[Strawbale] plwood vs. osb for arch gussets

[Rob Tom]

On Fri, 23 Feb 2007 15:16:14 -0500,
Ken Simonelis <farmhill at telus.net> wrote:

>  I'll be building either the 24' or 32' span straight wall truss.Either  
> 8' 0r 10' leg height and using 2"x10" Spruce.I sawed these a full 2"x10"  
> and since our mill is a band saw type theyare quite accurate  
> dimensionally so I won't be planing them.


Ken;

I'm not that concerned about the capacity of the OSB to do the job, so  
long as the OSB was not a "B" grade (or worse) and the OSB is kept dry.

ie If the reason you "got a deal" on the lifts of OSB was because they  
were "seconds" what was the reason they were classified as such ? If it  
was defective adhesive or the sheets were cooked too long, the OSB may not  
be a suitable substitute for the plywood for gussets).

I've made plenty of box beams using OSB webs and after 20 some years, have  
yet to see anything resembling a problem as a result of choosing OSB vs  
plywood.

In fact, I have some box beams consisting of 4 pairs 300 x 300 x 11 mm OSB  
gussets used to space apart 2 -2x4s (using only drywall screws for  
attachment, no glue) to create 300 mm deep beams that I use to span 10  
feet on scaffolding, on top of which form-ply is used to create a 600 mm  
wide floor deck on which to work. Those box beams have supported   
wheelbarrows full of wet concrete + workers + tools and have been left  
outside year round in all kinds of weather (with the formply deck in place  
of course).
No problem.

The bigger concern IMO is the 2x10 lumber.

Since you've sawn them yourself, it's reasonably safe to assume that the  
2x10s weren't graded and there's nothing wrong with that *iff*  (ie the  
mathspeak "if and only if" )
you are well-versed in lumber-grading procedure and can adequately assess  
the quality of each piece using the same standards that a real lumber  
grader would use.

I'm reasonably sure that whatever plans you are using did specify lumber  
species and grades for the arch components.

For instance, if the 2x10s were plain-sawn from relatively young,  
small-diameter trees from which you were just barely able to squeeze out  
10" deep pieces, then my guess is that the chances that of the lumber  
being full of larger-than acceptable-knots in the wrong places is pretty  
good (but bad for you). My guess is that the plans specify a minimum of #2  
or better, S-P-F, light framing grade for the rafters.
The Building Codes in Canada (ie the worst buildings that the law will  
allow) do not permit the use of #3 or worse grades of lumber for flexural  
members.

The other thing is that if the rafters of the arches are only 2x10s, then  
10" is the max. available framing cavity to accommodate ceiling  
insulation.  I doubt that there's anywhere in Canada where 10" of ceiling  
insulation (other than pee-you foam etc) (minus ventilation space) is  
adequate from a Green perspective.

I think that it might be more prudent to look at turning the rafters of  
the arches at least, into parallel chord trusses to increase the depth of  
the rafter to at least 600mm
(ie 24" in Murrican).

That would enable you to use smaller dimension lumber (ie 2x4 or 2x6) of  
which you're likely able to get higher-grade pieces from the lumber you've  
sawn.

I have used rough-sawn lumber from the farm before for farm buildings,   
but it was usually sawn at a PTO-driven mill with a circular blade. That  
surface of the lumber from that sort of a mill would be too rough to  
provide good glue lines at the lumber-gusset interface. I don't know how  
coarse a blade you used in your bandsaw mill but if you plan on gluing the  
gussets and don't want to have wasted the expense of the glue for nought,  
consideration of the quality of the gluing surface presented by the  
unplaned surface should be considered.


=== * ===
Rob Tom
Kanata, Ontario, Canada
<A r c h i L o g i c   at  c h a f f y a h o o   dot   c a >
winnow the chaff from my edress in your reply