Rafter/ridge board

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[Jerry Peck]

Let's visualize a 2x12 rafter against a 1x6 ridge board, leaving over half of it unsupported.

The part which *is* supported against the ridge board will transfer its loads to the other, opposite, ridge board.

The part which *is not* supported will likely split away from the upper part which is supported. This is because the roof load (dead load) is a gravity load, working against the rafter vertically, trying to push the rafter down, the unsupported part of the rafter end complies, the supported part of the rafter end does not comply - causing the rafter to split.

Now, add snow loads, wind loads, etc., to those roof loads and the forces trying to deflect that rafter is even greater. And the force trying to split the rafter at the unsupported end is even more concentrated.

I'm sure someone has a good photo of a rafter splitting because it was unsupported by a too-small ridge board.

[Phillip Stojanik]

Please excuse my "artwork", but this is the kind of rafter damage that is best avoided by having a ridgeboard that fully accepts the bearing end of attached rafters. A fully bearing cut rafter end that is "tight" to the ridgeboard does not split but rather transfers its load to (and through) the ridgeboard to the complementary rafter on the other side (even when they are somewhat offset laterally at the ridgeboard).

With an engineered truss system, no ridgeboard is needed at all because there is no lateral offset allowed (by design) and the bearing faces of the rafters/upper cords are in complimentary alignment. But that’s another thread altogether!

[Jerry Peck]

Phillip,

Good art work. Shows just what I was describing.

[Jack Feldmann]

In my area, the most common practice is to just add a 2x4 or such added to the bottom of the ridge board to fill up the space.

I have had discussions with many structural engineers about this. In their opinion, there may not be that big of a problem if the span is not that great, and the difference between the rafter/ridge is not either. Such as when the span is maybe 12' total and the difference is 3/4".

I'll look thru my photos and see if I can find a split rafter.
JF

[Bob Harper]

a commonly accepted repair is to make plywood gussets. These are simply triangles of 3/4" CDX with a pocket cut out in the apex that fits the ridge board snugly. You apply construction adhesive on the rafters, slide the gussets onto both sides then screw in with deck screws about every 6". Most AHJs will allow this in lieu of collar ties if done on both sides of the rafter. What it does is transfer the loads all along the rafter ends. As long as the triangles are not too small, enough 'meat' of the rafter will handle the tensile loads along the bottom chord and compression load at the top where they oppose one another.

HTH

[Jack Feldmann]

Seems like a lot more work than what the AHJ's let them do around here.

[Richard Rushing]

Bob,

I've seen what you have described. The gusset was made from 1/2 CDX and about 16" long. Makes for a very strong sectional tie-in at the ridge.

Rich