[Greenbuilding] Alternatives to Pressure Treated Wood
Tim Vireo Keating
t.keating at rainforestrelief.org
Sun Oct 28 22:01:40 EDT 2007
At 5:39 PM -0400 10/17/07, Keith Winston wrote:
>Modulus of rupture? Never heard of that one. I
>think in terms of compressive and tensile
>strength, and those translate into bending
>strength. I'd assume at least one of those is
>quite poor, since it's so floppy. Where does
>modulus of rupture come into play on a worksite?
My understanding is that "tensile strength" is
the measure of how much tension under which a
round member of a material will load until it
separates (pulls apart). I'm guessing this would
be important when considering reinforced
concrete. But for a piece of wood or recycled
plastic lumber, Modulus of Rupture is a measure
of the point at which a member of a material will
fail when a bending load is applied. This is one
of two most important mechanical properties when
one is considering loading a board across a span.
The second measure that I have seen be of most
concern in this application is Modulus of
Elasticity (sometimes "elastic modulus" or
"stiffness"). This is the amount of deflection a
board will experience under a specified load.
Recycled plastic lumber is more elastic (or
flexible) than most woods. Even softwoods are
typically stiffer than RPL. But structural
recycled plastic lumber is stiffer than
non-structure, which is stiffer than wood-plastic
composite materials (such as Trex). Most
structural plastic lumbers on the market add
pre-consumer recycled fiberglass to the mix of
(usually) high-density polyethylene (HDPE, #2),
which makes them stiffer. Typically, to reach the
same M.O.E. for pine, one would have to use
members that are thicker than the wood members
one would otherwise have used. For instance, 2
1/2" structural RPL is as stiff as 1 1/2" pine.
This certainly adds to cost but when one
considers zero durability and maintenance (only
for actual RPL - not for plastic-wood
composites), there is a cost savings over time
with RPL.
There are lots of elements to consider when using
RPL instead of wood. But as a sleeper, RPL's
compressing strength is excellent, even when
ripped. It's durability, even in water contact,
is without equal, unless one has an unlimited
budget. One would have to go with titanium to
match the durability of HDPE (again, as opposed
to plastic-wood composites). RPL can outlast
concrete and steel in these types of applications.
tim keating
>
>Keith
>
>
>Tim Vireo Keating wrote:
>>It's simply not true that plastic lumber has
>>poor strength. Folks continually confuse
>>plastic lumber with plastic composite
>>materials. They're not the same and ASTM has
>>agreed.
>>
>>True recycled plastic lumber is stronger than
>>wood. More flexible, yes. But stronger (if one
>>is considering modulus of rupture).
>>
>>tim keating
>>
>>At 12:15 AM -0400 10/9/07, Keith Winston wrote:
>>>Dan, I think me and everyone else are stalling because there aren't very
>>>good options ;-)
>>>
>>>Yes, you're right sleepers are used all the time. Plastic lumber
>>>typically has very poor strength, so while it might be used for a
>>>sleeper design (as long as it's "full contact"), I'm not sure I'd trust
>>>it structurally, from what I've seen. I sort of like the concrete pier
>>>ideas, except that everything has to be quite high to make space for
>>>full-fledged structural lumber in that case. If you're using sleepers,
>>>you should probably try to align them with the underlying framing
>>>(unless they are across it), since if you've got 1/2" plywood sheathing,
>>>it might not support the weight of your deck happily between rafters.
>>>
>>>There is some borate & polymer treated wood that I've heard about but
>>>haven't seen. It can be found here:
>>>
>>>http://www.eswoodtreatment.com/
>>>
>>>Probably won't be too easy to get locally.
>>>
>>>There is cedar or redwood or ironwood/Ipe or whatever else you might
>>>find, none of which (well, maybe Ipe) could be considered sustainable
>>>(the Ipe distributors say it's grown sustainably, rainforest activists
>>>typically say there's no such thing as a sustainably grown rainforest
>>>tree, last I heard...). Ipe is extremely hard, has to be drilled to be
>>>nailed or screwed, and would be sort of painful to use. Oh, and it's not
>>>available in 2x lumber I don't think. Although it's extraordinarily strong.
>>>
>>>There's another option on the books for pressure treated wood,
>>>acetylated wood. I believe it is only currently available in Europe. Go
>>>on vacation and bring some home! Take a freighter and it might not be
>>>too ungreen! You can find out more here:
>>>
>>>http://www.titanwood.com/
>>>
>>>I've heard of a possible silicone treatment of some type for
>>>direct-contact wood, but I haven't seen anything develop for the last
>>>couple years.
>>>
>>>Good luck. Keith
>>>
>>>
>>>dantonioli at earthlink.net wrote:
>>>> Hi Keith,
>>>>
>>>> People keep wanting information about the design, which is understandable
>>>> but which distracts from the question. The only real alternative here that
>>>> I'm familiar with is to use redwood or other rot resitant woods. Sleepers
>>>> tapering down to zero are used all the time
>>>>quite successfully....it's only
>>>> one end of a tapered piece.
