Purlin Strength

[Michael McMurray]

I have died and gone to heaven with finding this Forum. Luckily I am self-employed or I would be fired, as I am going to be spending a lot of time on this page! What wonderful topics and information.

I am building a post and beam garden shed as an introduction to this type of construction that I have loved, from afar, all my life. I want to try this out, with the shed, and then will move on to bigger things.

My design is from my head, I have read Steve Chappells A Timber Framer's Workshop, but don't blame him for my errors. The design is a 10'x12' (WxL) shed with the sliding doors on the long side. It would be four bents creating three bays with the central bay being double width (3,6 and 3). Height is 8' at the wall. Posts are rough cut Pine 6x6 and rafters are 2x6 rough cut. I want to tie the bents together at their tops with one long beam or purlin (12')and have the rafters butt onto the purlin (five sets of rafters butting over the post tops with the middle centered in the double wide bay not over a post) rather than have beams cut and jointed connecting each bent at their top.
My question is: would the purlin attached along it's length at the top of the posts with a plain mortise and tenon at each post (1.5" according to one topic on this Forum about Tenons) be sufficient to resist the lateral or twisting/shearing force of the rafters and roof?
It is not a big structure, and I think I am over-building with the rough 6x6's but wonder if there is a better joint for this.
Also, Does Jim still have a boring machine available for sale to a Canadian?
Mike

[Jim_Rogers]

Please read the glossary of terms.

What you are calling a purlin is actually a plate.

And it should resist the thrust of the rafters depending on its size, and of course if you make the joints correctly.

We ship boring machines all over the country, to Canada, and over seas as well.
For tools sale information, send me a private email via the regular email system.

Jim Rogers

Oh, yea and welcome to the forum...

[witterbound]

As Jim said, what you're describing is a post and plate design, where a long plate sits on top of each post, and is connected to each post with a tenon.  What's going to resist the outward thrust is the beam that ties each post to the post on the other side.  These beams are also called tie beams.  With such a small structure, you should be able to get by with only putting the tie beams on the outside two bents, so that you've got more headroom in the middle of the shed.   I've also got to say that 4 bays seems like way too many.  I'd think that two bents on each end, with maybe a couple of posts to frame the door and one on the back to provide a nailer for siding would be all that you need.  But I'm no engineer .......

[Michael McMurray]

Excellent input. I think fewer bays would do it, but I'm attempting to learn technique and process for a larger structure. This will be a very "overbuilt" shed, looking almost like a building in miniature, but a learning experience essentially.
Is there a formula for establishing the placement and length of braces? I intend to brace the tie beams in the bents and wonder how to place the braces there. Also, do I need to brace the posts to the plate?
I am using a wedged, shouldered, half dovetail through tenon for the post to tie beam joint. How deep do I make the shoulder (angled cut to provide a "ledge" for the end of the tie beam to sit on) on the face of the mortice on a 6'x6' post? (I think shoulder is the wrong term, but can't find a reference to this part in the Glossary and so am open to correction).
Mike

[Mad Professor]

Why do you need FOUR bents for a 12 foot shed?  Am I missing something? How is the door on the long side , a slider, going to work with the bents 3' apart?  Maybe I'm missing something.....

I'd think a 12' footer could be WAY over-engineered with just four stout corner posts and good bracing, even in big snow country.


WHOOPS!!! Welcome!

[Jim_Rogers]

Is there a formula for establishing the placement and length of braces?

No, there is no formula that I know of, only standard brace layout lengths. On a good or old framing square on the back side of the tongue of the square in the middle running along is a series of numbers. Such as 24 over 24 with 33⁹⁴ beside it. And 27 over 27 with 38¹⁸ beside it, up to 60 over 60 with 84⁸⁵ beside it. These are standard brace layout dimensions.

To read this you see 24 over 24 that means the right triangle has two legs of 24". One leg is the vertical leg and one leg is the horizontal leg. These two legs connect to form a right triangle making a 90° corner. The diagonal or hypotenuse of the right triangle then measures 33⁹⁴ inches. Now we use tools to layout the brace stock that has measurements marked off in 16ths of an inch. Next we need to convert the dimension 33 point 94 to a dimension we can use with our tape measure, in 16ths of an inch. To do this you use your standard pocket calculator and enter point 94 and then press the times button and enter 16 as we need to know this dimension in 16ths. The answer is 15.04. We round down and drop off the point zero four and that leaves us with 15. That means 15/16ths. Our diagonal is 33 15/16"
As a side note if you wanted the answer in 32nds you'd multiply point 94 by 32 and get 30.08 but again you'd round down dropping the point zero eight and that would be 30/32nds but which would be 15/16ths moving to the lowest common denominator. And in this case does not improve the accuracy of the layout. However in other layouts it may.

My point is that these numbers, on the framing square, give you the standard layout lengths for braces.

.... do I need to brace the posts to the plate?

Usually the answer is: Yes......

I am using a wedged, shouldered, half dovetail through tenon for the post to tie beam joint. How deep do I make the shoulder (angled cut to provide a "ledge" for the end of the tie beam to sit on) on the face of the mortise on a 6'x6' post?

First of all if the shoulder is cut at an angle, it is called a diminished haunch. Next the shoulder for the tie beam to sit on should follow the "general frame rules" of the frame design. Unless you intend to place heavy loads on the top of the tie beams such as a deck for storing heavy materials up there in the shed. In that case you may want to increase the "ledge" for the tie beam to sit on.

Jim Rogers

[witterbound]

I'd suggest you tackle the 2 plates, 4 corner posts, 2 tie beams, and 8 braces first.  My hunch is that you'll feel you've got enough experience in these 16 pieces that you'll be brave enough to tackle a bigger job, without cutting 4 more posts and 2 more tie beams (not to mention the rafters).

[routestep]

There has been a few posts concerning Jack Sobon's garden shed. It is a little larger than your proposal being 12ft by 16ft and made out of 8 by 8 stock. It uses 3 bents I think. If you have the chance you might search for some posts on it or get his book on it.

With four bents made of 6 by 6 stock you will have very small braces in the two 3ft bays going up to the plates.  Two feet of your 12 foot length is taken up by the wood in the posts, your braces would be on the order of 12inches by 12 inches legs. If you just put braces on the corner posts going up to the plates you could squeeze a 24 inch brace leg.

Wouldn't three bents with two bays serve you?

[Michael McMurray]

Wow,
I am begining to see the light. I am rethinking this to possibly just two bents. I was going to have a larger middle bay to accommodate the sliding doors. Perhaps four corner posts, two door framing posts, tie beams and plates is good experience, as suggested.
Thanks for the framing square info Jim. I have read your post on the uses of the square. Now I understand it a little better!