Ceiling joist strengthening

[spikyspikyspiky]

My 1934 2-car garage has 2x4 ceiling joists with floorboards on top, intended for light storage in the "attic." The garage is 20x20. The 2x4s are 24" apart, and originally had 20-foot spans across the garage. Then a more recent owner added a 4x6 beam under the center of the span and supported it on 4x4 posts - so these old 2x4 ceiling joists now span 10 feet of open space at the longest.

As soon as I saw 2x4s up there, I was leery of anyone going up into that attic - even despite the last owner having added the new support beam down the center. I've since confirmed in span tables that 2x4s spaced 24" are not enough for such a span, even if you *didn't* want to use the attic for light storage. That said, the whole building is still in excellent shape, no sagging or wood deterioration or anything - even while apparently the kids used the attic for years as a clubhouse of sorts. But I'd still like to improve the joist situation, so grownups could go up there with confidence too!

It would be quite difficult to replace all the joists with 2x6s or 2x8s, as there's only room for 4" depth atop the wall plates and the newer center support beam. Also, I couldn't add more support beams like the last guy's, as their posts would block the garage car entrances. The joists are all exposed, by the way - no actual ceiling was ever attached to them, just the attic floorboards laid on top.

Can I strengthen these joists by sistering new 2x4s beside each one? That would be kind of like 4x4 joists, if you see what I mean. (I suppose it's the same volume of wood, but I don't know if two 2x4s sistered together is considered to equal the strength of a 2x8, like the original builder ought to have used.) I could sister on both sides, for that matter. Then they'd look like 6x4s lying sideways.

Or, instead of sistering, I could simply double or triple the number of 2x4 joists, thus halving or "thirding" the ultimate joist spacing to 12" or 8" apart.

I'm pretty sure either approach would make me feel better about my heavy friends walking around upstairs... but is this a crazy solution? Is there another I don't know about?

What say you, the folks who know about this stuff? I appreciate any advice.

 

[beakie]

if it's that old I'd guess they are true 2x4's so a bit better than what you buy today.

if you want to walk/store up there I'd go with 2x6.
PL & screwed to each existing 2x4

don't forget bridging, especially with the 24"OC

 

[readhead]

Is the 4x6 only supported on the ends? If so it is not contributing much support since it is way over span also. If there is a double top plate I would consider using 2x4 hangers placed below and to one side of each existing joist to support a 2x8 which would then be sistered to the existing joist. Remove the 4x6 and use it for something else. This plan would reduce the ceiling height by about three inches.

 

[plumbing101mike]

Even gluing and screwing some good 3/4" plywood to both sides of. Each 2x4 would help tremendously. Be sure to stagger any joints. I like the idea of using a larger joist and a hanger on the ends. Because of the small area I would cut (notch) the to fit up above the wall plate and then also add a hanger somehow.
Anything will help. Many different methods to get it done. You mentioned adding more 2x4 joists. This should work also.

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[plumbing101mike]

Is the 4x6 only supported on the ends? If so it is not contributing much support since it is way over span also. If there is a double top plate I would consider using 2x4 hangers placed below and to one side of each existing joist to support a 2x8 which would then be sistered to the existing joist. Remove the 4x6 and use it for something else. This plan would reduce the ceiling height by about three inches.

Even gluing and screwing some good 3/4" plywood to both sides of. Each 2x4 would help tremendously. Be sure to stagger any joints. I like the idea of using a larger joist and a hanger on the ends. Because of the small area I would cut (notch) the to fit up above the wall plate and then also add a hanger somehow.
Anything will help. Many different methods to get it done. You mentioned adding more 2x4 joists. This should work also. Good luck!

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[theoldwizard1]

The easy thing to do would be to install 2x4 half way between the existing joist making the 12" O.C. Check that in you span table. 10' is a pretty short distance. You could even go 8" O.C.

 

[bczygan]

Using #2 hem-fir at 12"o.c. and L/240 with a LL of @40 and a DL of #10, you can span 7'3".

