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CCarlson 10-04-2008 04:45 PM

Garage Ceiling
I am trying to make my garage workshop all-season ready, which will involve adding a ceiling and insulation to withstand MN winter cold.

The existing, detached garage is 24' wide by 20' deep, framed with 2x4s 16" o.c. throughout. The roof is 6:12 pitch, 2x4 rafters 16" o.c. with existing 2x4 joists 48" o.c.

The existing joists are at a height of only about 7'2.

As you might expect, the 24' span of the 48" o.c. 2x4 joists don't inspire confidence to add drywall for an insulated ceiling (setting aside the low ceiling height). They are further supported by a 2x4 tie from the rafters.

So, the question is, can I add new joists above the top plate and install insulation and drywall on these joists? I'd like about 8-9' height, which leads to about 16' total span for the new joists. (or do they become rafter ties at this point?)

I would not plan for storage above, so the primary concern is whether the 2x4 rafters can support 2x(6?) lumber hanging from them, with insulation, drywall, lighting, garage door opener (no storage above).

Also, with these in place, could I remove the existing 2x4 joists?

Does this sound like something I'd be approved for at the building permit counter?

Termite 10-04-2008 07:00 PM

Not sure I understand what you're asking but I'll give it a shot.

You currently have 2x4's spanning 24' center supported to the roof in the middle. You're wanting to add additional ceiling joists to reduce the spacing and hopefully make the span?

If so...

2x4's will span approximately 8'-6" (depending on species) in a ceiling joist application....Spaced at 24"oc. At 48" centers, no way.

Supporting the ceiling to the roof framing may be an option and may not be, depending on how the roof is framed.

Marvin Gardens 10-04-2008 07:54 PM

Wow. This is interesting. Some pictures would be nice.

I am not sure what you want to do. The whole building sounds shaky to me especially with the Minnesota winters (I am from there and know what it can be like). At least the roof is steep and will shed snow load.

CCarlson 10-05-2008 10:53 AM

4 Attachment(s)
Thanks for your help- hopefully this helps explain. I attached some photos and a quick illustration. The close-up photo is the typical wall-rafter-joist framing arrangement.

If possible I'd like to remove the existing joists, but if they can stay and the ceiling project works, that's fine too.

The roof boards don't look great, but there's a new roof on top of them from (about 4 yrs old, we moved in 2 yrs ago). It's not a great building, but it's what I've got for now. In 10-12 years I hope to build a new garage with engineered trusses.

Marvin Gardens 10-05-2008 11:26 AM

Well this is a rather flimsy garage and the short answer is no, you won't be able to do what you propose.

The current 2x4 joists are there to hold up the walls. Removing them would put your roof on the floor and might kill you in the process.

Putting some newer joists up higher is depending on the 2x4 rafters to hold it up. These rafters will not hold up that kind of weight and the end result is the ceiling on the floor and the possibility of death or serious injury.

I can see what you want to do and appreciate the desire to use the space for more than just storing air.

The fix would be complex but would work well as long as you did it carefully.

Option one is to remove the roof and increase the height of the walls and then put in your heavy duty joists and you would have a nice second floor.

Option two is a little more creative and might be more to your liking.

Build a new wall in from the old wall far enough to get your ceiling height where you want it. Then put in your heavy joists to handle the load you want. My guess is that would be a span of 18 feet and would need 2x14 or 2x16 or some engineered TGI's without having a middle bearing support.

Then put in some supports to the ridge like the existing 2x4's that are there now. This will support the ridge.

Once this new wall and joists are in tie in some 2x4's to the old wall so that when you remove the current 2x4's the walls won't fall out and the roof come tumbling down.

Now, there is always the question of the loss of space between the old wall and the new wall. Make this a storage area and put in closets and shelves.

This building cannot be modified much in any way without severely affecting the structural stability.

I am surprised that it passed code with the snow and wind load in Minnesota.

CCarlson 10-05-2008 03:53 PM

Thanks for the info and ideas.
A workshop-only garage would be great, but not too realistic... yet.

Based on the poor structure of what's out there- is there something I should be doing just to hold up the existing building?

Marvin Gardens 10-05-2008 05:07 PM


Originally Posted by CCarlson (Post 168659)
Thanks for the info and ideas.
A workshop-only garage would be great, but not too realistic... yet.

Based on the poor structure of what's out there- is there something I should be doing just to hold up the existing building?

Well if it was permitted then it is code and nothing needs to be done. But my guess is that there was no permit. I can see lots of areas that are deficient like the few numbers of joists to hold in the walls, the lack of significant roof support.

Do you know if there was a permit on this?

A center support to the floor would be good under the 2x4 joists. My concern is that if there is heavy snow (unlikely since we are in a world threatening global warming if you believe Al Gore) that builds up on the roof the weight would be too much. Even with the 6/12 pitch if there is some that sticks and then more snow falls it will not come off easily.

Pressure on the roof would put pressure on the walls and the joists. Since there are just not enough 2x4 joists which don't even show up on span tables for 10lb/sq ft of load for distances over 6 feet with 24 inch centers it is hard to calculate the weight that is allowed in this setup.

A span table will not allow a span of more than 6 feet with a 30 pound live load with centers of 2 feet. You have a 10 pound (estimated but probably more) load on 2x4's with 4 foot centers spanning 24 feet. Even a 2x10 will barely span 24 feet with 24 inch centers.

The rafters should be 2x10's and that doesn't consider any snow load. They are 18 feet with a 6/12 pitch and a 12 foot peak.

Center posts would go a long ways to keep the roof up but the building is still way under spec'd.

Get back to me. I have to know if this was a permitted structure. I am real curious. If it was then I am way off base here but I don't think so. I have put up too many buildings in my life and this one just looks too "interesting" to be a legal building.

Sorry to be the bearer of bad tidings.

bjbatlanta 10-05-2008 05:25 PM

If you are REALLY serious about this, I would hire a structural engineer. There MAY be a way to do what you want (I doubt it), but I would not attempt it without consulting someone who would advise me and stand behind it in writing. It doesn't seem feasible to me, but I'm not an expert in the field........ As "Marvin" speculated, I doubt it was permitted and pretty sure it's not up to code even in an area where snow load is not a factor. If you had actual trusses (2"x4" framing is common) it might be different. I don't see any truss plates which, I understand, help give the trusses their structural integrity. And trusses are ENGINEERED for a specific application. Overall, it may be cost prohibitive to do properly and safely.

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