but I know these terms tend to vary a bit based on locality.

Thanks everyone for your answers!

 

[phamENG]

Rough opening is just that - the width of the opening for the rough framing. So it's measured from jamb to jamb. If you have a 3' wide door, it typically requires a rough opening of 38.5". That's you're RO. Your design span is 38.5"+0.5(required bearing length at left end+required bearing length at right end).

Ron is talking about the same thing we are. His point is that for wide openings, you may need 3 or 4 jack studs/trimmers at each end. If you need 4, then your design span would be RO+6".

 

[gte447f]

Engineeringisthebest, yes, a rough (pardon the pun) estimate of design span would be door/window size + 3" + 3". This would be pretty accurate for openings with 2 jack studs at each end of the header, slightly conservative for openings with only 1 jack stud, and unconservative for larger openings with more than 2 jack studs. Or at least that's my take on the issue.

 

[NorthCivil]

I used to work in wood frame, within my old practice i was the "alterations" guy.

most of my work was on projects where nobody really knew what was behind the walls, so we took an educated guess, produced a set of structural plans for permitting, and once we opened things up, we were always re-doing the plans in some way. in buildings anywhere from 5 to 150 years old.

The things i have seen 2x4s do, and function as fit for purpose, for decades, negates any argument for considering an extra 3" span on a new timber lintel.

 

[XR250]

The things i have seen 2x4s do, and function as fit for purpose, for decades, negates any argument for considering an extra 3" span on a new timber lintel.

Agreed. No need to sharpen your pencil too much

 

[gte447f]

Northcivil and XR250, I agree that there is no reason to get up in arms about a few inches of span for a wood header, but the OP seemed interested in a best practice for a design protocol. With that in mind, I think the OP would be better served to add 6" to the window/door size as a general rule to account for rough opening size plus bearing length, rather than using the just the window/door size, which we know is shorter than the actual span and unconservative.

 

[phamENG]

The things i have seen 2x4s do, and function as fit for purpose, for decades, negates any argument for considering an extra 3" span on a new timber lintel.

This attitude doesn't sit well with me. Wood design values are, by their nature, conservative. Wood is incredibly variable, so we end up with a whole lot of wood that's stronger than we give it credit for initially in our buildings. It's important to understand this when you see that 2x4 holding up entirely too much load, but I think it's also important to understand that trying to use that "extra capacity" also reduces the factor of safety and potentially reduces the reliability (at least theoretically/statistically) of the structure.

I agree that a couple inches on the header isn't the hill to die on here, but I try to stamp out that thought process whenever it creeps up in the back of my mind lest I try to justify something that I really shouldn't.

 

[XR250]

I think it is all situation dependant. However, for 99.9999% of the cases, 3" or 6" isn't going to matter for a header as there is so much redundancy in a typical wood structure that is never even considered. Floor bands above headers, diaphragm action in a trussed roof etc.
For me, after doing this for 31 years, I have found the places to be conservative are sheathing, long floor and LVL beam spans, beam stability and crushing of wood at bearings.

 

[IceNine]

For me, I'd have a hard time sleeping at night if 6" would make or break any of my designs. Maybe I'm more conservative than most, but I don't think so.

 

[dold]

So since we've got the razor blades out, you'd have different spans for strong-axis and weak-axis bending of the header. Strong-axis being center of jack stud bearing to same, weak-axis being face of king stud to same. I wouldn't ever consider that but just for sake of discussion.

 

[phamENG]

dold...what have you done...

Is the header composite for out of plane, or do we assume inter-ply slip? Do you count filler plywood strips? So many things to think about....

(In case anyone is wondering, I am joking. Maybe.)