Using a LL of #30 you get 8'.

If existing "joists" are old size 2x4's they might have more strength, but then you will have to shim the new smaller ones to fit.

You could also go to an 8" spacing, and/or increase strength by choosing a different species and/or grade of lumber.

In any case, the floor will feel springy and I would be careful to not overload it, especially in the middle.

Note that you could get rid of the posts holding that beam up, by replacing it with an LVL. Place the LVL above the ceiling line and hang the 2x4's from it and you also eliminate the beam hanging down below the ceiling.

That 20' LVL would need to be a 1.9 made up of two 1 3/4 x16's for a #40 LL. 14" deep ones for a #30 LL. Both are L/360.


Bill

 

[theoldwizard1]

You could also go to an 8" spacing, and/or increase strength by choosing a different species and/or grade of lumber.

So Bill, are you holding out on us ? Do you have a supply of milled and dried 2x4 white oak or maple ? Or are you eyeing some of those tree in Detroit parks ?

 

[DougWil]

Is the ceiling finished?

If not or you don't mind hacking I would consider running a HHS (tube steel) retangular beam down the middle replacing the 4x6 wood beam. The beam could be underneath the existing joists if you could stand the loss of headroom or flush at bottom if you cut the 2x4 joists and hang them from the beam.

The beam would have to be supported at the ends, I wouldn't want posts in the way in elsewhere.

Additional 2x joists then could be sistered or placed between the existing joists.

 

[teal95]

If you don't have a ceiling you could sister/add (12" centers) 2x6 or even 2x8 just notching the ends to fit over the existing top plate and center support.

 

[Crazyjake8493]

I have the exact setup in the main 2-bay part of my garage, which was the original garage before a few small additions. Rough cut 2x4s spanning 20' with a triple 2x8 beam across the center. It looks like there was originally a post in the center of that beam, which was removed and another beam (triple 2x12) added below the original beam, crossing it perpendicular. Not the best setup, and you lose some headroom under it, but it works and I don't have the time/energy to change it.

I do use my attic for some light storage, and I go up there often, especially around the holidays getting decorations out and setting them up.

 

[spikyspikyspiky]

wow, thanks everyone for all the suggestions.

Since the ceiling isn't finished (the joists are exposed), and since that middle beam is doing a good job of halving the old 20 foot span (yes, it is supported by a post in its middle as well as the ends), therefore:

I am concluding my quickest simplest solution is to add 2x8 joists sistered to the 2x4s (and hanging a bit below them obviously), attaching the ends to the top plates and to the center beam using 2x4 hangers and notches.

So that's not going to fall down on my head right?
=-)

 

[Firebrick43]

Cheapest would be to screw and glue a clear 2x4 to the bottom of the existing 2x4. They don't have to go all the way to the sill plates just close. Glue is the most important part to prevent shearing. Screw just hold it in place until the glue set. A construction adhesive like liquid nails is sufficient. Adding this 1"1/2 nearly doubles the strength of the beam. Adding additional joist the same size will only increase strength 50%. Adding a 2x6 will do the same as glueing on a 2x4. A 2x8 will be twice as strong as a 2x6. End could be notched two the set up in the sill where the 2x4 sets but it will be more expensive. Also the original 2x4 then would be doing nothing but adding weight.

If the joist sag use a jack in the center to hold the 2x4 to the bottom of the joist and screw from the center out. Don't remove Jack until the glue sets. Without glue the bottom 2x4 can move in shear in the joint and you don't gain anything. An no the screws are not enough to prevent shear.

 

[spikyspikyspiky]

glue a clear 2x4 to the bottom of the existing 2x4. They don't have to go all the way to the sill plates... Adding this 1"1/2 nearly doubles the strength of the beam.
.

Wow, didn't realize gluing onto the bottom adds so much more strength than sistering. I guess it's like creating the 2x8 that should have been used originally. Didn't know that approach was ok.

