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Author Topic: Rafters sliding under shrinkage  (Read 333 times)

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Offline Iommi

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Rafters sliding under shrinkage
« on: July 18, 2021, 08:27:04 AM »
Hi Guy,

I see a lot of videos of log house building with rafters nailed to the Ridge, purlins and top beams of the walls (plate log). To my comprehension, when the Ridge and purlins are sit on full log gable end, rafters angle will change because of the different  settling under the Ridge vs under the purlins vs under the walls. This may cause lateral sliding of the rafters under shrinkage, in the firsts couple of years, and that is why they say you should only nail rafters at one extermity. Does that make sens ? Do you have ideas of connectors between rafters and purlins that will allows sliding ?

N.b. I know that a framed gable will fix this issue, but I really like full log gable.

Offline Don P

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Re: Rafters sliding under shrinkage
« Reply #1 on: July 18, 2021, 09:00:56 AM »
Yes that is a real problem, I've seen it push the upper log outward. Poorly tied rafters can also have the same effect.

The best way is to let form follow function, frame the gables. If you can't do that full length purlins from gable to gable with no rafters is one solution for modest spans. For rafters you could fabricate slotted angle iron plate to rafter clips that allow for some sliding however the ridge would need to be designed as a beam since ties would not allow for a changing angle. Any midspan support posts would need to be on screw jacks to allow for lowering the ridge beam as the gables drop.

From your dowel question yesterday. The dowels would need to be perfectly plumb to allow settlement. A couple of divergent angles would "lock up" the ability to drop. Drive a couple of toenails at different angles between boards with a gap between them and then try to drive the gap tight if you need a visual on that. If a log twists on a dowel it will also hang up. That connection improves lateral but does nothing for uplift, one of the requirements of modern codes. Allthread rod running from top to bottom through looser holes would satisfy that requirement and depending on detailing can provide for snugging the stack down as it settles. You can get rod in HDG, hot dipped galvanized if corrosion is a concern.
The future is a foreign country, they will do things differently there - Simon Winchester

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