lumber grading: SPF vs SYP

[LD1]

Good info so far.

New favor to ask.

Those of you that have 40' spans or close do it, can you post some detail of your trusses? What size cords, spacing, span etc. Perhaps even a pic if visible. And if you happen to know the load ratings....

Any all all info is helpful in me making my descions about what to do.

 

[LD1]

Another possible change now.

I am considering 12' truss spacing. Need to call and get some quotes. But fewer poles, and I would drop to 12' doors on the eaves instead of a 16', and save having to do a large header.

Can't really find any tables for purlin spacing. But would think a 2x6,on edge would work. Probably stronger than 2x4's spanning 8' on edge.

The 8' oc trusses were $161. If 12' trusses are $290 or less, it will be less money overall.

 

[PoorPlowboy]

12ft OC trusses never saved me any money because I was always instructed to use no less than 2x8 No2 purlins on edge, on 24" centers with spans that far.

 

[s219]

Can you run the load calculations to see what the deflection is of a 2x6 over 12' with your design psf?

Anecdote: when building carrying beams for my boat house roof system, I used triple-bolted pressure-treated 2x6s over a 12' span (they only support load near the ends, not in the middle). Anyway, when assembling the beams with through bolts, for which I had pre-drilled ahead of time, I noticed the single boards slightly sagged under their own weight when hanging over the 12' span. I had to wrestle them a bit to get the holes to line up. These were fresh/wet pressure treated boards, so they were much heavier than a dry 2x6 should be, but it impressed upon me how little stiffness a single 2x6 has over 12'. A sopping wet 2x6 would have about 3# per foot of loading from the liquid, which isn't a lot.

Another factor to consider is uplift. You should be able to start with the roofing metal and get a requirement for purlin spacing, then work your uplift loads down to fasteners for the purlins to the trusses, then trusses to header beams, etc. What I found with my boat house roof is that there was a manageable spacing for which I could properly attach purlins to rafters, and then rafters to headers with readily available screws and hurricane straps. I needed two #10 3-1/2" star drive screws for each purlin/rafter attachment (that's for a 2x4 purlin laid flat), and then each rafter needed about 600# of uplift resistance at it's seat (which I got by putting two hurricane straps at every rafter/header attachment).

When you start spacing out the rafters or trusses farther, it means each will carry more of the uplift load. And it could reach a point where you can't properly attach them to the headers with readily available hardware. Or you may run out of places to nail the straps on if you need more than one. Just something to consider.

How were you planning to attach the 2x6 purlins standing on edge to the trusses? When I have seen 2x6 and 2x8 purlins used, they were sitting between trusses supported by hanger brackets. Seemed like a lot of extra hassle.

 

[LD1]

Weather I go 8' with 2x4's or 12' with 2x6 purlins, the plan is to go between trusses with the hangers.

I am not a fan of on edge on top with a huge nail. Just don't like it.

 

[GPintheMitten]

Wow. 12 foot O.C. trusses. Just make sure you do the calculation s for the purlins. Angle bracing of trusses will be harder.

There will be some handling and assembly challenges. I don't remember if you're building this yourself or having a contractor do it.

Personally, I wouldn't do it to avoid the need for a beam or two.

 

[LD1]

No I am doing it.

Will probably call one of them metal truss companies. I know they go 12' OC frequently. Weather or not I buy their trusses, I will ask them what size they use for purlins.

12' OC isnt just to avoid a beam or two.

12' trusses would take 7

8' would take 10

So fewer trusses = less $$
Fewer posts = less $$$
Fewer holes to dig and concrete = less money
Fewer purlin hangers = less money.

Now weather or not all of that savings is offsets the larger and longer purlins and heavier more expensive trusses...depends on just what size is needed. If 2x6's will span the distance, it should be a good bit less, just depends on how much the trusses are.

Should go up quicker too.

 

[s219]

At some point, fewer trusses and posts will begin to drive up the sizing of the trusses, posts, headers, and footings, as well as drive uplift resistance requirements of them all. That also starts to place more importance on individual structural elements as failure points. Typically, we design away from those fringe situations by distributing the load as much as possible, with closer spacings and lighter materials.

There is obviously an optimum with regards to managing loads from purlins all the way down to footings, and the associated costs. Might take a lot of calculating and pricing to sniff out that optimum. I bet the easy way is to copy whatever some of the better Amish barn builders have done. I had CHA Polebarns put up my barn, and they had it all down to a science, from design/plans to fabrication (I learned a lot of new tricks watching them, and this after 30+ years around the construction industry).

 

[GPintheMitten]

No I am doing it.

Will probably call one of them metal truss companies. I know they go 12' OC frequently. Weather or not I buy their trusses, I will ask them what size they use for purlins.

12' OC isnt just to avoid a beam or two.

12' trusses would take 7

8' would take 10

So fewer trusses = less $$
Fewer posts = less $$$
Fewer holes to dig and concrete = less money
Fewer purlin hangers = less money.

Now weather or not all of that savings is offsets the larger and longer purlins and heavier more expensive trusses...depends on just what size is needed. If 2x6's will span the distance, it should be a good bit less, just depends on how much the trusses are.

Should go up quicker too.

Good points LD1. You're putting a lot of good thought into this. It will be good to hear what you finally decide on... and of course, to see your build thread too.

 

[LD1]

Got a quote on some 12' OC trusses.

$270 ea Using 2x10 2400msr tops and 2x8 2400msr bottoms

The program actually gave using a 2x8 top cord. But I requested an upsize as the CSI was at 94% max capacity with the 2x8. The 8' OC trusses were also in the 90% range, so not much room for error or that ocassional heavier than designed snowfall.

Upping to the 2x10's gave 64% max load @ 12' OC. And at $270 per truss, desiging everything else around that, will save ~150 bucks and make a stronger roof too:thumbsup: