There may be better approaches to the problem at hand, but I'm
Ok, well, I did not mean to insult you, as I've made some decisions I
regretted in the past, but I think if you build something out of OSB
and put it outside for a while you'd discover while it is not bad
outdoors, its certainly not like PT wood for durability. The beam you
describe is basically indoors so it would not be a valid comparison in
my opinion. I live along the gulf coast where we have year round high
humidity and everything deteriorates fairly quickly, even my Trex
decking has some mold stains and what not. In a different climate,
you might be ok for a while, but I stand by the recommendation for PT
wood for outdoors, or steel as one poster recommended for meeting your
low profile requirements.
I wasn't insulted. I knew it was "out of the box" thinking, I just
didn't know how far out of the box I was. BTW, I'm in central
Mississippi, only 150 miles from the coast. Plenty of humidity here
too. The PT box I intended to build around the beam should have
protected it to about the same degree as the roof over my present
beam. But the question is somewhat moot at this point since the young
couple's budget just wouldn't cover the materials for a thick beam.
They decided to just install a center post, which isn't what I would
have done, but does simplify life considerably.
"Even an old blind hog finds an acorn every now and then."
While the immediate structural needs (bending in this case) can be met
(generally) with less material than is normally used, like Stephen
noted, there are other considerations (lateral buckling) that need to
be considered. When one is talking big construction - many thousands
of dollars, it is appropriate to investigate trimming things down to
precise materials - because much money can be saved. Smaller projects
obviously reach the point of diminishing returns rather quickly.
However, there may be other ways of looking at things here...
What would make the most sense is what is called a 'stressed skin
panel' - wherein the rigidity is built up thru the use of 'skins'
serving the purposes of 'flanges' (of your standard 'I' beam). By
this, *both* surfaces of your framing are 'skinned' with plywood / OSB
- anchored at precisely specified intervals (typically in the 2" to 6"
spacing range). This produces a panel product that has some pretty
surprising rigidity. Another (similar) approach are the SIP
(Structural Insulated Panels), wherein the 'skins' are bonded to a
foam core that does the same thing - and adds insulating value as
The 'trick' here for SIPs (if there is one) is that - by bonding the
surface skins to that foam, then every square inch of the inner
surfaces of the skins contributes to carrying the stress - reducing
the individual unit stresses down to a very low level (thus, the foam
can handle it).
While Stephen appropriately notes 'blocking' for lateral support,
placing that 'skin' panel on both sides of the framing, this panel
then becomes your lateral support (special circumstances may, indeed,
require blocking as well - concentrated loads, etc).
It is not inconceivable for this to be done by the homeowner on
smallish projects - but getting an engineer involved probably makes
sense for more elaborate undertakings.
As an example of a 'non-analytical' application, one of my hobbies is
rocketry (not the small stuff - some of it gets pretty big). I had,
in times past, acquired a fair stockpile of some 0.030" G-10
fiberglass panels (like circuit boards are made of). We are always
exploring other approaches that can provide strength capable of Mach 3
undertakings while still being lightweight. Stressed skin panels have
some real application here. I made up a test panel (about 11"x14")
with some 2" foam (that standard, white 'beadboard' product) as a core
and the 0.030" G-10 epoxied to both surfaces. Obviously, the bare
foam could carry very little - equally, something (anything) 1/32"
thick (the G-10) couldn't carry much either. Together (and held
rigidly in place with the epoxy), I could set that panel on top of two
bricks (13" apart) and it could carry my entire weight (200lbs+) with
almost zero deflection - and this for something that weighed less than
a pound. Such is the nature of things when the right material is
placed in the right place.
The APA (American Plywood Association) has an extensive library of
publications - all manner of things - that could lend insight into
this -- see...
You'll have to register (and I think the public can do this - no
cost). Look for publication W605 "Structural Insulated Panels" as one
resource. There are others on that website as well.
Every once an awhile, leftover panels from projects show up in your
'shopper' mags (and, probably, online) - can occasionally find some
pretty good deals.
This got a little long - sorry,
<with much snippage - I've saved the original>
| What would make the most sense is what is called a 'stressed skin
| panel' - wherein the rigidity is built up thru the use of 'skins'
| serving the purposes of 'flanges' (of your standard 'I' beam.
| As an example of a 'non-analytical' application, one of my hobbies
| is rocketry (not the small stuff - some of it gets pretty big). I
| had, in times past, acquired a fair stockpile of some 0.030" G-10
| fiberglass panels (like circuit boards are made of). We are always
| exploring other approaches that can provide strength capable of
| Mach 3 undertakings while still being lightweight. Stressed skin
| panels have some real application here. I made up a test panel
| (about 11"x14") with some 2" foam (that standard, white 'beadboard'
| product) as a core and the 0.030" G-10 epoxied to both surfaces.
| Obviously, the bare foam could carry very little - equally,
| something (anything) 1/32" thick (the G-10) couldn't carry much
| either. Together (and held rigidly in place with the epoxy), I
| could set that panel on top of two bricks (13" apart) and it could
| carry my entire weight (200lbs+) with almost zero deflection - and
| this for something that weighed less than a pound. Such is the
| nature of things when the right material is placed in the right
Interesting! I've looked at SIP panels, but only within the context of
thermally-efficient home wall construction.
| The APA (American Plywood Association) has an extensive library of
| publications - all manner of things - that could lend insight into
| this -- see...