BUT, you mean glue the narrow edge of the new 2x4 onto the bottom edge of the existing 2x4, right? I'm confused that you say [adding 1.5"] -- it would be adding 3.5" wouldn't it? In other words my screws would need to be over 4" long, rather than over 2" long. (Or are you saying, glue it on the flat 4" side, creating sort of an upside down T-beam?)

Also, should I put joist hangers at the ends of the new 2x4s, to attach them firmly to the top plates and center beam?

 

[CTyankee]

BUT, you mean glue the narrow edge of the new 2x4 onto the bottom edge of the existing 2x4, right? I'm confused that you say [adding 1.5"] -- it would be adding 3.5" wouldn't it? In other words my screws would need to be over 4" long, rather than over 2" long. (Or are you saying, glue it on the flat 4" side, creating sort of an upside down T-beam?)

Yes, he means gluing a 2x( the wider, the stronger) flat along the bottom of the existing joists. You can find lots of discussions and debating about this on the web, but I believe the general consensus/science is that it will strengthen the joists considerably. That's my understanding anyway. Maybe an engineer here can verify it. Never tried it in practice, but as mentioned, the gluing/screwing procedure needs to be done correctly for it to be effective.

 

[Flail]

I might put a 2x4 on top of the existing 2x4s in the vertical orientation and then glue and nail 7/16 OSB to sides of these. In essence makes a box beam and is an inexpensive way to get a lot more strength. Think how I-joists are made. They are essentially a top and bottom 2x3 with one piece of OSB spanning them. You get a lot more strength from some vertical elements than paralleled elements. If you cut the OSB sheeting you could cheat 7 strips out of each. You would use about 2/3 a sheet for each joist. So at $12 bucks a sheet plus $8 for 2x4x20' it would cost 13x$12 + 20x$8 or $316 plus some glue and nails. Call it $400 and you could probably get rid of the center beam. Downside is you lose 3 1/2" height of the attic however you loose no headroom in the garage.

 

[Firebrick43]

Wow, didn't realize gluing onto the bottom adds so much more strength than sistering. I guess it's like creating the 2x8 that should have been used originally. Didn't know that approach was ok.

BUT, you mean glue the narrow edge of the new 2x4 onto the bottom edge of the existing 2x4, right? I'm confused that you say [adding 1.5"] -- it would be adding 3.5" wouldn't it? In other words my screws would need to be over 4" long, rather than over 2" long. (Or are you saying, glue it on the flat 4" side, creating sort of an upside down T-beam?)

Also, should I put joist hangers at the ends of the new 2x4s, to attach them firmly to the top plates and center beam?

Ctyankeye, is right. Upside down T. Adding the 2x4 this way makes it 1.7 times stronger than a 2x4 on edge or close to a 2x6. A 2x8 is nearly twice as strong as as a 2x6 or 4 times stronger than a 2x4 for only twice the weight


I have done it to two structures. One was a house with 2x10 that the floor was ok but right on the edge of being to flexible for tile. Couldn't believe the difference.

A friend has a pole barn that he put a storage loft in above the office he built. Problem was the truss running through it so one had to bend nearly double to get under it. After putting in a center post(thankfully lined up with the office wall below) we cut the truss bottom chord and its webs out. So now we had 2x6 rafters support by a king post. To keep the rafter from being over loaded by snow loads we glued/screw the 2x4 t fashion to the bottom of the 2x6.

As far as building a box beam with sections of OSB be careful that the vertical joints at the end of the ply are staggered from one side to the other. A clear 2x2 would actually be as strong as a 2x4 although clear 2x2 seam to be like hens teeth here and 2x4 harder and harder to find. In and beam the top is in compression, the bottom in tension and the center is resisting shear. I would probably pick exterior ply instead of OSB to prevent water damage and therefore delamination but also because OSB fail spectacularly and quickly in a fire. This is one reason I will never use I joist.