Another strengthen ceiling joists question
I’ve lurked for some time and read the similar requests/response. I’m a retired computer engineer – and know enough to know I don’t know anything about structural engineering.
I have a 20×24 garage with a 10/12 pitch roof using 2×6 rafters 16″ OC. I have 2×6 ceiling joists that are 24″ OC. The attic has a plywood floor and we store a few lightweight items up there like christmas decorations.
My goal is to put radiant heat in the ceiling (about 170 total pounds spread evenly with the joists) then covered with drywall. My main concern is drywall popping. Sometimes when you walk up there you can feel the joists give a little.
The builder tied the rafter to the joists every four feet as in the following sketchup picture:
I’m reluctant to go to the expense to put up the radiant and drywall only to have it cracking. I can’t get additional joists to fit (or at least I can’t figure out how). I was wondering if the following beam type thing (sorry, I don’t know the terms) above the joists would sufficiently tighten them up?
We would still like to be able to store small items in the garage attic.
Replies
Truss
That'll work but there' s things to be careful of. Where you place the open web trusses will need a support column down to the foundation and you don't want to place them over any of the openings in the walls like garage doors etc.
The truss company will probably need to make a site visit and they should be able to spec the hangars required to attache the ceiling joists to the truss.
Gonna have some fans blowing that heat down to the occupants?
steve
Where are your door/window openings? Can any posts be installed in the 1st floor open area?
We've put laminated LVL's under whimpy joists in a garage a couple of times. Sized right for the span and and/or added columns, that'll do much to stiffen the attic.
Of course, you should run this by a real lumberyard (so they can send it to an LVL company for engineering) or consult a licensed eng.
The of course, there's always steel.
There's a point in time you shouldn't hang the ceiling from the roof frame. Drywall and heating elements with some christmas lights isn't that much. But, that's a large space available for storage-might think that way b/4 you start framing.
There are two windows and a door on the side. Garage door is at front of iso view. Thanks for mentioning the posts. If I could do that then I'd just put up a couple of beams and be done but I use all the space for a wood workshop and I must avoid posts.
calvin, when you say you've put LVL's under joists - I don't want to put anything under the joists but the heat and sheet rock. Yes, that's a large space for storage but my wife and I understand the implications and avoid it for that reason. (The garage was built when we bought, otherwise it'd be different.)
john7g, I was going to make the "trusses" myself. When you say support down to the floor - I want to avoid that. I was hoping the truss would spread any temporary load from walking up there to adjacent joists to avoid drywall popping. As a side note, the ceiling heat works better in many cases for radiant. Think of radiant as light shining down - not heated air going up (although some eventually does of course). We have radiant in the floors on the main house and in the ceilings in the bedrooms. The ceiling warms the bed up nicely.
View Image
The posts will need to be in the end walls creating the laod path to the foundation/footers.
If you're going to do homebuilts surf over to the APA and read up on box beams. They might work for this.
You still need to figure out the connection of the joist to the beam and propably where you need to focus your attention first. Makes no difference if you can build a beam to carry the load if you can't attach the load to it.
Talk to a structural engineer
There are several ways to approach the problem.
The one that comes immediately to mind for me is to glue and nail plywood both sides of the existing floor joist, essentially making a large torsion or sheer box out of the whole ceiling.
You might also be able to sister onto, or adjacent to the existing joists. You might lose a little headroom, but it shouldn't be too much.
I'm just a dumb ol civil engineer, who got A's in my structures classes, twenty years ago. I sort of know what can be done, but don't generally do structural design, because the folks who do it all the time have dedicated software, that allows them to quickly generate answers that it would take me weeks to do by hand. And, they are current on all the current code requirements.
Get a good structural engineer on board, and they can give you solid options, that will get past the building department.
I can't figure out how to sister or put in adjacent joists as they rest on the top plate and under the rafters so there's no way to get a full length joist in without removing the roof. (At least when I tried it in sketchup it couldn't be done.)
I'll think about the torsion.
