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Old 05-19-2010, 03:27 PM   #1
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Raising ceiling (rafters) in detached garage


Hey guys.....

First post here

I am buying a house next week that has a detached once car garage, 20 X 14. The garage was built with the house in 1938. Of course the roof is not trussed, just built in place. The bottom of the rafters is pretty low, probably 8 feet and thats being generous. Can I cut the joists (I might be calling them something wrong) and move them up a foot or two and then resize and reattach to the roof rafters. This would give a lot more headroom in there.

I'd also like to make the garage bigger down the road by expanding it out to 20X20 and replacing the whole roof rafters and all with a porch off the side. I realize this would probably require trussing to accomplish.

I have attached a simple diagram that shows what I'd like to do. Blue is the current position, green shows how I'd like. I know the higher they go the less effective they will become.
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Raising ceiling (rafters) in detached garage-garage.jpg  


Last edited by acme54321; 05-19-2010 at 03:41 PM.
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Old 05-19-2010, 04:28 PM   #2
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Raising ceiling (rafters) in detached garage


The joists don't seem to really be needed except to keep the walls tied together while the rafters were installed. I would say you really dont need them, as you wouldn't have them there if you had a cathedral ceiling. If you are just going to move them up it might be fine.I am no expert on framing nor am I an engineer, This is only my opinion based on what i know

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Old 05-19-2010, 04:40 PM   #3
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Raising ceiling (rafters) in detached garage


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Originally Posted by Millertyme View Post
The joists don't seem to really be needed except to keep the walls tied together while the rafters were installed. I would say you really dont need them, as you wouldn't have them there if you had a cathedral ceiling. If you are just going to move them up it might be fine.I am no expert on framing nor am I an engineer, This is only my opinion based on what i know
Incorrect
The ceiling joists keep the walls from spreading out
They are needed after the rafters are built & roof installed
You can move them up, depending upon rafter size, snow load etc
I think they still need to be installed in the bottom 1/3 of the rafter length

In a cathedral ceiling the ridge bean is a BEAM properly sized that supports the roof
In most instances theree is only a ridge BOARD installed, not sized to hold the weight of the roof

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Old 05-19-2010, 04:52 PM   #4
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Raising ceiling (rafters) in detached garage


Thanks guys, I'm actually a Mech E, just enough knowledge about structures to get me in trouble I just want to check with people about building knowledge. I know they are only there to keep the walls from bowing out. I may just raise them but leave a couple in down low for piece of mind, not really sure.

What I am more worried about are my later plans of expansion and how I can do the roof. This thing is really simple contruction and I was hoping that I could just expand on it somehow.... not sure about a 20 ft span though as far as the joists go.

Turns out that statics class actually has become slightly useful!
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Old 05-19-2010, 05:22 PM   #5
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Raising ceiling (rafters) in detached garage


If the plan is to extend the garage I'd just leave them until you redo the roof
Are you going to have 2 garage doors?
Are you taking the existing wall on the left down ?

You are better going off with ane equal angle roof over the whole thing
Rather then the 2nd lower slope you show in your diagram
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Old 05-19-2010, 08:01 PM   #6
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Raising ceiling (rafters) in detached garage


Steve,

You mention a ridge beam for a cathedral ceiling....

Is that essintially where the ends of the roof are trusses with a full legth beam across? Then the end trusses and beam position the roof and the rafters maintain the tie in with the walls?
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Old 05-19-2010, 08:07 PM   #7
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Raising ceiling (rafters) in detached garage


A Ridge Beam runs the full length of the ridge & is SIZED to carry the roof load
For a 20' run it would not be one board
It would be LVL or something similar

For a ~23' distance my ridge beam was sized as (3) 16" LVL's

The rafters do not tie in the wall
The beam supports the weight which prevents the walls from pushing out
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Old 05-19-2010, 08:14 PM   #8
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Raising ceiling (rafters) in detached garage


Ah, got it. So what do the rafters tie to on the bottom end if not the wall?
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Old 05-19-2010, 08:25 PM   #9
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Raising ceiling (rafters) in detached garage


The rafters attach to the walls top & bottom
But they do not (fully) tie walls together
Weight presses down, without the rafter ties to tie the walls togethe rteh walls will eventually bow out
How fast depends upon rafter size, span distance, snow load, weight of roof....etc
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Old 05-19-2010, 08:39 PM   #10
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Raising ceiling (rafters) in detached garage


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Ah, got it. So what do the rafters tie to on the bottom end if not the wall?
The rafters lay on the wall and are nailed in place to prevent the roof from lifting off from wind!

