I'm looking for some opinions on rafter spacing on common rafter style timber frames. I understand that structural design partially governs the spacing; however, the designer usually has the option for a wider spacing with a stout rafter or a narrow spacing with a skinnier rafter. Any input on what looks best aesthetically? My guess is a 4ft spacing is a bit much and 24" might look too busy. This would lead me to a 32" to 36" spacing as optimum. Any comments?

Mine are nearly identical to those described in Jack Sobon's "Build a Classic Timber Framed House". 5x7 rafters tapered to 5x5 at the peak (reduced to 4.5" fork and tongue joinery) set 32" OC (the two center most pairs are set with a 32" gap between them to allow chimney clearence). The rafter's in Jack's book span 20ft on a 9:12 pitch, mine are pitched 10.5:12 so they are just over 13ft. in length.

These proportions look really good and the 32" OC leaves a reasonable span for sheathing to bridge. 36" would make my rafters look (and possibly be) a bit undersized, 6x8 or 9s though impressive might look too fat for their overall length and too large for the space. So I'd say spacing in part depends on span

I agree, 24" OC works for purlins broken by principle rafters but starts to look crowded in long rows of common rafters.

My first question is what's going over the rafters?
If it's sips that one thing to consider.
If it's boards or planks that's another.
You could consider the rafter spacing to fall where it works out right for each... or for the intended decking materials.

I sold a beginner timber framer some boards for his barn. He spaced his purlins at 30" apart, on a principal rafter purlin roof system.
This meant that if he bought 8' lumber he'd have to cut it back to 90", not much waste. 10' lumber worked right on 120", but 12' lumber had to be cut back to 120" wasting 2' of boards, 14' lumber wasted 18" of lumber cutting back to 150". And with 16' lumber being cut back to 180" he wasted 16" of lumber on each board.
If he had keep the spacing to something that divides by 4' then most of all the lumber I had on hand could have worked out for him, except the 10' and 14' lengths. If he had kept it at a 2' spacing every length would have worked. I know 2' is very close spacing for purlins so it probably wouldn't have worked.

About sips, usually if you're using common rafters then the sips would run from gable to gable, and if you space them out to 30" then you'll be paying for a 8' panel that they'll cut back to 90". Ten foot panels would work.
You may want to discuss the rafter spacing with your panel supplier if you're going with panels.
They might have a recommendation that will help you reduce costs and waste.

Hope this helps...

Jim Rogers

Gents,

I'm putting 2x6 decking over the rafters so 32" spacing would work well. Also, I have a couple of 8' wide gable dormers so the 32" spacing works for that also. Thanks for the feedback.

Regarding SIPS.. We typically run common rafters on 4' centers and use curtainwall panels running up and down the roof. There is minimal waste and very little finishing work. The problem with running panels horizontally is you have a lot of seams to clean up in a vaulted area.

And for the customer that wants a wood ceiling, a chamfer along the top edges of the rafters makes a nice little pocket to drop in 1x6's in horizontally and then apply the panels over them in any direction you want.