Need Ceiling Joist help - Long Span

Pooh BearMarch 17, 2012

We want to add a garage to our house. 28' x 42'

How do I span 28 feet with ceiling joists?

This is what the add-on to our house will look like.

Actual pictures of our house can be found here.

This is the foundation layout for our garage.

The garage is 28x42. A little mudroom connects it to the house.

Because the mudroom floor is at the same height as the house, the ceiling of the garage

has to be raised where the door comes out of the mudroom and into the garage.

The mudroom will be divided into two spaces. One will be a laundry room.

Because of the pitch of the room and the different floor heights,

one section of the ceiling in the mudroom will be sloped with the room.

The divider wall will be on the right side of that door.

Another door will exit the mudroom to the back yard.

That little deck will wrap around the back side of the

house to meet the back deck.

So how do I construct the roof. I have to span 28 feet

for the garage. I can't use trusses because I have to match

the original house roof, approximately 4.5/12

The garage walls will be 2x6 framing. I wanted to use

I Joists for ceiling joists but because of the span the

I Joists would be too tall and I couldn't bevel them to

match the rafters. I'm going to put a steel roof on it,

and the house while I'm at it. Do I need OSB on the roof?

I keep looking at span tables for I Joists but I don't

know how to figure the loading. Nothing will be stored

in the attic. It will just be blown in insulation up there.

So how do I frame this. How do I span 28 feet.


Pooh Bear

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I'd go 10' walls (or what ever you need to reach top plate of house after rim is installed). Then run a built up LVL from the 8 foot wall to the 36' wall. then one at 90 degrees to that one from the 8 ' wall to the front of garage, over the door. (door header LVL also to carry load). Then you have 20' joists, and 22' joists. Run a rim all the way around, Ladder the edges, ply in to 2nd joist (or whole thing) and frame rafters on top of them. Some plywood up the center and you COULD have a little storage if you wanted it.

    Bookmark   March 17, 2012 at 10:26PM
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Pooh Bear

That helps a little. I didn't look at it that way before.
That would split it up into 2 shorter spans to work with.

I would rather not go any higher with the walls.
It would mess up the look I'm going for where it meets the
house. (top picture, gable end with kitchen door)

run a built up LVL from the 8 foot wall to the 36' wall.

There will actually be a full wall across there.
It just doesn't show up on the foundation view yet.

How about 2 LVLs on 14ft centers, 28 feet long. Or I Joists.
Use those as beams to set the ends of 14ft ceiling joists on.
Run the beams from 42' wall to 42' wall, and then run the
ceiling joists from 28ft (36') wall to 28ft wall.
And what size LVLs would I need for that.

I was quoted $98 for 28' long I Joists. I wasn't here when
they returned my call so I didn't get to ask about specifics.
How do LVLs compare to I Joists in price.

Pooh Bear

    Bookmark   March 17, 2012 at 10:55PM
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I only said 10 as an example. When I looked at you home picture, there is some exposed foundation. Add to that the thickness of your floor joists in the floor system of your house. You'll likely have a wall taller than 8'. The gist of the whole thing is your joists will run perpendicular to your rafters. (less than ideal) but can be done if you sheath some to keep rafter load from spreading walls. I won't claim to know your codes for loading or pricing in your area. For here I'd use three 1 3/4 x 11 3/4 glued and nailed together for a ceiling. That should be fine, but look into it for your area. using two front to back could work good and considering you'd cut down length of joists you could make them smaller. The LVLs will hang below if less than 2x12 are used for joist.

    Bookmark   March 17, 2012 at 11:20PM
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Trusses are the traditional way to span these longer distances. It's what we used for our 30'x54' shop/garage. The truss company did all of the engineering calcs, manufactured them and delivered them. It saves a lot of labor if you are DIYing but you will need a bit of help to swing them into place. It also saves on having an engineer's stamp on the support plan as they have an engineer on staff that will provide that for you for your permit.

    Bookmark   March 18, 2012 at 12:33AM
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Pooh Bear

Yes, I was thinking of the LVLs being under the joists.
Of course then I have exposed beams running across the ceiling.
I'm hoping to get it all up into the ceiling out of sight.

I was just looking at build-on-site glue-lams.
I could build them to any spec I needed.

We don't have a building inspector.
But I do have an insurance agent to answer to.
It will be built right. None of my younger shenanigans.

Although I am thinking about using beams and at the three
places they have to be bevel cut, placing a wall under
them that sticks out far enough to give the required support.
I'll have to draw that and see how it works.

Pooh Bear

    Bookmark   March 18, 2012 at 12:47AM
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Pooh Bear

I'd love to use trusses but there is no way to know the
measurements in advance. And I have to match the existing
house. I did think about using floor (flat) trusses though.
They would add a little height to the roof but would be strong.

Pooh Bear

    Bookmark   March 18, 2012 at 1:12AM
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I don't know exactly where you live but you should ask the local building department for the applicable Snow Load and determine how it should be applied to your roof especially where it might drift against the existing building before trying to size any long-span engineered wood structural elements whether they be I-joists, I-rafters, LVLs or any kind of trusses.

