Roof spar orientation

[Bezoar]

I have seen several posts debating the difference in putting roof spars flat versus vertically. Last night I was basically done in the shop for the day and decided to play around and maybe contribute a tiny bit to the knowledge of the group.

I have laminated poplar for my roof spars. They are 78 inches long, made with 2 pieces of poplar glued together with Titebond. The individual boards for the lamination are 1.5 inches wide. I picked the straightest boards I could from a local sawmill and they were planed to uniform thickness that ended up being 0.95 inches. After ripping from wider boards, before gluing, I flipped one of the two laminations end-over-end, hoping any bowing would "cancel itself out".

For the test: I grabbed the heaviest thing I had at hand that I could rest on a spar - the concrete cap of an old chimney - I estimate it weighs about 50 lbs (such scientific rigor!) I laid the spar supported only at the ends and measured the distance to floor with no weight, then with the weight in center with the spar oriented in both directions.



And the results are...

With No weight - 36 7/8 inches
Weight, spar on edge - 36 1/8 inches (-3/4)
Weight, spar on side - 35 3/4 inches (-1 1/8)

So no surprise that putting the boards like "beams" deflects less than putting them like "planks". My goal was to see the size of the difference. It turned out to be 3/8 inches more, or an additional 50% deflection.

[Pmullen503]

From your description I gather the beams are 1.5 x 2"?

[Bezoar]

Yes I plan to orient vertically with 1.5 inches insulation in spaces between them

[dmdc411]

I used single poplar 1"x2"s, stood upright at 10" centers on 5' wide. Covered the roof with 5mm luan, and 1/8" bendy on the interior. I can walk across the roof of mine with no deflection. Of course I don't do it now that it's completely finished!

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