Roof options

Neil_B · December 04, 2003, 04:07:47 AM

Hi guys, still working on my building drawings but I need help on the roof.

The dilema is that I want to go with 8 or 10 foot walls but I need almost 12 feet of height for the mill. Without having to buy expensive trusses that will allow that kind of height with the shorter wall, and also keep the walls from spreading, what kind of rafter type designs would I need to do this?

The building will be 15' deep and 50' long, so the rafter span is 7.5 on the horizontal, 10' something on the diagonal. The pitch is 8 in 12. The opened ceiling needs to be almost 30 feet not the entire length.

I had started drawing it just with the rafters and gussets at the peak but then realized that I needed to keep the walls from spreading somehow. Can the collar ties be kept up towards the middle of the rafter and still keep the walls from spreading? I was sure I read somewhere that the collar ties have to be within the bottom third of the rafter but now I can't find that description to verify. Beginning to wonder where I even saw that ???.

Any ideas would help and if you need more info just ask.
Thanks

BTW - In updating my post on beam span, I am going with steel. All the info has been very eye opening.

beenthere · December 04, 2003, 06:49:10 AM

There are different truss designs shown here that may help you decide. I believe they will give sidewall support that you are looking for.

http://www.phillipsbuildingsupply.com/Products/Trusses/Designs/body_designs.html

I couldn't tell how wide you wanted the 12' height to be, that you wanted 30' long. Hope you get it worked out.

Don P · December 04, 2003, 07:10:44 AM

A scissor truss might get it. That is a rule of thumb for collar ties. Under heavy loads the lower portion of the rafter can still flex and thrust on the walls. The connection also has greater demands on it the higher the collar is fastened.
I was at my brother in laws hunt camp in Alabama over the holiday. He had built equipment sheds using open web steel trusses made of welded angle iron. The collar across these parallell chord trusses is in the upper third.
On the header beam problem, I looked in an open web steel joist table I have. A 16" deep one would span the opening at the loads we had.
Here's another link that might be of help.
http://www.ag.ndsu.nodak.edu/abeng/plans/Construction1.htm

Don P · December 04, 2003, 12:31:36 PM

Looking at the Phillips Building Supply link above they've got a couple mixed up. The scissor is labelled fan-fink and vice versa.

Jim_Rogers · December 04, 2003, 02:43:37 PM

Don_P:
Nice link to truss site. Thanks.
Jim Rogers

Neil_B · December 05, 2003, 04:47:37 AM

Thanks for the links guys.

Will check through to see what my options are but I'm thinking it may just be easier to go with the 12 foot walls. Depends on what I have for logs kicking around.
Basically trying to do this with what I have here for as little money as possible. Not cheap, just broke :(

Neil

Norwiscutter · December 05, 2003, 08:54:16 AM

Was looking at the Socket Systems site and was hopeing that someone could clarify the difference between ground snow load and roof snow load. My area has ground load of 60, Roof Load of 42, and Live load of 20. How does the total load of a truss system relate to these requirements?

Don P · December 05, 2003, 09:09:40 PM

It sounds like they have given an average total roof load for design of 42 psf.

Or you can tally up your actual dead loads and add 20 psf for snow.

Or,
Slope factors are often used to reduce ground snow loads to more accurately reflect what really sticks up there.

4/12	80%
6/12	80%
8/12	72%
10/12	60%
12/12	50%
14/12	42%
16/12	34%
18/12	28%

What does Socket System rate their system for various spans and species? It gives clear span with high collars, though it felt kinda wimpy to me.

Norwiscutter · December 06, 2003, 09:45:57 AM

Their snow load information shows how spacing and span relate. IE 60lb snowload section that spans 30 ft must be placed on 4 ft. centers. No specification of species type which makes me wonder how accurate the specs can be... I would assume that the assumption is made that materials will be of the same quality as conventional framing plans. Also, the plans assume the use of 6x6 timbers. called the place and no engineer certified plans avalible for 8x8 connecters, although they said they use half inch steel for the brackets. I would imagine that one could use 8x8 timbers and assume the 6x6 specs. Obviously it gets alot more complicated than that but it could be a good starting point. If I was building a house I would probably go with conventional timber frame and avoid the ugly brackets...but for putting up a garrage, barn, etc. looks like a much better option than the typical metal pole buildings that ya see everywhere.

Neil_B · December 06, 2003, 10:21:51 AM

I had actually called the Socket systems rep here in Canada as they are only 25 minutes away from me. Unfortunatly I never heard back from them so don't know what happened there. Maybe lost my number, I don't know.
Anyway, was looking for the price and specs for the 8 inch post to concrete connectors. Not really interested in the other brackets after reading some of the posts from others that did use them.

When I phoned, it sounded like I got the house but the lady said they sold the brackets. Took her a while to find a pen to write my name and number down ???. Didn't seem to professional to me but maybe will try to get in touch again this week coming.

ARKANSAWYER · December 06, 2003, 05:30:53 PM

New-Sawyer,
Look at "Another frame". It is made with 6x6 timbers and and connectors that I make my self. I have free spaned 30 ft with them and 15 ft would not be any hill to climb. This frame is 12 ft side walls and 20 ft free. Cost is low.
ARKANSAWYER