Since such elements are not included in 1 & 2 family dwelling codes, they usually must be designed by an engineer. Even if no code applies, an engineer would be cheap insurance against a roof collapse. Usually a lumberyard can get the manufacturer to design and stamp a design for you for free so there is no reason to try to do it yourself or rely on amateur advice on the internet.

Roof sheathing is likely to be required to resist the usual lateral forces (wind & seismic) unless an engineer can provide the necessary resistance by some other means.

    Bookmark   March 18, 2012 at 2:35PM
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Pooh Bear

We don't get snow, none to speak of, just flurries usually.
I went out today and laid it out a bit so I could see where
everything would be. I didn't like it. Too close to road.
I think I'll change the garage depth from 28' to 24'.

I think I'm gonna have to put OSB sheathing on the roof.
No seismic activity, but we have been getting some wicked
winds over the last few years, tornadoes to close for comfort.
Been a lot of damage here in the Chattanooga area lately.

Pooh Bear

Here is a link that might be useful: Where I live, 25 miles north of Chattanooga

    Bookmark   March 18, 2012 at 10:01PM
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Make sure you understand how much movement a regular 1/360 joist 23 feet long moves.

You are looking at 0.7 inches of allowed deflection.

This may result in a floor that feels bouncy.

If you use manufactured joists they have stiffer ones available, but they must be taller than a 1/360 deflection joist.

    Bookmark   March 19, 2012 at 11:27AM
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If you talk to a real lumberyard they should be able to put you in touch with a truss manufacture that can make up almost anything you want.

Trusses are not a 'standard' product, though most truss manufacturers have every design they have done previously available.

    Bookmark   March 19, 2012 at 11:29AM
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Sophie Wheeler

Trusses would be SO much easier to do. All you would need to worry about is getting the walls the right height and the truss manufacturer will do the rest. You will lose storage space in the "attic" area, but even then, you can have them do a section of "storage" trusses if you absolutely need that space to hold stuff.

    Bookmark   March 19, 2012 at 12:28PM
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"I'd love to use trusses but there is no way to know the
measurements in advance."

Sure there is.

It is called a set of plans.

    Bookmark   March 19, 2012 at 3:33PM
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Pooh Bear

I don't know the exact pitch of the existing roof to use trusses.
The roof was supposed to be 5/12. But we could only get 18'
dimensional lumber at the time. So I just told the carpenter
to "cut 'em as long as you can" to get the most pitch.
It came out about a four and a half - twelve, but not exact.

I don't understand why deflection is such a big concern.
It's a ceiling. Nobody will be walking on the ceiling.
Just OSB and insulation up there. I understand that it has
to also support the roof but we rarely get more than a
dusting of snow. Our biggest problem is wind.
All of the span tables I find are for rafters and floor joists.
I expected to find something for ceilings too.
Unless the ceiling serves as a floor above it why does it
have to be built to the same specs as a floor.
That's the part I don't understand.

Pooh Bear

    Bookmark   March 19, 2012 at 3:55PM
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Pooh Bear

I drew the original plans and I'm drawing these.
I would have to get out there and be climbing all over on a
ladder to get lots of measurements. And it would have to be
done after the concrete blocks foundation is in place to know
exactly how its height relates to the house. The carpenter
that is actually doing the work would be held up while all of
this is done. His crew can stick frame it faster than that.
I don't know what putting a steel roof on the house will do
to the roof measurements. It would mean putting that roof
on before I start taking any measurements.

Otherwise I run into problems such as What if the roof came
out higher than expected, or What if the garage floor turned
out lower than expected. I have to remove about 8 inches of dirt.

I can't be certain enough until it's actually being built.
I'd love to use trusses. I just can't see it happening.

Pooh Bear

    Bookmark   March 19, 2012 at 4:09PM
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"I don't know the exact pitch of the existing roof to use trusses."

It is not hard to measure.

Just a few minutes on a ladder that reaches the low edge of the roof with a level and a tape measure.

You can eve measure from in the attic of you want.

    Bookmark   March 20, 2012 at 9:21AM
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I don't see how the raised section of ceiling over one zone would make it conducive to trusses. The engineering workaround and extra material (if they are even conquerable) would make the trusses close to the cost of stick-framed.
If you could work out a better stair placement (contained inside the mudroom ) then get away with a flat ceiling, you'd be moving toward trusses.
To keep the raised ceiling area means stick framed, and engineering to figure out how to raise the ceiling and simultaneously support it clear-span.
A homeowner using span tables isn't going to be able to puzzle out this ceiling.

    Bookmark   March 21, 2012 at 8:00AM
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Pooh Bear

.....would make the trusses close to the cost of stick-framed.

Stick framed is already cheaper than trusses on this job.
Trusses would only be more convenient to use.

If I can draw out how we are planning on doing it, I'll post it.
We came up with a simple solution using two glue-lams.

Pooh Bear

    Bookmark   March 22, 2012 at 4:20AM
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