Roof Truss Construction

[JoefromKY]

I'm planning to build a 20'x28' cabin, and considering using roof trusses.  I think that it would be less expensive and easier to build using trusses versus rafters ?  Anyone have experience with both options ?  
We have a pole barn constructed with roof trusses spaced every 4ft using 2x4 purlins and a metal roof.  I plan to put a metal roof on my cabin and wonder if I can use this same light weight method keeping the trusses on 4' centers, or is it recommended applying OSB sheathing instead of the 2x4 purlins (thus increasing the load) and requiring 2ft spacing for the trusses ?  I assume what I use to finish the inside ceiling also changes the loading on the trusses and effects their spacing.  Obviously my goals are to build something as economical as possible, yet will last the test of time.  Insulation comes to mind also, is it more difficult to insulate a ceiling if the framing is spaced 4ft ? I've only seen insulation widths up to 23".  Please tell me if I'm being penny wise, pound foolish !
Thanks for any feedback,
Joe

[Amanda_931]

We did the rafter/purlin thing on the barn.  screws through the metal roofing.  And since I use the very low loft area there for storage, there's a painted OSB floor to the loft over the little room.

No problems so far.

BUT!!

there's a guy on another list with me with a skylight from which the condensation on the inside runs down to the low place and drips off onto his floor.  He's solved the problem to his satisfaction by putting a plant there. (I think John's plans tell us how to avoid plants as drip catchers, but....)

That's what you will tend to get with a metal roof over a heated space--condensation onto the under surface, and it's got to go somewhere, and if there's enough it's going to drip on whatever--insulation? probably going to be too irregular to put a plant there--window boxes maybe.

And metal roofs done this way are NOISY, especially when the sun first hits them in the morning.

So this is why the manufacturers say to use the standing seam type roofs for applications you care about, AND to put a solid layer between them and the living space.

I've been reading up on this to try to see how low a slope I can use for my Little House.  (4 in 12 unless the metal roof manufacturer says otherwise.  Looks like rubber roof for me.  Although storage warehouses typically use very very low slopes and metal roofs).

[John Raabe]

One solution is to put down a good lapped membrane roofing for the waterproofing and then do whatever you want on top of that. This outer roof is really a sun and wind screen for the membrane and need not be a perfect roof itself.