Purlins

I have a question about purlins. I am building a 1 1/2 story post and beam structure and the 3x6 purlins are designed to lay flat and nail on top of the rafters instead of on end like a stick framed roof. I like they way they look in a finished room laid flat and also the additional head room as opposed to laying them on end and nailing them through the rafter.
My question is has anyone had any experience with sagging over time whan they are nailed flat with the roof decking over top?

Derek, Thanks for the reply. You're right about that!
I guess I'm just looking for structural wisdom on this thing.
I'm new at this type of building and I'm trying to decide if I should regroup now and turn the purlins on edge.
It's engineered to be 3x6 purlins at 18" oc for our 40#psf snow load here in Colorado. That's a lot of purlins and it seems like it should be fine but my construction buddies are questioning it. It's probably just that it goes against what we've been used to in stick built roofs that's buugging me about this. But like I said I like the way it looks finished. Thanks.Ron

Deflection is proportional to the width, and proportional to the cube of the depth. So a 3x6 on its side deflects half as much as a 3x3. A 3x6 on it's edge deflects 1/8 as much as a 3x3, or 4 times better than flat. This crdue analysis does not take into account the slope of the roof, though. CB.

CB,
Thanks for the good technical information on deflection.
I noticed some photos you included in the thread about a King Post question a couple of days ago. I noticed the purlins.in that shot. That's almost my same layout. What size are those purlins?

Derek, Thanks for the good information as well. That's helpful.

Thanks for the replys. I appreciate it. Ron

Ron:

The purlins in that frame were 6x6s. The bent spacing was only 11'. In a more typical frame, we would use 6x8s. But of course, you must always do the math (or hire someone who can). CB.

You know daiku I did hire an engineer and I'm double checking his math. That's what this thread is all about. Imagine that. Hiring someone to do the math they put a
You know what I was right. He had blown it and didn't calculate the snow load right for the size and orentation of the purlins he specified. go figure...
I think you have rely on your better judgement as well and not just rely on the calculator. see ya