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Re: RESIDENTIAL: Rafters Not Meeting At Ridge Plate -- Staggered Spacing

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Let's take this even further.  If the rafters don't line up, is the collar tie even installed???
I suspect that the sheathing is resisting the thrust.  Do you want to analyze a plywood
folded plate?
 
Dan Goodrich, P.E.
----- Original Message -----
From: Neil Moore
Sent: Friday, October 19, 2001 11:25 AM
Subject: RE: RESIDENTIAL: Rafters Not Meeting At Ridge Plate -- Staggered Spacing

Let's see how this thing looks.  If I remember correctly, rafters are usually spaced either at 16" or 24" are part of a simple gable system, with the bottom being a 2x something at about 48" spacing acting a tie.  A free body diagraom (remember those?) will show that there is compressive stress in the top chords (rafters) and to make the little triangle work, they are generally abutted with each other at the ridge member.  Sometimes the ridge member is even left out.

Returning to our little F.B.D., it seems that if the rafters on each side of the ridge don't meet up, then there is bending in the ridge member across the weak axis as well as shear.  Even if the ridge member calc outs, there will be possibly some deflection at each span.

So is this good practice?  Is there are future problem?  And will someone be liable for accepting this practice in about eight years when the lawsuit might come?

My reference is Page 22 of the Architectural Graphic Standards, 5th edition.  Can't find my 3rd edition.


Neil Moore, S.E.
neil moore and associates


At 11:16 AM 10/19/2001 -0500, Bill Polhemus wrote:
Not the sheeting directly, but I do think the stiffness of the roof assembly is less, at least in part because of the deformation of the ridge plate.
-----Original Message-----
From: Matthew Stuart [mailto:m.stuart(--nospam--at)aespj.com]
Sent: Friday, October 19, 2001 9:34 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: RESIDENTIAL: Rafters Not Meeting At Ridge Plate -- Staggered Spacing

If by "lateral load resisting element" you mean the capacity of the roof as a diaphragm, I don't think the offset spacing of the rafters would impact the shear capacity of the plywood sheathing.