Replies

1. 
   Guest_ | Jun 28, 1999 12:30am | #1

   *
   Hi Tedd,

   I think you're starting to push the envelope here.

   How much wood do you intend to add to the structure in order to stabilize the purlins? The 2x4's on edge only give less than 1 3/8 flat bearing surface on your 2x8s and toe nailing the assembly will lessen the shearing strenght of your nailing.

   Gabe

1. 
   Guest_ | Jun 28, 1999 01:40am | #2

   *The purlins will do nothing to strengthen the roof framing, so why put them on edge? Put them flat and get better nailing/screwing areas all around.

1. 
   Guest_ | Jun 28, 1999 02:54am | #3

   *Hells Bells,Do you blokes think wood grows on trees??( sorry george )Heres some figures from our (engineered) Code Book ( this covers wind speeds up to 41 metres per second).Rafters 2x8 @ 4' centresBatten (purlin) 2x3 @ 4' centresThese are calculated on unseasoned timber. Make it seasoned and the size reduces again.Now I'm not saying use these figures, 1. It's not your code and 2. These are maximum and I feel pushing the envelope, but just look at the differance in the amount of timber.A standard scenario here for this would be,Rafters 2x8 @ 4' cBattens 2x3 @ 2' or 3' c.The only reason we reduce the batten spacing is because we don't sheet with ply like you guys do, and it makes it a lot easier to sheet the roof when your battens are closer together. ( sheet roof will span up to 4' unsupported.)

1. 
   Guest_ | Jun 28, 1999 04:21am | #4

   *The American Wood Preservers Institute (Pole Building Design) says 2X8's spanning 14'3" can be spaced at 28.5 "'s with 2X4 purlins above (on low pitch roof).I'm trying to figure this out ?tedd

1. 
   Guest_ | Jun 28, 1999 08:37am | #5

   *Mark ...What span is the rafters/battens system rated for with the 2X8's and 2X3's ? If your not using sheathing, what are you toping off the rafter bays with ? and what are you roofing with as well ?tedd

2. 
   Guest_ | Jun 28, 1999 11:36am | #6

   *tedd,maybe I shoudn't have stuck my nose in here.I'm from Australia so a lot of what we do just won't apply, however lets work backwards...Sheet roof here can span a maximum of 4' unsupported.The batten size is determined by the rafter spacing.The rafter spacing for a sheet roof can be a maximum of 4'.The span for 2x8 is a maximum of 14' (based on unseasoned timber and a stress grade of f8 though that probably won't mean anything to you as I believe you have a different grading system).We don't sheath at all. Never.( I am alittle curious as to why you do... is because of the climate differance??)Oh yes, sheet roof I'm talking about is steel, however here are some figures for a tile roof for the same span.Rafters 2x11 @ 3' centresBattens 2x3 @ 1' centresBoth of these specs are for rafters of single span ( that is no underpurlin supporting them...supported at top plate and ridge only ), if they are continuous spans then the size reduces even more.RegardsMark

3. 
   Guest_ | Jun 29, 1999 12:53am | #7

   *Mark ...Why don't you post some of your engineered span tables on this site/string of posts. To keep the lumber and manufacturing industries of North America robust, the building codes force us to over-build. Engineers, architects and builders use commercial industry conventions not truely engineered designs.I found a table today that will shock everyone over here. It says rough cut/sawn lumber (full dimension 2X's) is 25% to 50% stronger than dressed/finished lumber depending on size - 2x4's were around 50% stronger when fully dimensioned (rough cut) and 2x10 were around 35% stronger. Over here light framing lumber is 1.5" x for width and 1/2" less than its named size on breadth -- so 2 X 8" 's are actually 1.5 x 7.5 inches. This applies in Canada and US whether we are in metric or imperial measurements of course. The interesting thing is that as the retailed finished dimensions shrink over the years, the span ratings have changed little (in my books anyway). THis is evidence that we are overbuilding. The quality of lumber has declined corresponding to this evolution in smaller dimensions being rated the same as full dimensions.I also was forwarded conversion factors by an engineer in the Timber Framing Guild this morning. His info says that a rough cut plank is actually 1.9 X's stronger than the finished dimension. That's pretty signigicant for stiffness/deflection and span.What is also conspiciously absent from the load and span books and from builders minds is that when one sheaths rafters or joists, it creates a box beam with a compression flange. This turns the floor or roof into an engineered system with extraordinary spanning capabilities. If a structural panel/stressed skin panel made of 2X4's and 1/2' or 3/8" plywood/OSB can be site built with nails to span 8 to 16 feet unsupported, how come 2x8 rafers/joists with ply/OSB nailed on top isn't understood or graded as a system for even longer spans. When we get to 2x10's for rafters and joists, which is common, what kind of spanning strength do we really have.When guys tell me that 2x4 purlins horizontally on top of 2x8's, sheathed, only add dead load (but no strength), I realize just how much the wood industry has taken the builders for a ride and that trranslates into fooling home owners/buyers who are paying premium dollars for structures that wil never be stressed to their limits. Post your span and load charts.all loaded up and no where to go ...tedd

4. 
   Guest_ | Jun 29, 1999 04:44am | #8

   *The lumber industry explanation for shrinking but still strong lumber is that grading has become more sophisticated, so there need be less margin for error.Not vouching for this at all, they're up there with the tobacco industry in my book, but interesting.With the ply box beams, you mean boxes of considerable depth with the 2x4's as the flanges? This works for the same reason a truss can work with small wood -- arranging the load-bearing and distributing members in a more efficient manner than stick building.

5. 
   Guest_ | Jun 29, 1999 11:55pm | #9

   *Tedd - The design of the rafter is based on wether or not it can handle the live and dead loads. The purlins make little difference in the design of the rafter. You need to be consulting a rafter chart to find out what you need for rafters, then worry about the purlins.