View Full Version : Sizing a beam
02-24-2009, 07:01 PM
I started cleaning out the garage and got to thinking about the ceiling. The problem is the ceiling joists are 21' long 24" OC 2x4s. The big problem is the one of the previous owners put 1/2" sheetrock up there and then 10" of blow in fiberglass on top.
As you can imagine the whole ceiling is sagging about 6" in the center. I want to mud and tape the seams on all the drywall out there but I know this needs to be done first.
The span for the beam will also be 21' the garage is 21x21. I will have to build this beam myself out of dimensional lumber for a few reasons. I would love to just use a gluelam.
On one side I have a window and the other a door. I can't just slide a beam up into place because of these obstacles. I plan to use scaffolding and assemble a beam with dimensional lumber above the window and door.
I know I will need to build a beefer header above the window and door.
The attic above this garage will never be used for storage because of the insulation and the low roof.
One solution I thought about was to attache the ceiling joists to the rafters. I feel that the roof is already overloaded. THere are two layers of asphalt and a concrete tile roof sitting on those rafters. Yes I am planning on doing a tear off on the roof.
Sorry for the long post, I am no good at being short and sweet.:)
02-24-2009, 07:08 PM
Will this be supported anywhere in the middle?
If you have 2x4 ceiling joists then you probably have a truss system
To span 22' I needed 2x 12" LVL
I'm not sure what you would need for dimensional lumber
I'm not sure you can even span 21' with reg lumber
Is it exactly 21' ? No chance of using 20' 2x12's ?
You need an engineer to size this
02-24-2009, 07:15 PM
Nope it is not a truss roof, just a rafter roof with 2x4x21 ceiling joists. Strange it is still standing huh? Believe it or not this is not the dumbest thing the people that built the house did.
Last time I had to hire the engineer it cost me nearly $800. On the bright side my foundation is fixed, my back sure hurts though.
02-28-2009, 06:00 PM
Perhaps it's just me, but I'm not quite sure where you are intending to put this beam. I'm assuming mid-span of the 2x4's? But you reference headers and such that interfere with it... So the door and window are centered on the side walls (parallel to the joists)?
Anyhow, I can give you a few tidbits:
1) It is not permissible to splice the parts of a beam unless over a bearing point. I'm guessing you don't want posts throughout your garage, so this is really not an option. You can build a beam by laminating two smaller members together, but not to lengthen. In other words, there is no rule of thumb for overlapping. Beams must bear on both ends (except very modest cantilevers) unless you get an engineer to stamp it otherwise. Your best bet to build it up would be to splice at a center post.
2) The load tables are pretty easy to figure out. First, you only have a ceiling load, not a floor load. I'm guessing the 2x12 LVL's were for a floor load. You can probably assume the 10psf dead load (no storage) for insulation and drywall. I don't know which code you are under there, but in the IRC it's table R802.4, which simply says you probably should have 2x8s@16 for a 21' span (depending on grade and species). You can use 2x6s@12 if you want if you buy SS-DF (well, with a bit of "rounding" of 20'8"). The other important thing to note is that 2x4@16 is only rated for 9'5 - 11'11" (depending on species), so you might still be a bit undersized even if you do cut the span in half.
3) It's probably easier to tear off the drywall (or enough to slide up new joists), replace/sister the joists and rehang the drywall. You'd have no beam to deal with and a straight, flat, and structually sound ceiling.... that is if you can find 21' 2x anything -- straight and flat might be asking a bit much! :) Some kind of engineered lumber could be used though, and maybe @ 24" centers (means 5/8" drywall). Just be sure to leave the 2x4's not on layout to keep the walls straight. You might have to get creative on how to use the door or window to get the lumber in, but most garages I've seen this can be done. Your local lumberyard should be able to size any engineered lumber (would suggest LVL's or I-joists which are much lighter to work with). If not, call the manufacturer that your lumberyard deals with. You have to pay a bit of attention to the end cuts and manufacturer's instructions on connections when used ceiling joists.
And finally... If you use the code books they are generally "prescriptive" so you can avoid the engineering as long as you follow their specs. You can also find prescriptive guides for the beam, but you need to calc the loads. This is fairly easy though... In your case you are basically dividing a 21'x21' @ 10psf into two 11'6"x21' @ 10psf. The center beam will obviously be carrying twice the load of the walls as it is carrying one half of both sides. The load on the beam is going to be 21' x 11.5' x 10lbs which is 2415 lbs. Your problem isn't in the sizing of that member (which can be found in a few places) but that you then have to transfer that load down to the ground (continuous load path). Your footings and foundation may not be happy with a sudden point load (much different from the current distributed load) of 1200 lbs, so you might end up dealing with footing problems.
This is probably where the good intentioned solution breaks down. It's easy to look up to the problem and figure it out in the sky, but gravity keeps bringing us down! I would suggest leaving the distributed load distributed if you can.
You might just want to tape it and appreciate the design "feature" of a reverse-coved ceiling! ;)
Definitely not short (or sweet)!
02-28-2009, 06:39 PM
A 2x6 to span 21' ??? :eek:
I don't think so, lucky to get a 2x10 to span 21'
And then only select/structural grade, not your normal #2 wood
LVLs were for a ceiling & supporting the floor above
What is holding the roof up? Normally, there would be braces or props down to the ceiling joists, but if they are sagging then the roof must be also. I am guessing you need more than just a "put-together" beam in the middle.
03-01-2009, 10:07 PM
Nothing needs to hold the roof up... other than the rafters. There doesn't have to be purlins (braces), though they can be used to reduce the span of the rafters. They still would not rest on unsupported joists, but connect over bearing walls. As there are no walls in a garage, there would usually not be any purlins. I can't specify the rafter size for an 11'6" span without knowing the snow load, but it could be as small as a 2x6@16" or even a 2x4@12", but that would be with a 20psf snow load (Seattle is 25psf, so it wouldn't fly here).
Does "I don't think so" supercede the code books? :) I mean, I gave the code, table #, and even the species/grade disclaimer. Granted that's the IRC (of which I only have the Seattle modifications -- though there are no mods to that table) and that may not be the code in Mike's area. I 'm not an engineer, but I am a carpenter and I do rely on the code books to tell me the "minimum for public safety." Would I use 2x8s? Probably, but more is definitely overkill and basically a waste. I'm curious if 21' 2x8's are readily available anyway :) Engineered may be the only practical way to do it.
Anyway, you might want to take a look at the code book, or give a reason more informative than "I don't think so" :) BTW, For a standard floor load using solid lumber, it would have to be SS 2x12's of almost all species to span 21'. Ceilings usually tolerate more deflection (L/240) than a floor (because L/360 is for comfort more than necessity). L/240 is only 1" in this case though... he's got something like L/40 deflection! Eek!
Again, it's the Seattle code, but you can see it online (table R802.4):
Or check the AWC calculator:
But for the original poster.. I'm assuming the garage ceiling has been "hanging" around for some time, and if it's not a matter of immediate safety, it's hard to justify the cost/labor of ripping it all out. There probably is no simple fix, but if you did go the engineer route, they could possibly suggest fixes that don't conform to "normal" building situations.
Hj's idea of hanging the ceiling from the roof is interesting (you'd even gain ceiling height!), but I'd be sure that the roof could handle the ceiling load or you definitley will end up with roof problems. BTW, the code books show rafter calcs when ceilings are suspended from the rafters (usually by lifting the lid up and using the joists as rafter ties, not by hanging it down).