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Planning lap scarfs on 36 foot purlin plates

Started by iancorey, October 25, 2023, 12:39:38 PM

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In 6x6 white pine. I have to be able to mill this and place it alone with A-frame and hoists. Therefore, I'm splitting it into pieces and scarfing them together. There are four posts supporting the plate. Each bay is 10'. If I split the plate into three pieces (black and white illustration), the outside members are partially unsupported. If I split it into two, the scarf is completely unsupported (color illustration). 

Would making each plate from four members and splicing over the tops of the posts be ok?

Thank you for any advice.


One thing you have to consider when designing a long plate and a frame is "how are you going to put it together."
Which timber goes up first? which will go up second and which will go up last.
These facts sometime help you to decide which scarf to use and where they should be located.
Another factor is what is the length of the longest timber you can use.
If you're milling your own that's one thing. If you buying timbers that's another thing.
Of course, you can buy 40' long timbers shipped in from some far away mill/lumber yard. But do you want to pay for all that.

A while ago, I did a 40' long wing on to the side of a hay barn.
Here is a drawing of that wing:

As you can see each scarf is supported by a brace. You frame design doesn't show any braces. You need braced along the plate.

My design I put up the first two posts with the long plate section. (Posts 1 and 2). We then added the plate with post 3 and braces attached. Afterwards we did post 4 with the plate and brace attached.
Lowering the plate down from the top is easy and works great. And of course we test fit all scarfs on the saw horses before we assembled anything.

When we build the hay barn, we did a different plate design.
Shown here:


Above you can see the plate. We raised all four bents. Then lowered the plate down over posts 1 and 2. The lowered the plate down over bents 3 and 4. Then lastly we lowered the connection plate between these two sections. We measured from scarf to scarf ends to make sure we had the required 9' span. On one side we didn't. But the posts were not secured to the foundation concrete yet, and we just bumped them with a commander mallet and moved the entire end section out 3/4 of an inch until we had the 9' span. Then the third section dropped right in. With draw bored peg holes we pull the joints together. And it came out to 40'.

Bottom line, again, is you need braced in your design. I hope this helps you to understand how to do scarfs supported by braces.


Jim Rogers.
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension


Thank you for the detailed response and the plan examples. Those are worth a lot to understanding how to split a plate. I particularly like the pair of symmetrical plates with the joining plate in the center.

Quote from: Jim_Rogers on October 25, 2023, 01:12:55 PMYou frame design doesn't show any braces. You need braced along the plate.

I'm moving forward from the principal members and, after I design the plates, I will add the braces to the plans.

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