[GSBN] SB Overhangs

[Bob Theis]

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,
>
> I’ll 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 Bruce’s 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 “good practice” or “best practice”.  I don’t know if they are  
> at the level of “minimum practice”, 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
>>
>>
>>
>> On 23/08/2010, at 5:32 PM, martin hammer wrote:

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