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Old 10-15-2018, 07:49 PM   #1
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19-1/2" joist spacing


I purchased a custom built 1980s home back in May, after some work on other areas, I am finally getting to the garage. I've been working in the Attic of the garage re-wiring some things and general improvements. As there were only 3 lights in the entire 38 X 21 garage, lighting is a big thing I'm working on. As I'm laying out my design, I've noticed that the rafter spacing seems odd. I measured it and it is about 19-1/2" for the 2x10s.
1. Has anyone heard of this? Everything I can find says this is not right, it should be 16 for a 20'+ span

2. I am noticing some sagging in the center of the garage, can i use a jack to lift the ceiling and sister every joist or every other joist to take care of this issue?
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Old 10-15-2018, 08:32 PM   #2
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Re: 19-1/2" joist spacing


19.2 centers is done sometimes. Ever notice the little diamonds on a tape measure? Those are for laying out on 19.2 centers, which allows 5 joists per 8 feet instead of 4 with 24" centers or 6 with 16" centers. It does sound like your garage is over spanned, though we would need to know more details to offer any advice on fixing it.
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Old 10-15-2018, 08:52 PM   #3
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Re: 19-1/2" joist spacing


Quote:
Originally Posted by Marson View Post
19.2 centers is done sometimes. Ever notice the little diamonds on a tape measure? Those are for laying out on 19.2 centers, which allows 5 joists per 8 feet instead of 4 with 24" centers or 6 with 16" centers. It does sound like your garage is over spanned, though we would need to know more details to offer any advice on fixing it.
Sure thing, what details do you need?
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Old 10-15-2018, 09:23 PM   #4
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Re: 19-1/2" joist spacing


For starters, how is this roof built? Is it flat or a gable roof? What is this attic? Are these trusses? What is the snow load in you area?
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Old 10-15-2018, 10:12 PM   #5
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Re: 19-1/2" joist spacing


20 ft span with 2x10s is the bigger question or are we dealing with trusses?
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Old 10-16-2018, 09:23 AM   #6
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Re: 19-1/2" joist spacing


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For starters, how is this roof built? Is it flat or a gable roof? What is this attic? Are these trusses? What is the snow load in you area?
it is a Gable roof, no trusses. No snow load in our area, we are in Houston Tx.
Here is a picture of the attic area (i'll try to get a better pic later), the issue in question is the floor, while it is covered in decking, the Joists are 2x10s


This is the underside
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Old 10-16-2018, 10:08 AM   #7
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Re: 19-1/2" joist spacing


What type and thickness of decking?

Many times folks will want to use an area for storage without doing the due diligence to find out what that area was designed for in relation to psf.

There are more things to consider than just snow loading... such as wind loading, seismic, etc. Also, the type and grade of lumber used, and if it is spliced and how it is spliced, are also things to be considered.

Here's your best bet at this point. The home is new enough that you should be able to go the local jurisdictional authority and get the drawings for the home. If it was designed as a storage area, then the drawings will specify the load rating and the design specifications.

This is a handy calculator you may find interesting... https://www.awc.org/codes-standards/...tware/spancalc

Good luck..

Last edited by HenryMac; 10-16-2018 at 10:17 AM.
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Old 10-16-2018, 11:05 AM   #8
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Re: 19-1/2" joist spacing


How is the ceiling attached to the walls on the sides?
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Old 10-16-2018, 02:36 PM   #9
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Re: 19-1/2" joist spacing


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What type and thickness of decking?

Many times folks will want to use an area for storage without doing the due diligence to find out what that area was designed for in relation to psf.

There are more things to consider than just snow loading... such as wind loading, seismic, etc. Also, the type and grade of lumber used, and if it is spliced and how it is spliced, are also things to be considered.

Here's your best bet at this point. The home is new enough that you should be able to go the local jurisdictional authority and get the drawings for the home. If it was designed as a storage area, then the drawings will specify the load rating and the design specifications.

This is a handy calculator you may find interesting... https://www.awc.org/codes-standards/...tware/spancalc

Good luck..
Decking is 1/4" to 1/2" plywoood, I can tell it has been there from the beginning based on the way the attic latter was cut out of the ceiling.

Based on the plans, lumber used was fir or dried pine. The building plans called for 2X12s on 16" centers, but that is not what was done. Past that there is not allot of information on construction of the Garage. I'm going to have to remove the decking to see if it was spliced or single plank.
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Old 10-16-2018, 02:39 PM   #10
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Re: 19-1/2" joist spacing


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How is the ceiling attached to the walls on the sides?
Ends of the rafters are placed on 2 x 2x4 plates, I believed it is nailed in.
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Old 10-16-2018, 02:46 PM   #11
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Re: 19-1/2" joist spacing


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Ends of the rafters are placed on 2 x 2x4 plates, I believed it is nailed in.
So the studs don't go all the way to the roof, I don't see a bottom wall plate up stairs?
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Old 10-16-2018, 05:08 PM   #12
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Re: 19-1/2" joist spacing


Quote:
Originally Posted by stone150 View Post
Decking is 1/4" to 1/2" plywoood, I can tell it has been there from the beginning based on the way the attic latter was cut out of the ceiling.

Based on the plans, lumber used was fir or dried pine. The building plans called for 2X12s on 16" centers, but that is not what was done. Past that there is not allot of information on construction of the Garage. I'm going to have to remove the decking to see if it was spliced or single plank.
If we run the numbers using the above calculator the 2x12's are good for your application, assuming grade 2 southern pine, 16" o/c, no splices.

https://www.awc.org/codes-standards/...tware/spancalc

Last edited by HenryMac; 10-16-2018 at 05:38 PM.
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Old 10-16-2018, 05:22 PM   #13
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Re: 19-1/2" joist spacing


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If we run the numbers using the above calculator the 2x12's are good for your application, assuming grade 2 southern pine, 16" o/c, no splices.


The 2x10's are not, also assuming grade 2 southern pine, 19.2" o/c, no splices.

https://www.awc.org/codes-standards/...tware/spancalc
We had an engineer that thought double 2x10s 20 ft span would work, they sagged under their own weight then we put the beams in.
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Old 10-16-2018, 05:36 PM   #14
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Re: 19-1/2" joist spacing


Stone150 and Nealtw,

Sorry for the edit above. Let me clarify.

If we drop loading to a live load of 10 psf and a dead load 10 psf... the 2x10's work in the 19.2 o/c configuration.

That's pretty light duty. You need to use the sheathng, drywall and any insulation as part of the dead load. Bottom line, you are left with super-duper-light duty storage.

But you need to verify non incised lumber and the other factors shown below.
Attached Thumbnails
19-1/2" joist spacing-stone-150-ceiling-joists.jpg  

Last edited by HenryMac; 10-16-2018 at 05:42 PM.
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Old 10-16-2018, 05:40 PM   #15
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Re: 19-1/2" joist spacing


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Stone150,

If we drop loading to a live load of 10 psf and a dead load 10 psf... the 2x10's work in the 19.2 o/c configuration.

But you need to verify non incised lumber and the other factors shown below.
I was thinking it was balloon style with full height studs, he said the floor sat on the walls so the top walls are added, that would explain that one rafter tie. Just something to think about before he removes that.
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