[GSBN] SB Overhangs

Thu Aug 26 04:42:38 UTC 2010

Bob,

Thanks for your good points about the durability of the finish being an
important factor to consider when deciding what if any walls should be
required to have an overhang in an SB standard or code.  Maybe such a
requirement would be only for walls with clay plasters, and would take into
account effective reduction of wall height by finishing the wall with a more
durable plaster close to the ground.

I suppose some might argue that irrespective of durability of the finish,
there are ways rainwater can into a strawbale wall, including through-cracks
in the plaster and at any wall penetration, therefore the roof overhang
requirement should apply to all SB buildings, with the possible exception of
buildings in climate where there is virtually no rain.

I appreciate Graeme North¹s e-mail as well, and his advice to avoid
prescriptive language and use performance language.

Thanks.

Martin

On 8/25/10 7:54 PM, "Bob Theis" <bob at bobtheis.net> wrote:

> Hello All, 
> 
> Based on my experience with clay plasters on bale walls in northern
> California,  it seems like mandating protective overhangs  - specifically for
> clay plasters  - is a good idea.
> 
>  With a material as soft as clay plaster,  the standard notions of how much
> overhang is enough are, fundamentally, inadequate.  I've had earth plasters
> that,  in spite of sealers and rooflets on the gable, have had their finished
> surface on the lower wall eroded off in a few years' time, because I simply
> didn't know the wind-driven rain from that direction would be so strong.
> 
>  John Straube showed a combined wind and rain rose in a talk years ago,
> showing where the rain blows in from over the year.  I'm still wishing they
> were readily available. If they were, it would make decreeing what measures to
> use where a great deal simpler.  ( And I wouldn't have a plaster repair to
> schedule. )  
> 
> Worse,  a LOT of designers carry notions about overhangs not having that
> "edgy" look they cling to, and that hubris has no place with this material.
> 
> Since we are arguing for Codes that reflect long-term thinking, this is a good
> example of doing just that.
> 
> What should be mentioned in the standard is that armoring the lower wall with
> something more weather-resistant is an available option to reduce the height
> to overhang ratio. It's an approach I am increasingly using. Ground splash can
> do a lot to the walls,  even when the overhang is generous.
> 
> Bob
> 
> On Aug 25, 2010, at 5:49 AM, martin hammer wrote:
> 
>>  Hi Graeme,
>>  
>>  I¹ll send you the SB code off list.
>>  
>>  Re: the overhangs, what do you do with multi-story or tall walls?  Do you
>> have roof at typical first floor ceiling height in addition to the upper
>> roof?
>>  
>>  I will definitely consider putting such a table into the SB code.  I saw
>> Bruce¹s vote of agreement, and wonder if anyone else strongly agrees or
>> disagrees that overhangs be code-mandated for strawbale.
>>  
>>  Overhangs are generally a very good idea for SB, and I consider them ³good
>> practice² or ³best practice².  I don¹t know if they are at the level of
>> ³minimum practice², which is typically the threshold of code language,
>> although all of that is open to considerable debate.   Also in your table you
>> account for wind as a factor, but not rainfall.  If your table were applied
>> to desert climates, it might be unfairly restrictive (although some desert
>> climates receive concentrated periods of rain).
>>  
>>  In an earlier iteration of the code I prohibited strawbale parapets, but for
>> a few reasons decided to instead be silent on the issue.
>>  
>>  Martin
>>  
>>  
>>  On 8/24/10 4:11 PM, "Graeme North" <graeme at ecodesign.co.nz> wrote:
>>  
>>  
>>>  HI Martin
>>>  
>>>  firstly my congratulations -
>>>  
>>>  and yes, I would really appreciate a word document I can make comment on
>>>  
>>>>  >From my neck of the woods, one of the overriding issues I encounter time
>>>> and time again is that of good weather protection from wind driven rain in
>>>> our decidedly pluvian and humid climate - something that gets skittered
>>>> around in most books and references.
>>>  
>>>  I think we need a prescriptive starting point.  In the NZ Earth Building
>>> Standards NZS 4299 we relate wind zone. eaves height (vertical exposed wall
>>> height), and roof overhang width to give  MIMIMUM roof overhangs as follows
>>> - 
>>>  
>>>  
>>>  Wind Zone   - Ratio of eave height to width
>>>  Low  (at ULS 32m/s) 4:1  (600mm over a 2400 wall)
>>>  Medium (37m/s) 8:3
>>>  High (44 m/s) 3:2
>>>  Very High   (50 m/s) 1:1  (or in other words a full verandah)
>>>  
>>>     
>>>  We developed this table after leaks and some degradation of material in
>>> some earthen structures and I can report that there has been no reported
>>> problem since we adopted this.
>>>  
>>>  It is my contention that straw buildings are at least as moisture sensitive
>>> as earthen buildings and I would suggest that this sort of table be regarded
>>> a good starting point for calculating minimum roof overhangs for
>>> direct-plastered external strawbale walls, a recommendation that could
>>> possibly be worked back (or exceeded) after local weather or site
>>> assessment, or possibly a service history of locally developed techniques.
>>>  
>>>  It may seem a but draconian to some but for my money the biggest problem
>>> with strawbale buildings, in humid wet climates at least, seems to be that
>>> of providing adequate primary weather protection, in the form of eaves, or
>>> rain screening, and lack of good practical prescriptive guidance on this
>>> subject.  
>>>  
>>>  
>>>  Comments welcome
>>>  
>>>  
>>>  
>>>  
>>>   
>>>  Graeme (in bossy standards writing mode) North
>>>   
>>>  Graeme North Architects
>>>  49 Matthew Road
>>>  RD1
>>>  Warkworth
>>>  tel/fax +64 (0)9 4259305
>>>   
>>>  graeme at ecodesign.co.nz
>>>  www.ecodesign.co.nz <http://www.ecodesign.co.nz>
>>>  
>>>   
>>>  
>>>  On 23/08/2010, at 5:32 PM, martin hammer wrote:
> 
> 
> 
> _______________________________________________
> GSBN mailing list
> GSBN at greenbuilder.com
> http://greenbuilder.com/mailman/listinfo/GSBN

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.sustainablesources.com/pipermail/gsbn/attachments/20100825/1fcda62a/attachment.htm>