[Greenbuilding] Growing Media Embodied Energy

[John Salmen]

Something I'm not following here. Water on a roof fills voids and either
ponds and or hopefully drains. Most any roof built in the last century under
some standard is capable of holding a live load of about 6" of water (or 12"
of dry solid wood).  Not saying its wise to leave those materials sitting on
a roof over time just that that is a general capability.

4-6" of woodchips will be the same density if compressed to 1". If
compressed and decomposed the weight will probably be even slightly less. If
compressed and saturated the weight still will not exceed the weight of
whatever the height of water is plus the wood density. Over time fully
decomposed the woodchips could probably form about 1/8" of a soil material
but more likely a anaerobic biological mat of some sort. Definitely less
weight than gravel or broken brick.

The concept is that this is not a green roof. It is a place to put woodchips
which may be more plentiful and have less embodied energy in an urban
environment than gravel. Call it a beige roof. It would support biological
activity and moss growth and some moss like mold does not die when dry and
can hold and thrive on very little moisture. Moss can filter heavy metals
and acidity in water and probably numerous other benefits.

I love planted green roofs but like any artificial landscape they have to
really well tended. The carefully designed and prepared energy intensive
soils and plantings fail quickly so many continually get 'redone' or become
moss zones. If we look at it simply we want a sacrificial material that has
the fewest deficits (expense, weight, embodied energy, toxicity, etc.) and
the most potential benefits. A material (like woodchips or possibly other
bio or mineral waste) that holds and releases water and supports life while
decomposing is pretty good - and the end product is soil for some future
roof purpose.


-----Original Message-----
From: Greenbuilding [mailto:greenbuilding-bounces at lists.bioenergylists.org]
On Behalf Of RT
Sent: October-13-13 11:46 AM
To: GBioEL
Subject: Re: [Greenbuilding] Growing Media Embodied Energy

On Sun, 13 Oct 2013 10:52:32 -0400, Topher <topher at greenfret.com> wrote:

> On 10/12/2013 11:09 AM, Gennaro Brooks-Church - Eco Brooklyn wrote:
>> The wood will decompose and shrink in depth, requiring the addition 
>> of more. Each time you do that the weight increases. Is that a valid 
>> concern?
>
> No, it's not.  Wood is mostly C5H10O5, when it decomposes it does so 
> into things like H20, C02, etc.  All of those things subsequently 
> leave a roof environment.  So decomposition, and shrinking in depth, 
> reduce the weight in direct proportion to the loss in height.  
> Compaction does not however, so I would make your load calculations 
> based on a fully compacted mass of the given depth.


I'd say that weight increase *is* a valid concern but not for the reasons
stated.

Fresh, large-chunk-size wood chips drain water quite well but as the
material decomposes, it's ability to hold water for longer periods
increases.

The wood chips will eventually decompose into a rich humus and that material
holds water like a sponge, for a long time.

Assuming that the wood chips are being placed on a relatively flat older
roof not really designed to be a Green roof, the increased weight of the
waterlogged humus may result in deflections of the rafters, possibly
creating scenarios for ponding to occur.

Once ponding begins to occur, the problem exacerbates itself because
increasing amounts of water will start running to the sagging area. More
than a few roofs have collapsed for this very reason.

Another potential issue is that as the wood chips decompose, the particle
size decreases from inch-plus coarse particles eventually over time to
pin-head-sized particles.

If the drainage system was not designed to anticipate a large volume of
fines, the drain inlets would be inundated and the problem of ponding could
present itself.

While the idea of growing moss on a NYC roof sounds nice and natural, I had
to wonder how realistic it is.
NYC is not the Wet Coast of the Pacific NW.  Moss does not stay green for
very long in dry environments as I would imagine a rooftop in treeless NYC
would be.

Without moisture, it becomes a fragile, dry crunchy material.

My guess is that in NYC, the primary source for woodchips would be ground-up
shipping pallets rather than ground-up tree limbs. While I'm no doubt sure
that one could order "black locust wood chips" if one is concerned about
embodied energy numbers for various growing media, then importing designer
lo-rot wood chips wouldn't make much sense.

OTOH, I would imagine that broken clay brick and such-like coarse granular
material from building demolition would be a plentiful locally-sourced
resource in NYC. It would provide the permanent "structure" for the  
growing media.   Mix in some locally-sourced organic matter (say discarded  
produce from green grocers, composted) and you're good to go. The organic
material would be replenished regularly over time, as one would with any
garden soil.

=== * ===
Rob Tom					AOD257
Kanata, Ontario, Canada

< A r c h i L o g i c  at  Y a h o o  dot  c a  > (manually winnow the chaff
from my edress if you hit "reply")

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