[Strawbale] Green roofs on strawbale buildings

Derek Roff derek at unm...
Fri Sep 17 20:37:51 CEST 2010

While Rob Tom's proposed experiment is entertaining, evocative, and a 
good illustration of why we shouldn't build a wall with uncompressed 
broadloom carpet, I think there is some merit in looking at the 
original question in light of both laboratory testing and case studies 
of existing strawbale buildings.  My observation is that the 
loadbearing element of a plastered strawbale wall can vary somewhat.

I am a believer that the stiffest element of a composite will take the 
loads when the force applied exceeds the capacity of the more resilient 
elements.  Bruce King's book "Design of Straw Bale Buildings" has 
references to multiple structural tests that confirm this view.  "The 
Last Straw Journal" has published a number of these tests over the 

I am also a believer that unplastered strawbales are subject to creep, 
that is, that they will continue to compress under a static load over a 
long period of time.  Thus, one would expect that even if the plaster 
skins are not strongly loaded initially, over time they would come to 
take on more and more of the load, even without the dynamics of snow, 
ice, and soggy green roofs.

On the other hand, if you visit a number of loadbearing strawbale 
buildings, I suspect you will observe, as I have from time to time, 
walls where there is a continuous gap between the plaster and the roof 
bearing assembly (RBA), both inside and out.  This gap exists, because 
plaster shrinks as it dries/cures.  Without special attention and 
detailing, as RT recommends, wet-applied plaster is likely to recede 
slightly from every plaster stop and edge, including the bottom plate 
and RBA.  It is fairly common, in my experience, that a continuous gap 
is present a couple of years after construction and building occupation 
in owner-built homes, and in some cases, the gap has been filled with a 
non-bearing flexible caulk.

I know of two post and beam strawbale houses, where one or more of the 
posts does not touch it's pier at the bottom.  The post is suspended 
from the beam, hanging with a small amount of clearance above the stone 
that is supposed to be supporting it.  My guess is that the beam shrunk 
enough as it dried, that it lifted the post off of its support.  Or it 
may be that the pier settled, or both.  One of these buildings shows 
the continuous gap between the plaster and RBA described above, while 
the other has a variable, non-continuous gap.  So both of these 
buildings in fact have load-bearing strawbale walls, even though roof 
loads were intended to be carried by the post and beam frame.  In the 
case with the continuous gap, it appears that the bales themselves are 
taking the load, while in the case with the variable gap, I suspect 
that the plaster and the bales share the load.

This doesn't contradict Rob Tom's thesis that the stiffest elements 
tend to take the load, but it does point out the difference between 
strawbales and uncompressed broadloom carpet.  Strawbales are pretty 
dense and stiff, and are able to support some percentage of the roof 
loads by themselves.  Potentially 100%.  There are historical records 
of strawbale buildings remaining unplastered for over a decade of use. 
We don't know how much creep the walls experienced, but not enough to 
be commented on in the historical descriptions.

In plastered strawbale walls, the plaster may be taking all of the roof 
loads, or it may be that the bales are taking all of the roof loads, or 
each element may be taking some of the loads.  As the loads increase, 
and as time goes by, I would expect the plaster to take on an 
ever-increasing proportion of the roof loads.  I would also argue as RT 
recommends, for careful plaster application and detailing, so that the 
plaster takes the load as soon and as completely as possible.  But I 
think evidence is good that in a fair number of real-world cases, the 
bales themselves may be taking a significant part of the roof loads 
during some periods of time.

OK, RT, tell me where I've gone wrong.


Derek Roff
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek at unm...

--On Friday, September 17, 2010 11:38 AM -0400 RT <ArchiLogic at yahoo...> 

>> Would you mind providing a reference for that? I have no trouble
>> believing that the stiff plaster takes dynamic loads, but I don't see
>> why it would take the static loads, given precompressed bales and all
>> static loads applied before the plaster is applied. So experimental
>> evidence would be very valuable.

> Just curious as to how you think a SB wall assembly differentiates
> between   the dead and live components of the gravity loads to which
> it must respond   and then proceeds to direct the straw portion of
> the wall to deal with   only the dead load component of the applied
> loads ?.
> (And I don't think it's reasonable to assume that all of the dead
> loads   will be in place at the time of plastering. ie think of
> multiple storey   structures as a "for instance")
> To anyone who has any doubts about the harder/stiffer elements taking
> the   loads I would suggest a simple demonstration they can do on
> their own to   confirm:
> Find a piece of deep pile broadloom or carpet and place a few small
> stones   into the carpet in a manner that the carpet strands stand
> proud of the   small stones.
> Then place a chair or ladder next to the piece of carpet, take off
> your   shoes and socks and climb up and then jump onto the
> stone-studded carpet.
> When you recover, comment as to which element (hard stone (analagous
> to   the plaster in a SB wall assembly) or the compressible carpet
> (analagous   to the straw) took the load .
> As to official in-lab test data, any of the compression resistance
> tests   done on plastered wall panels (in North America, Europe, AUS
> or NZ) will   provide the same "evidence".
> Those same tests will also show the importance of proper detailing of
> the   plaster to deal with the expected failure modes (ie Euler
> buckling,   localised crushing, delamination etc.)

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