I am building a 20'x30' pavillion in our orchard I want to do it timber frame. I live in easten PA.

I am sawing my own oak beams and had some plans drawn. They call for 8x8 posts, three down each of the 30' sides. Then 8x18 cross beams. The plates are 8x12. The rafters directly above the posts are 8x8 and then the common rafters are 4x8 on 36" centers. The king post is 8x8 with an 8x14 ridge sitting on top of that. The roof pitch is an 8/12. The decking on the roof is 1.5 inch poplar of varying widths. Do these sound like large beams? I am new to timber framing but have done a good deal of reading on the forums and a few books but I have never read about any beams being this large to span 20'.

The other issue is after getting these plans I started cutting the logs that we have at out house and found out that they are 12.5-13' long. So I was thnking about expanding it to 4 posts down a side and scarf three pieces together to make the plates. I would locate the scarfs over top of the braces on the two middle poles. Would this be strong enough?

Any advice would be greatly appreciated
Thanks

Chris Becker

Well I don't recommend designing the timber frame structure without calculating all of your joint stresses. You cannot just guess what will be strong enough. The number of pins needed at a joint is determined by the stresses at those joints. A kingpost design especially needs to be sized according to the stresses. This is tension joinery which is the most precarious so it must be designed correctly. Steve Chapell's book explains the calculations, but if you are not an engineer, you may get confused at some points. A lay person can learn this, but it really helps to have someone to go to when you get stuck or have questions. And you absolutely need someone to check your work. As an engineer, I always want someone else to check my work. You can look at the same mistake 10 times and never see it. It always takes a fresh set of eyes to spot some mistakes. I took Steve Chapell's intro course which is a week long. We spend 2 days on design and joint analysis. I was ok with that because I was already comfortable with the math and stress concepts, but for others, they struggled a bit. In truth, you could make the joinery design/stress calculation a several week course. Don't be intimidated though, because once you are comfortable with the basics, other joints are just slight variations on the same calculations. You are calculating resultant loads due to your snow and wind loads (and weights of the structure and any other added weight). Then you are calculating streass and how much shear area you need to handle it, which will lead to the sizing of the joints(mortise and tennons) and the number of pegs using the material properties for the species you want to use.
I am assuming by the way you wrote that you are new to timberframing and have not had a course. Please forgive me that that is wrong as I am just going by the way you wrote and the way you seem to be going about the design. I am a newbie myself but have had an intro course and am comfortable with the joint design and the math.

Have you discussed your concerns with the guy/gal who drew your plans?
Did he/she do all the calculations?
He/she should have in order to correctly size the beams, posts, rafters, king post, and ridge beam.
Are you planning on having a second floor deck?
Is there storage being planned up there?
There are lots of questions about your design that you need to discuss with him/her and see if he/she understands your loads and wishes.

I am new to timber framing and have not taken any courses yet.

The plans were drawn by my girlfriend's father who is an architect in Michigan and checked over by the engineer at his firm. It just seemed that the beams were of rather large size for the span that they cover. The plans do not have sizes for the tenons on them or the number of pegs. They have the cross beam tying in even with the top of the posts. The distance from grade to the bottom of the cross beam is 8' 6". I am not planning on putting a second floor or any heavy storage up there. The snow load for my area is 40 psf.

Hope this info helps

Chris

No disrespect to them, but if they are not familiar with timberframe techniques and design, they may have just calculated some bending and shear across the whole cross section. You cannot do that with a timber frame (so I've learned). You will have tennons and mortise shear areas that will bear all the load and not the whole cross section of a beam. I know it may seem like I'm complicating things, but it's important to use proper timberframe design techniques. Before you ever start laying out things on a timber, you will need dimensioned drawings for every member. They don't have to be separate drawings for each, you can have a drawing of a bent and a few detail drawings where needed. You need to have everything drawn and dimentions so that you can first visualize each member, where and how it's oriented in the frame. This will prevent many mistakes and serious ones. I haven't got all the books I'd like to yet, but I'd recommend Chapell's book for starters. You need to work though the design of joints sections and thoroughly understand. There are also tips and tricks that we picked up in the intro class that we wouldn't have gotten from any book, many time saving and technique improving things. If I were you I'd get the book at a minimum and ask questions on this forum when you get stuck. It's helpful if you know how to scan and post pics when you ask your questions. Also, see if you can find a good intro timber frame class for next spring. I can recommend foxmaple.com (Steve Chapell). The intro class is end of May/beginning of June. I also think there was another school in Tennesee earlier in the spring but don't know the website. These are excellent skills to learn and will help you in other building areas and projects as well. I don't want to discourage you from doing this or drive you to another method of construction because this seems to hard. I don't think it is once you are informed.

I got Steve Chapell's book and read it. I learned alot of if info from it. I still have a few questions though. One question I have is since the pavillion is not going to be enclosed is it still appropiate to use step lap rafter seats to hold the rafters in place or is there a concern with wind getting up under the whole roof and lifting it up? The roof boards are 1.5 inch popular. Also does anyone have any experience with ash or locust in timber framing?

Thanks
Chris

A step lapped rafter tail and matching seat still need to have a fastener to hold it together. A pair of timberlok screws from FastenMaster will do the trick, no problem...

Is there a specific reason that the tails of the rafters are adzed down to a smaller depth? Is there any reason why you couldn't notch the rafter to fit the step lap seat and still keep the rafter tail the full size of the rafter. In my case 4"x8". That would leave more wood to make a stronger tail.

Thanks
Chris

The reason that the tails are adzed down is to reduce the size and not create a "point of fracture". That is why it has a curve cut in the bottom.

If you only notch the rafter to fit the step and allow the tail to overhang the side of the plate at nearly full depth you'll have a gap there between the top of the rafter and the top of the plate.
Usually the distance from the top of the plate to the bottom of the sill is a standard board length so that you won't have to buy longer stock and cut of a large piece of waste to make up an odd height of vertical siding.



In the above image the top of the plate is 12' from the bottom of the sill. So 12' vertical siding works great.
And the roof decking is flush with the top of the plate so there is not gap there to fill.

If you raise the rafter you'll create a gap that will have to be filled and fitting boards around the overhanging tail could be a problem.

The tail usually only overhangs maybe a foot or so. And with 2" tongue and groove decking there isn't much load out there that a 2" thick tall couldn't support....

In this picture can how are the rafters joined to the plate?
http://www.lancotf.com/windows/leiberman_lighted_kingposts.html Is it a birdsmouth? Or is it something else?

Thanks
Chris