In my area we have to consider, not only the pressure exerted outwards on the walls, from the weight of the roof. But, also the snow load.
Our snow fall exerts 30 p.s.f. so our collar ties cannot be raised as high they could be in the south.
We think of the joists that run from the tops of the walls as collar ties and if they are raised, as rafter ties.
Other regions may refer to them differently!
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Old 05-19-2010, 09:21 PM   #11
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Raising ceiling (rafters) in detached garage


Dave, "You are better going off with ane equal angle roof over the whole thing
Rather then the 2nd lower slope you show in your diagram" ------- I believe that his plan is a 20' wide garage with the 8' ? porch roof attached in a truss. (The low sloped roof in his diagram).

"The beam supports the weight which prevents the walls from pushing out" ------- the ridge beam supports 1/2 the roof weight with the walls supporting the other 1/2 divided at the walls.

"For a 20' run it would not be one board
It would be LVL or something similar" ----- It could easily be a 6x12, D.F., Select #1, which would safely carry his load in FL. (no snow load). Many types of engineered lumber beams are able to replace solid lumber beams, but one of many could handle it. (and greener)

"The rafters do not tie in the wall
The beam supports the weight which prevents the walls from pushing out " --------- The rafters tie the walls to the ridge for shear flow against racking and seismic loads down to the ground. Ridge blocking and positive rafter fastening required per local code.

OP, " So what do the rafters tie to on the bottom end if not the wall? " ------ The walls.

Check with your local Building Department for FL hurricane tie downs before you build to warrant your Homeowners Insurance liability.

Be safe, Gary
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Old 05-20-2010, 06:15 AM   #12
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Raising ceiling (rafters) in detached garage


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Originally Posted by Scuba_Dave View Post
If the plan is to extend the garage I'd just leave them until you redo the roof
Are you going to have 2 garage doors?
Are you taking the existing wall on the left down ?

You are better going off with ane equal angle roof over the whole thing
Rather then the 2nd lower slope you show in your diagram
Well, the expansion won't come for a little while, and it shoudln't be much work to move them. Currently it has one small door, which is a two piece barn door style. I'd like to replace it eventually with a normal overhead door, but I will probably leave it one car size. This garage will never be a "garage." Just a workshop/man space/W&D area. One thing I am not sure of is how the washer and dryer are plumbed, I'll have to look at that. I am pretty sure it isn't through the slab though so that is a plus.

The 2nd roof is just a porch roof. The neighbors have something similar on their little garage and they have a grill under it and whatnot, looks nice.
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Old 05-20-2010, 06:22 AM   #13
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Raising ceiling (rafters) in detached garage


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Originally Posted by GBR in WA View Post
Check with your local Building Department for FL hurricane tie downs before you build to warrant your Homeowners Insurance liability.

Be safe, Gary
Gary,

I had this concern also. The garage is fairly shielded though from the wind. There are treelines right up against it to the east and north and the house to the south. I will deffinately look into what is required though as far as wind issues when I come to that point. I do figure though that just mimicing what has been there for 70 years will probably be ok, it is still standing rightside up after a few storms
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Old 05-20-2010, 10:42 PM   #14
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70+ years ago the building codes were quite different. I currently have remodeled house that I framed 35 years ago and things have changed a lot. It is all for your safety, and code is the bare minimum. Visit your County's web site for your local conditional changes from the IRC.

Be safe, Gary
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Old 05-21-2010, 06:57 AM   #15
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Raising ceiling (rafters) in detached garage


CLICK HERE for a calculator that will give you a fair (though not totally exact) estimate of the forces your rafters apply to your side walls.

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