I'll give 'em a visit and look for W605. I think this could turn out
to be a valuable resource. Thanks!
| Every once an awhile, leftover panels from projects show up in your
| 'shopper' mags (and, probably, online) - can occasionally find some
| pretty good deals.
I'll be watching - methinks these might be worth 'playing' with and
have broader application than I'd imagined...
| This got a little long - sorry,
I'm not. Your response is lucid, thought-provoking, and very much
DeSoto, Iowa USA
Morris Dovey wrote:
> I'm not. Your response is lucid, thought-provoking, and very much
If you want to talk about sandwich core construction using structural
foam with epoxy/glass skins, contact me off list.
Morris Dovey wrote:
> A bit of playing with my calculator has me wondering why not use 1x
> stock for rafters and joists, or at least _some_ rafters and joists?
> It would appear that if I resawed a 2x rafter and edge glued the two
> halves, then the resulting rafter could carry a greater load.
> What am I missing?
Lateral stability, fire safety factor, lack of uniformity of
construction lumber, etc, etc.
Time to start thinking about metal studs rather than wood.
"Don" (I'm just just this moniker for convenience)...
Well, your further elaboration of the conditions sheds more light on
possible solutions (as does your engineering degree).
Height limitations always rear their ugly head in everyday design -
and, when that is all you have to work with, well, that is all you
have to work with. Shallower beams can be used - it just takes more
material (as you obviously know) to accomplish the same result - i.e.
less efficient, but then, as noted, that's life. (Lew is correct and
I mistated about the Ix varying as the cube of the depth).
You might consider a smallish (i.e. low slope) truss construction -
even a simple 'king-post' truss (i.e. 1 vertical strut connecting top
& bottom chord at mid-span) would probably work in this case. This
would provide considerable rigidity over a simple beam and even a very
low slope (say, 2 feet of height at mid-span) should work and would
lessen material needs - as well as getting the end "under" the fascia
(as I understand the situation). That may not fit with your aesthetic
desires, tho -- just have to look the situation over.
Just more thoughts.
My gut tells me this would be overkill. Just remember with this new
ACQ pressure-treatment the corrosion issues with steel (over the older
(and now banned for residential) CCA treatment). These concerns (with
corrosion) are real.
Well I knew a guy that did something like this. He took two 2X8's & routed
out the center of them & glued in a plywood lamination like you speak of.
Then glued up the whole thing so it looked like one beam. I can't remember
how thick he made them, but I think it was three pieces of plywood. Since
your going to do this anyway, why don't you glue up three and see how it
feels? Then you'll know if it feels right. Also I would use Titebond III,
it's a great glue for outdoors.
You really want to check out the cost of an engineered beam like Anthony's
GlueLam ... they may be cheaper in the long run than your materials and
These are similar to the ones I use:
(and I just had an extra 18' one hauled off the other day (Habitat for
My gut agrees. 1/2 the ply is going in the wrong direction to offer any
strength. Of course the middle doesn't do much anyway; it's the top an
bottom of the beam that are exposed to all of the force. 18' is a pretty
I too think you should check with your local lumber yard about pre-made
Posted via a free Usenet account from http://www.teranews.com
If I'm understanding correctly, your patio roof will be 18 feet long by
16 feet wide (not counting overhang on the ends), with two beams and four
Typically, you would calculate the loads using 40 lbs per square foot,
but since this is an open structure, I'd probably just use 10 lbs per
foot for the dead weight of the materials (Keep in mind this doesn't
account for any snow loads, weight of the vines, etc.).
So you would be looking at roughly 2880 pounds on the roof structure
(18x16 x 10psf).
Half of that weight (1440 pounds) is carried by each beam.
According to the charts I have, you would need a MINIMUM 4x10 beam to
support that weight over an 18 foot span. If it were me, I'd go with a
4x12 beam. 18 feet is a long distance to free span.
If possible, I would span the beams across the 16 foot distance, rather
than the 18 foot distance. This would let you use 4x8 beams, though I'd
still opt for the next size up, 4x10 beams.
Of course, adding additional beams and posts would make a huge difference
in reducing the spans and the corresponding beam sizes.
Sounds expensive and a lot of work. In addition, for outdoor projects you
really want as few joints and seams as possible. I'd go with solid beams,
or a couple of 2x10's nailed together as a bare minimum. Plywood isn't
really a good material for outdoor use.
20 foot long 2x12's would be fairly easy to set in place by yourself,
though it would help if you could get another person to help balance
things while you nail things together.
I'd also recommend metal post caps or hurricane straps to hold everything
together. You don't want the whole structure coming down in a strong wind
or earthquake. And, you'll need to sink the posts deeply in the ground
and/or install some kind of diagonal lateral bracing to keep the whole
structure from toppling over.
You might want to pick up a book on outdoor roof structures. I have one
by Sunset books called "Patio Roofs and Gazebo's" that is pretty good
Why not buy glulams already made up. That's a pretty
wide "clear span (18') and you need to be solid for a
porch swing. You can always "dress" the glulams with
a better looking wood.
One idea no one has mentioned is to laminate the beam out of 2x4's laid
flat. Stack them as high as needed so you end up with a beam 3-1/2" wide by
how ever many 2x4's tall. Probably not cost effective, but possible. When
you get it done you could plan it flat and smooth, as long as you have the
tools and support to do it!
Have you noticed that the 2x4s you can buy are generally of lower quality
portions of the tree than the 2x8, 2x10 and 2x12?
Yes, you could save a buck or two, maybe. I'd look elsewhere, though.
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