[Greenbuilding] Growing Media Embodied Energy

[RT]

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

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