Built my Garage Wrong -- PLEASE HELP w/ADVICE!

[Ronbo]

I recently built a 24’ W x 30’ L detached garage with an 8/12 pitch gable roof resulting in a storage attic with 9’ of headroom at the center. The garage doors are on the 30' L end (one 16' door, one 9' door). I used a built-up center beam of two 2x8s bolted and nailed together that spans the length of the garage (30’). The center beam is supported at the joint which is approximately 10’ from one side wall using a steel column rated at 20,000 lbs, plus. The center beam is unsupported for the remaining two-car portion of about a 20’ span.

I used 2x6 joists, approx. 11’ 9" long, 16” on center, tying the outside walls to the center beam. The center beam is also tied into the roof rafter at three evenly spaced points using 2x4s configured in an L shape. The tie-ins are spaced at 8’, 16’ and 24’ on the center beam. The attic floor is ½ OSB throughout. Plans are to sheetrock the garage walls and ceiling – not the attic space.

The local framing inspector passed the construction but said the attic should only be used to store empty boxes and Xmas ornaments, as it will not support heavy loads. He really didn't do a thorough check -- only peeked into the dark attic and may not have realized the center beam was 2x8s as it was flush with the bottom of the 2x6 joists inside the garage. He also noted that the tie-ins from the center beam up to the roof rafter serve no load-bearing purpose. That information bummed me out as it will be costly to tear out the beam and change the joists to wider/thick lumber.

MY QUESTION: Considering the three tie-ins to the roof rafter and the steel beam support, what can I do to support a decent storage load in the attic without major reconstruction or adding a steel column in the middle of the 2-car bay to support the center beam at another point? I don't care about the floor flexing as long as it won't crack the drywall.

Thanks,

Ron

[TnAndy]

Must not been much of an inspection.

If you have a 20' clear span center beam ( girder ) of 2 2x8s with 2x6 ceiling joist/ floor joists, personally, I sure wouldn't even park a vehicle in there, much less put anything upstairs.....cause that whole deal is liable to come in on you !

Is it not sagging now ??......it ought to be. Stretch a string and see.....

You've got a mess, which really ought to be taken out and redone.....what you 'should' have done was gotten an engineered truss that would have a storage space boxed out in the center, and given you clear span across the 24' like you're trying to get by assembling a bunch of pieces like you've done.

Personally, short of taking the whole roof off, I think you're gonna have to beef up the center beam, and live with a minimum of one more support post in your 16' door bay......you really ought to have 2x8s for joists if you plan to load much of anything up there, but could probably squeak by with the 2x6's.

Clear case education not being cheap.

[Ronbo]

TnAndy,

Thanks! Not what I wanted to hear -- but thought that might be the case. I did some measurements this a.m. The doubled 2x8 center beam spans 18' until it is supported by the steel column; then it goes for another 10' to the wall. The 2x6 joists span 11' 6" to the center beam.

Without tearing the whole thing down, what do you think of this option:

1. Sister 2x6s to all the current joists and use double hangers at the center beam. Not sure if double 2x6s equal the strength of a 2x8.

2. Replace, or beef up the center beam. What size beam would you recommend for that span with the ability to have a good load in the attic? Could I sister 2 additional 2x8s to it, or just pull it out and put in a combination of a few 2x10s, or 2x12s, or ?

Thanks again.

Ron

[TnAndy]

Ronbo,

You're welcome.....and I didn't mean to come off smartazz.....just remembering back on some of my own world class screwups with the part about education not being cheap.....ahahahaaaa

OK...to the problem....

The 2x6 joists 'might' work.....that's really stretching it on them, but the bigger problem I see is your beam and trying to clear span 18' with all that floor hanging on it.

Here's what I'd do if faced with it.

Glue laminated beams ( you can buy them at most decent building supply places in lengths up to 40' ) are pretty awesome for strengthening up 'boo boos'......I've used them in a bunch of 'this old house' situations to take out design flaws. ( When people say "they sure don't build 'em like they used to" I am generally muttering "Yeah...thank God ".......ahahahahaaaaaa )

I'd use a couple of glue-lam beam in the 12" height range.....if you can get them 30' long AND handle them once you get them on the site, I would slide them in the one gable end of the building, and run them down ONE side of your existing center beam....then bolt thru your existing beam and the new glue lam beams. This means you will have to take the joists off only one side of the building to do this.

I 'assume' you have a couple inches sticking up on the beam now into the attic floor space, ....since you said you used 2x6 joists, and a 2x8 center beam AND have a flush ceiling on the bottom side ?.....this is gonna make about 6 inches sticking up in the center of the attic, but whats a few more inches at this point.......or drop the glu lam to the bottom side and box around it when you sheet rock.

You're gonna be tempted to just do 18' with a glue lam, or more 2 x whatevers.......from one gable wall to the steel post, I know......and it would be easier.....but the extra 12' from the post ON TO THE OTHER END is going to have a 'cantilever' effect on that floor....by making is ALL one pc, like a 30' glue lam, you'll get an offseting effect of that extra 12' that will help the 18'....think kids 'see saw' .

Now the joist deal.......... the flooring your have in that attic...you said it's floor with 1/2" OSB.....did you carry the flooring ALL the way out to where the rafters come down ?? or did you come from the center like 2 sheets wide ( 8' ) and leave the other 4' where it gets close out under the rafters unfloored ?