>>>>
>>>> A typical "floating deck" on a roof has to account for slope. Even "flat"
>>>> roofs are sloped. The point is to make a level deck. If you have plenty of
>>>> room to work with, then you can cut 4x4 or 6x6 posts and create a shoulder
>>>> to fit under sections of girders, underneath a typical framed 2x6 joist
>>>> package. The posts sit on additional roofing material and there are no
>>>> penetrations. I've built several and they're holding up just fine after a
>>>> number of years, but I'm sick and tired of using pressure treated wood in
>>>> this way....all the extra ripping and treating with copper green is
>>>> non-green and toxic to work with. I'm old enough to remember when all we
>>>> used in California was redwood....but it's limited, politically incorrect,
>>>> and third-growth material rots out after
>>>>five years or so....it just doesn't
>>>> hold up like old growth.
>>>>
>>>> The only pitch for plastic lumber is coming from people who have no
>>>> experience working with it!
>>>>
>>>> Dan
>>>>
>>>>
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: Keith Winston [mailto:keith at earthsunenergy.com]
>>>> Sent: Monday, October 08, 2007 3:10 PM
>>>> To: dantonioli at earthlink.net
>>>> Subject: Re: [Greenbuilding] Alternatives to Pressure Treated Wood
>>>>
>>>> Is the corner drain under the new deck (when built)? If I understand your
>>> > context, sounds tailor made for drain clogs. What type of roof?
>>>> EPDM maybe? Wood won't be strong when ripped down to nothing, like
>>>> everything else. You can hold the deck back from the edges, or, raise it a
>>>> bit, as others have suggested, and use
>>>>larger framing material. Sounds like
>>>> a large deck, 20' x 40'? Or maybe I'm misinterpreting.
>>>>
>>>> Keith
>>>>
>>>>
>>>> dantonioli at earthlink.net wrote:
>>>>
>>>>> The roof deck is sloped from three directions down to a corner drain.
>>>>> The difference in elevation is approximately a foot, spanning a
>>>>> distance of twenty feet. You need a sturdy frame of joists to support
>>>>> an elevated deck, but as I mentioned in many areas the clearance is
>>>>> minimal and, thus, the need for sleepers.
>>>>>
>>>>> I've yet to see plastic lumber that can be ripped from, say, zero to
>>>>> three-and-a-half inches and hold its structural integrity, let alone
>>>>> be attached to a frame to create a rigid platform. But maybe there's a
>>>>> product that someone on the list has actually used in this capacity.
>>>>> Always checking the possibilities.
>>>>>
>>>>> Dan
>>>>>
>>>>>
>>>>>
>>>>> -----Original Message-----
>>>>> From: greenbuilding-bounces at listserv.repp.org
>>>>> [mailto:greenbuilding-bounces at listserv.repp.org] On Behalf Of Speireag
>>>>> Alden
>>>>> Sent: Monday, October 08, 2007 9:00 AM
>>>>> To: greenbuilding at listserv.repp.org
>>>>> Subject: Re: [Greenbuilding] Alternatives to Pressure Treated Wood
>>>>>
>>>>> Sgrìobh <dantonioli at earthlink.net>:
>>>>>
>>>>>
>>>>>> I'm considering a roofdeck project where I'll have to use a lot of
>>>>>> pressure treated wood as "sleepers." The surface is uneven and
>>>>>> there's only a few inches of clearance between the bottom of the deck
>>>>>> boards and the roofdeck....which means a lot of long rips through
>>>>>> pressure treated wood, and a lot of copper green over all of the
>>>>>> exposed untreated surfaces. The last time I built a deck this way I
>>>>>> told myself that I wouldn't do it again because working with PT this
>>>>>> way is unhealthy--there's a ton of pt dust, too much copper green,
>>>>>> and I can feel the physical effects and it sucks. I'm most likely
>>>>>> going to turn the
>>>>>>
>>>>> job down but thought I'd look into options.
>>>>>
>>>>>> The question is: is there a rot resitant, structurally sound material
>>>>>> that can be cut and tapered? FSC wood? Exciting new composition lumber?
>>>>>>
>>>>> There are structural plastic lumbers out there. You should be
>>>>> able to find them via Google.
>>>>>
>>>>> However, I'm confused about why you need them. If the clearance
>>>>> is that small, I'm picturing a flat roof. Why not use decking as
>>>>> sleepers, with shims to even things out? Then you wouldn't have to
>>>>> cut anything at all, except for length.
>>>>>
>>>>> What am I missing?
>>>>>
>>>>> -Speireag.
>>>>>
>>>>> --
>>>>> Anger, if not restrained, is frequently more hurtful to us than the
>>>>> injury that provokes it.
>>>>> --Lucius Annaeus Seneca, philosopher (BCE 3-65 CE)
>>>>>
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