Span tables for 4" x X" ?

[Drew]

Morning all,

I can find joist and rafter span tables for 2" x X" lumber (2"x6", 2"x8", etc.).  Are there tabels for four-by lumber?  My roof design requires me to put in 2"x6" on 16" centers and am wondering if I can get cute and do 4"x6" on longer centers.  If there is no table, is there a rule of thumb or even some math?

The project involves a shed roof with a 15' span for the rafters between the headers.  My memory is telling me 2"x6" on 16" but I have it written out in my plan folder.

If that works out, all I need is someone big enough to lift my rafters into place.

[PEG688]

You could do that BUT your roof sheathing materials will be the issue if you open the distance between centers your sheathing will need to be thicker to carry itself and the roofing materials over that increased span.

There  is a SYP span chart PDF down loadable file here ,

http://countryplans.com/smf/index.php?topic=2773.0

Located in the Free stuff section of the forum.

[John_C]

There are also some  pdf downloads here

http://www.awc.org/technical/spantables/index.html

that might help.  As  PEG  said the roof sheathing will need to be adjusted accordingly.  Locally it's quite common to use 2x6 T&G with rafters on 4'  to  6'  centers, although we don't have any real wind, snow, seismic considerations here

[Redoverfarm]

Drew you might want to reconsider the 2X6 for the rafters.  Even if you double 2"rigid and add 1/2 styro you will not get enough R factor in the roof.  You would have to have a closed cell sprayed on to gain enough R's $$$$$.  The price of the rigid 2"X 4' X 8' is $29 here.  The other option is to use 4X6 or 4X6 with 2" T&G and then build your insulation cavity above the T&G. Just my $.02 worth.

[John Raabe]

The main reasons to go to 4x beams and 2x6 T&G decking is to have the beam and deck exposed to the inside. For a rough guide you can turn the rafters into 4x rather than 2x and space them twice as far apart. 2x6 T&G can go 4' or more on a roof span.

For insulation on such a roof you usually go topside with layers of 2" or more of foam sheathing. In a cold climate you will probably need 6" of this which causes its own problems with fastening the roof, etc.

This is both costly and time consuming. In the days when insulation was optional or a token effort at best, this could be done. These days I only use beam and decking for loft floors where plumbing is not poking through the floor. 2x rafters and full depth insulation with drywall or wood boards (over black poly) on the bottom of the rafters is my suggestion for an insulated roof.

However, the beam and deck roof described might look great on a porch or other uninsulated space.

[mvk]

John
Is the black poly a vapor barrier? why Black?

I have done the foam bit to winterize porches that were beam and 2x6 T&G. 4" foam 2x4 strapping with another layer of 1 1/2" foam cut to fit between the strapping. It wasn't that hard to do but I never felt that good about it.  That's why SIP's are such a good marriage with beams. I also built roofs on top  with 2x12 rafters and fiberglass to do the same thing.

Back when I was reading about super insulation there were folks using TGI's in Alaska with post and beam roofs and walls I think.

I always thought that the roof was one of the drawbacks with straw bale. You had R 40-50? walls and R 19 roofs!

If I wanted the beam and plank look and didn't have to worry about code I think I would explore site built Larson trusses and blown in cellulose. There are some green builders in VT who say you don't need a vapour barrier because the cellulose stops almost all infiltration and can absorb a lot of water and let it back out to the interior. I first saw this on the fine home building site. Anybody have any thoughts on this about the vapour barrier

mike

[glenn kangiser]

One thing is that black is more UV resistant, but if exposed to direct sun black breaks down in about a year, clear - maybe 6 months.

Protected from UV - shaded should be a very  long time - not sure if its the same as buried which is near forever as our landfills show.

[John Raabe]

You need an air and vapor barrier on the underside of a single rafter ceiling. When you do drywall you can airseal the joints carefully and use paint as the vapor retarder. If you want a more rustic rough board ceiling finish use the black poly as the air vapor barrier and it will make any cracks and knot holes disappear.

[glenn kangiser]

Then with the boards over the black poly, UV damage would be non-existant.

[Drew]

Thanks for the information, guys.  I took another look at my plans and the span charts and it looks like I have to do 2"x8" for the 15' span but can have them on 24" centers.  It's a shed roof, and insulating between the rafters looks like my best bet at this point.  The good news is that Palermo, CA has no snow and rarely gets a frost.  The heat, on the other hand gets a bit rough.