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08-25-2018, 02:31 PM
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#1
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Moderator -- Wisconsin Kitchen & Bath Remodeler
Join Date: Feb 2005
Location: Oak Creek, WI
Posts: 22,362
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Skylight rough opening width dimension?
I’ve been looking at skylights for a house with 24” O.C. roof trusses.
And I am puzzled at the manufacturer’s offerings that include both a 21.5” and 22.5” rough opening width measurement. If you’ve got 24” O.C. trusses, for example, you’d have a maximum rough opening width of 22.5”. Why do the companies offer a 21.5” width, as well? I’m missing the reason for this narrower width. 
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08-25-2018, 02:41 PM
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#2
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Registered User
Join Date: Dec 2015
Location: Berkeley, CA
Posts: 1,104
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3x trusses? Or I-joists with 3x flanges?
Cheers, Wayne
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Wayne
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08-25-2018, 05:27 PM
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#3
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Moderator
Join Date: Nov 2006
Location: NW Arkansas, Ozark Mountains
Posts: 11,741
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In case the framers were hungover from the night before.
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Kevin
The top ten reasons to procrastinate:
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08-26-2018, 08:17 AM
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#4
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Moderator emeritus
Join Date: Jul 2001
Location: Boerne, Texas
Posts: 91,854
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If they offered only the 21 1/2" size, Goldstein, I'd be inclined to go with Kevin's thinking that it would be to accommodate poorly placed framing members, but if they also offer a 22 1/2" size, I'd hafta discount that thinking somewhat.
Can't go with Wayne's thinking on accounta with a 3x member you'd need a 21" unit or a 20 1/2" if the 3x was nominal. Perhaps you're in an area where full 2" engineered members are common for roof rafters? Either LVLs or I-Joist type? Be a PITA to finish out if they used the I-Joists, though.
Bottom line? I dunno. 
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08-26-2018, 08:25 AM
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#5
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Registered User
Join Date: Dec 2015
Location: Berkeley, CA
Posts: 1,104
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Quote:
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Originally Posted by CX
Can't go with Wayne's thinking on accounta with a 3x member you'd need a 21" unit or a 20 1/2" if the 3x was nominal.
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The OP referred to rough opening width, not frame size. And a 21.5" RO width is what you'd get with nominal 3x framing at 24" o.c.
Cheers, Wayne
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Wayne
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08-26-2018, 09:20 AM
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#6
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Moderator emeritus
Join Date: Jul 2001
Location: Boerne, Texas
Posts: 91,854
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Yeah, 21 1/2" is what I meant, Wayne. Old fingers mistake is what I'll claim.
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08-27-2018, 07:59 PM
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#7
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Moderator -- Wisconsin Kitchen & Bath Remodeler
Join Date: Feb 2005
Location: Oak Creek, WI
Posts: 22,362
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I need to make a correction. The smaller rough opening width is 21".
Around these parts, everything I've seen in residential is either nominal 2x roof trusses or 2x rafters.
Being a weekday, the company was open and I talked to tech support. And their response was, "The 21" width has been a traditional width for more than 70 years'.
Thanks, all. 
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08-27-2018, 09:01 PM
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#8
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Moderator emeritus
Join Date: Jul 2001
Location: Boerne, Texas
Posts: 91,854
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Quote:
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Originally Posted by Goldstein
"The 21" width has been a traditional width for more than 70 years'.
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Mmmm, that must be Wisconsonian for "I dunno." 
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08-28-2018, 11:43 AM
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#9
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Mudmeister
Join Date: Mar 2001
Location: Rosanky, Texas
Posts: 68,126
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The tradition might go back to when 2x4s were actually two by four inches, and some were a little more. A 21 inch skylight would allow for a little slop. 
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09-09-2018, 01:43 PM
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#10
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builder, anti-builder, rebuilder -- Retired Moderator
Join Date: Mar 2009
Location: oahu
Posts: 13,164
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may have to do with the way 1/2" drywall finishes up under the skylite.
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dana
"the road to hell is paved with osb, mastic, pre-mixed latex 'grout' or 'thinset', "
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