IF you did that ( boy, a picture would be wonderful here ), then "I" would sorta build myself a truss in place. Take a short lenth of 2x4....run from the top of the 2x6 joist where the OSB stops, and run up to the rafter. Then cut yourself a bunch of 1/2" or 5/8" plywood ( and I'd use PLYWOOD ) plates and nail the crap out of them on each side of the joint points ( making a gusset plate ) on the joist and rafter ( or couple-3 bolts thru the 2x4 and rafter IF the rafter sits to one side of the ceiling joist. I'd try just doing this on every other joist initially.....and if you feel you still have to much bounce, go back and fill in the others. EVEN IF you floored out all the way, you could cut a hole in the floor and run the 2x4 down along beside the 2x6 and tie it in anyway....personally, I think that would be stronger than sistering another 2x6 on the side of each joist.

Also, make sure you run collar tie 2x4's across the rafters at about the 7' ceiling level to help tie them to each other as well......the more 'struts' you can get in this roof, the better to tie the whole thing together and shore everything up.

Probably other solutions to this problem as well, but this is likely the route I'd go. Maybe a real carpenter will happen along here in a bit and help us out.....ahahahahaaaaaaaaa..

[Ronbo]

Thanks again! You've got the picture of it very correct! I did flush the beam in the garage with the 2x6s and have an inch or so sticking up in the attic. I also took the flooring all the way out to the rafters and can certainly take up the floor at the rafters or cut it to slide 2x4s down to the joists.

If I were to use the two lams dropped to the bottom side (inside the garage), would cutting the sill at the gable ends to fit the lams be a concern? Would I need to put in another steel column along the length of the lams since the one I have now would be too tall to fit under the 2x12 lams? Of course, I'd support the lams in the wall with additional 2x4s.

Right now, with the three tie-ins from the roof's ridge beam to the floor center beam, I don't have any bounce on the floor -- but there's also nothing in the attic yet.

Ron

[Ronbo]

Wasn't thinking ... If I cut the sill, I'd just make separate wall sections and tie them into the glulam and its support.

Ron

[TnAndy]

Or you could just cut out an "L" notch in the end of the lam beam at the point it goes in the wall ends.

On the post, you could simply cut it off to match the new lam beam bottom.....and if concreted in, weld a metal plate at the top to reach over to catch the lam, then use a fill block up to ther orignal beam.....you can rent a chain cutter from a rentall place that makes a clean cut in the post ( used to cut cast iron pipe )

[Sweep]

Those vertical ties are supporting just enough of the dead load of the structure to prevent bounce and limit deflection which gives the structure the feeling of being adequate but the addition of even a modest storage load might overstress the wood to wood connections causing the floor to fail.

If the rafters and joists are properly tied to the walls and there is a continuous tie through the floor sheathing, the vertical ties could be made strong enough using steel rods or angles to do the job but the design of wood tension connections for such large loads is NOT a DIY project.

Since the 2x8's are not even strong enough to support the original dead load of the floor you will need to reinforce them. For a 19 ft span (assuming only 2/3 floor area loaded), if you use dimensioned lumber, you would need at least five 2x12's for a modest total load of 27 psf. A better option would be two 1 3/4 x 11 7/8 LVL's. If you plan to ever occupy the floor you would need three 11 1/4 LVL's or two 14's. You can get this LVL beam designed at most lumberyards. Don't notch the beam at the support, use a "top flange" steel hanger from Simpson ( strongtie.com ). Pay attention to the required beam seat bearing dimension. The beam bottom must be well braced at it's ends and the top secured to the structure above.

The Section Modulous of a 2x6 is 7.56 and a 2x8 is 13.14 so two 2x6's are a bit stronger than a 2x8 although not quite as stiff. But you shouldn't have a problem with 2x6 joists if the total load is 30 psf or less. Code required total loading is usually 40 psf for sleeping rooms/attic with storage and 50 psf for living areas which would require 2x8 joists or double 2x6's. If access to the space is limited the lower load would be realistic but if it is easy to get into it I would use the higer loading.

To give you more accurate information I would need to know the exact spans and the intended loading.

An old friend of mine lost a Lotus Europa when a garage collapsed on it. A moment of silence please .... It is a common misconception that buildings give warnings before they collapse. When the members are undersized they usually sag first but when tension connections are overstressed they usually go with a bang.

If this were my garage, I would hire a structural engineer to design 4 or more vertical steel ties and their connections and avoid the beam (unless I planned to use the space for habitation later ... have any kids who play drums?). Each tie might need to carry 1,500 to 2,500 lbs so the connection at the ridge will be the biggest challenge. Double 2x12 horizontal ties between the rafters just below the ridge might give you enough surface overlap to get the required nailing/bolting. A threaded rod might work if it was supported at the top by a steel plate spanning the tops of the two rafter ties 6 inches or so below the ridge board. At the bottom it could be drilled through the 2x8's with a bearing plate and nut below. Be careful, the size of each of these elements must be designed properly since the weakest one will determine the strength of the structure.

That's a lot of work; perhaps the beam is easier.

It might be wiser to have a yard sale.