A good structural engineer would be nice. I live in a rural area and there is one I consulted when remodeling my home. I provided him with all my sketchup drawings, he charged me $3500 and sent me back my drawings. I did get affirmation but it was expensive. I don't mind paying someone to render a professional opinion but if every contractor had to pay that much they wouldn't survive and I'm the homeowner.
What you propose in your 2nd sketch will do little or nothing to strenghten the ceiling joists. All it would accomplish is put some added load in the middle of the rafters. They most likely weren't edsigned for that.
IMHO your best bet would be to add bearing under the ceiling joists (You've already shot down that idea) or sister deeper ceiling joists alongise the existing ones.
I don't see that.
What he is proposing, (at least the way I saw it), is installing a set of trusses, supported at the end walls, running perpendicular to the joists, and tieing the joists to the bottom of the trusses.
Depending on the trusses, it could add a lot of stiffness to the system.
Properly designed they could go in as knee walls, and define the attic storage area, while significantly increaseing the stifness of not just the cieling but the entire structure.
jigs
Say you home built these trusses (as proposed). The joinery is the key-you could perhaps line both sides with plywood with staggered joints and make up a box beam. As it looks now, it's just a bunch of lumber cut and nailed at angles to each other. Surely not a truss in my mind.
Then, you've got the connection to the joists to think about.
If the rafters would be stout enough, then there's another connection that just can't be nailed off..................
For Christmas lights.
TrusJoist
I interpretted the ones shown on his sketch to be commmercially manufactured open web trusses from Vulcraft, Georgia Pacific, or someone similar.
If I were site building them, I'd design them as knee walls or at least the bottom of a knee wall, and make them with doubled or tripled bottom plates, 3/4 structural plywood, and use 3X lumber at the plywood joints to keep the fasteners back from the edges of the panels, and 2X lumber for intermediate stiffners. Glue everything with uerethane glue similar to Gorilla Glue, for the long open time, and nail it together. Once the glue dries they are extremely strong and stiff.
The APA-Engineered Wood folks have some good materials on the design and construction, of built up box beams free on their website. You do have to register to get to their library, but they have a hundred or so publications available for free as downloadable PDFs. They also have some good dvds on the use of plywood in design and construction, that they will send you for free.
I don't see that he's installing any sort of "truss, nor is he actually forming any sort of truss.
The kneewalls in his 2nd picture will only connect the ceiling joists and rafters togather. The kneewall is not any sort of beam in itself. And connecting the rafters to the ceiling joists does not make a truss either.
So I stand by what I said - It will accomplish nothing, except maybe ading load to the rafters.
Well, the beams he's drawn in the second picture are technically trusses. Just not roof trusses.
"The beams he's drawn in the
"The beams he's drawn in the second picture are technically trusses"
BS. They're just stick framing with some boards tacked on.
Well, you're asserting that they will not be joined properly, which is entirely possible. But from a conceptual standpoint, any structural member that is composed of triangles is a truss, according to the standard civil engineering definition of the term.
reduced span?
Boss You may be seeing this differently that I am... If he finds a suitable attachment process of the joists to the beam above and the beam is adequately supported at both ends would it not reduce the 2x6 joist span? That's how I'm seeing it at least.
Summary at this point
Jigs-n-fixtures said what I was attempting to accomplish:
calvin and john7g bring up ommissions:
The truss needs to be strengthened which I can do with Simpson Tie Plates or site-made plywood gussets.
The truss needs to be joined to the joists. There are Simpson or custom steel ties for this.
The truss needs to transfer loads to support columns at the ends.
The last one poses a new problem. It's no issue at one end of the garage but the other end is the garage door which I'd like to keep. That means I need to determine the load capacity of this garage door beam header and consider replacing with a high strength laminated glulam header with additional end support.
The end result would give me a knee wall truss at about six feet from the joist ends with a center span of about eight feet which is significantly greater bearing capacity while reducing the the deflection.
This won't convert ceiling joists into floor joists but it should assure me of a durable drywall ceiling.
to build a truss
To site build a truss you have to guess at your load capbilities.
