Hello all:

Great topic, and might I add some comments:
In the older barns the rafter were supported on their centres by a purlin plate. Each rafter was pinned to this plate, and the purlin plates were fastened to each other by a tie beam at each bent. This would eliminate any outward thrust of the rafters to the main plates. These structures stood up well without any signs of stress to the sides of the building until after the early 20th century, at which time due to the advent of hay moving forks and tracks installed into the peaks of most barns, the farmers simply cut off these tie beams to facilitate the movement of hay. It was a very bad move because the wall then began to move outward leading to wall failure.
The analogy of this is in my opinion adding a purlin plate and their tie beams and fastening the rafters well will eliminate in my opinion any stress on the outside wall main plates--check it out!!
NH

I was debating whether to add the purlin plate in the middle of the rafters as NH suggests versus the structural ridge beam supported in the middle and decided that the center ridge beam was a better option for a couple of reasons:

1) The roof load would be transmitted directly to the footer through the center posts that extend from the ridge to the top of the main tie beams that run from side to side then through the posts that support the center of these main tie beams to the floor. The pulin plates would be suppported by the main tie beams midspan between the center posts and the posts on the side. Or, I would need to add additional posts to give the weight a direct path.

2) The ridge solution requires 40 ft less beam since there would be one ridge beam versus two purlin plates.

3) There would be only 2 posts in the space on the second floor versus 4

On the down side:

1) The ridge beam itself might be difficult to shape, and

2) the rafters may sag in the middle without the midspan support or I might need to add additional rafters.

The original barn was 30x76 and I'm reconstructing a 30x40 so I do have quite a bit of extra lumber. Unfortunately I am 4 ft short of having two extra plates to use as purlin plates.

I'll try to post some pictures later today.

-Dave

Dab,
If I've read your last posting correctly, I understand that the "Ridge Post" holding up the Ridge Beam will rest on the Tie Beam. And you are hopeful that the forces (weight) coming from the roof will be carried through the Ridge Post to the Tie Beam and then to the Wall Posts. Am I correct so far? If so, I have a short note for you.
I am currently involved in decking a roof with a similar arrangement. The first ( end wall ) Bent needed a replacement Tie Beam, so we found an aged, matching piece of wood and replaced the Tie Beam prior to raising the Frame. This replacement Tie Beam was crowned up by about 2 1/2 " when it was on the horses being laid out and cut. After raising, the post at mid point of the Tie Beam down to the main deck was short of the deck by about half of that and now that the rafter system and 3/4 of the roof decking is in place, the post has finally touched the floor. So, be careful about trying to get the weight to the outside walls without a post holding up the midpoint of the Tie Beam. What I am experiencing with this frame tells me that the weight will push the Tie Beam straight down if there is no main floor level post system under the Tie Beam.
Alternately, oversize the Tie Beam (a lot... )
And all of this settling has occurred without the weight of the SIP's or Shingles and without the snow or wind load on it.
Best of luck with your project...

Hi all:
One more comment on this topic:
It seems to me that the timberframers from years gone by would have used the central post arrangement if it was right to do so, but in my travels I have never seen a timberframe constructed with a central post to carry the load of the rafters in the way described above. The average size of the tie beam across a 30 foot wide barn in the early 19th century was 10" by 12", and upwards, and this was with the purlin posts situated a t 1\3 the distance from the exterior walls. This tie beam was usually supported by one vertical post at its centre. I suggest that such a tried and proven timberframe construction should not be varied from, and might I add that one should err on the side of caution when it comes to sizing timbers to do the job and stand up to the continued stresses from wind and snow loads and aging of the structure down the road. One other thought was the installation of bracing from the top of the purlin posts down to the tie beam would assume some of the outward push of the rafters if installed properly, although this placement of the braces is not necessarily historically correct
NH

I had the exact same thought about having rarely seen the center post supporting a ridge beam. Why not? (Although, Ted Benson shows this design in his 1997 book, The Timber-Frame home: design, construction finishing)

One possible explanation for the rarity of this design is that the savings in wood that you would get by having one full length beam (the ridge beam) over two mid-span plates is negated by the need for larger, full length rafters.

Since I already have the rafters in hand, I cannot see any reason why I wouldn’t take advantage of the ability to transfer the roof load directly to ground through the posts. This eliminates the need for over sizing the tie beams since they will be relegated to the principle task of supporting second story floor load.

-Dave

Actually, the roof system decribed above by others was exactly what was in the original frame I'm talking about.(Centre Mid Purlins) We reduced the span of the roof by eliminating an outside bay of the original Bent and converted to a "structural ridge", requiring the centre Main Floor Posts. Many other changes as well, but that's another topic...

Having one center post holding up a ridge beam was a common design in England during certain time periods. These frames are still standing. The center post is called a "Crown post" and it can hold up a structural ridge beam or a beam that runs from gable to gable under the collar beams, thereby holding up the roof and reducing the thrust at the plate. With collars you are also supporting the rafters near the sag point and this would be a good thing considering the load of your roofing materials.
Your design needs to be looked at very carefully as there are many factors in play.
Jim Rogers

Hi All:

Thanks Jim for that little tip about the central post being used at certain times in England, I wasn't aware of that. I guess we all get carried away by the techniques that were used and practiced right in our own areas. One other thought on the central post theory that may or may not be useful is that the use of 2 purlin plates would leave a good wide open central space area on the second story level. This might help out in the planning stage for future use.
Jim I wonder if you could expand on your statement of "reducing the span of the roof by eliminating an outside bay of the original bent and converted to a structural ridge requiring the central main floor posts" this sort of intrigues me. Do you mean that you narrowed up the building by a certain amount?
NH

Yes, we reduced the width of the building and also reduced the length of the building as well.

[img]http://i3.photobucket.com/albums/y85/jim4388/JerryS/roof005.bmp[/img] I hope this works... picture abovr shows what this "structural ridge" now looks like.