DL and read this on Box beams, build them out of 2x and plywood. You can build it in the attic.
Figure out your strap to connect the joists to the beam.
You'll more than liekly need to upgrade the header at the GD
APA home page if the link above does not work http://www.apawood.org/level_b.cfm?content=prd_ply_main
Thank you for the link!
That document has very good information. Note that the flanges have to be full length for the load tables but I think I could remove siding to get those long flanges in the attic .
john7g wrote:Figure out your strap to connect the joists to the beam.
That's an extremely important point that I haven't digested yet. If you place the beam on top of the joists then you're only as good as your fasteners.
a simple solution
My "down home" solution is to add additional vertical ties from the joists to the rafters... do this on both sides of the ridge, equally. Then glue and screw 1/2" plywood to these verticals. The ply panels ought to be 24" wide and run the length of space. There, you've created a sort of beam that will certainly add the needed strength to the ceiling. And its relatively inexpensive and painless to do.
Beefing up ceiling/floor joists
You can convert all your joists from 2 x 6 to 2 x 8 by simply laminating half a 2 x 4 to the bottom or top. If you are a woodworker, you likely have a table saw The new joist will be a saw kerf narrower than a standard 2 x 8 at about 7 3/8". A high strength adhesive is best. Clamps can be used for the addition or screws/nails. The adhesive might be a bit expensive, but perhaps the total cost will not be as high as an elaborate and far less useful system of trusses, fancy gluelam and whatever. Some bridging will also be very helpful and cost effective to help with load transfer.
I have done this in an old house remodel to reinforce 2 x 4 roof rafters. i used a slow set high strength epoxy, and the end result was nominal 2 x 6's that were just what our building inspector preferred. Later, trusses were fabricated using plywood gussets, screws, and epoxy for bonding. Several years later, they ae still rigidly in place, dead accurate. Slower than store bought, but stouter.
JB
It's not clear -- are those posts in the first picture running all the way from the peak to the center of the joists, or are they, as depicted, running from about halfway down the rafters, and hitting about the 25% spot on the joists? And, if the latter, are they only on one side, as depicted, or are there mirror-image posts on the other side?
If the posts are in the middle, or are mirrored on both sides, your strongback just needs to transfer the load from the "unposted" joists to the ones with posts -- it doesn't need to bear on anything other than the adjacent joists. And it doesn't need to me that much of a strongback -- a single 2x10 on each side, toe-screwed to the joists, would likely suffice.
Keep in mind that to prevent cracking you don't need to prevent movement so much as make sure everything moves together.
Afraid to transfer too much load to roof rafters
Sorry DanH for 3-D in single plane. Those are in about 5 feet from rim and run perpendicular to rafter. There is also a set on the other side that aren't shown.
You bring up a good point about making sure everything moves together and this would certainly be lowest cost in money and effort but since I have no knowledge I was afraid of transferring too much to the roof. There is currently a plywood floor and it does move when we walk on it so my inclination was to go overboard to transfer loads to the walls.
Steve,
I would sister some 2x8 to the joist using a ledger with the new joist notched for a let in. No need to sit on the top plate this way.
You could even use some Simpson double hangers if you wanted.
KK
I'm surprised nobody other than DanH mentioned using strongbacks. Look at image at http://www.sweethaven02.com/BldgConst/Bldg02/fig1201.jpg
Well, the OP's second drawing is basically strongbacks.
Sort of, but the bracing is weak, a sag in the ceiling could pull the nails loose. The other design has the nails in shear, they wont pull through.
He didn't specify a joining mechanism. Strapping would be better than the plank nailer in your diagram.
Pretty much any scheme for this sort of thing is highly dependent on the joining mechanism -- none of these schemes comes close to being a "stick framing" approach where gravity does most of the work and fasteners are mostly there just to keep things from slipping off of each other.
I was taking the scetch too literally, but if that box was properly made, my opinion is that it would not work much better than the usual strongback I showed. What are you saying about strapping?
By strapping I mean steel straps -- along the lines of hurricane ties.