[GSBN] SB Overhangs

Thu Aug 26 05:37:22 UTC 2010

Dear all

Thanks for the discussion.

There are many things to consider, but it seems to me that moisture  
vulnerable plasters such as earthen ones must have good overhangs for  
durability reasons.

The external water penetration issue needs to be carefully separated  
from issues of plaster durability - this point is very important.

The NZ Building Code puts these in quite different sections and the  
table I have given you deals with external moisture and always, in  
practice,  over-rides any requirement for any overhangs to meet  
durability requirements of earthen materials.

Leaks occur at joinery openings, cracks,  and many other  
penetrations, so I agree with Martin and would argue  for good  
overhangs as a sound building design strategy in all strawbale  
buildings that are direct plastered, and yes in a prescriptive way   
as a means of meeting performance requirements.
Then flashing details, and other details, joinery details, drips and  
sills, etc along with plaster mixes need to be devised that will cope  
with what (much smaller and less energetic)  amounts of moisture  
still reach the walls.

The performance criteria is simple - keep the straw dry.
A standard is helpful if it gives guidance as to how this can be  
achieved.

Eaves widths may be adjusted to suit  site-specific parameters such  
as rainfall, humidity, incidence of wind-driven rain, and local  
sheltering effects.  Personally I prefer to sleep at night  and tend  
towards the bigger the better when it comes to eaves.  Conservative  
maybe, but I don't apologize for that in this context.

(My conservatism in this instance can not and must not in any way be  
construed to think that I would have voted for Bush, even if I could  
have, and even if hell had frozen over.   No no no!)

The table I have suggested works really well in the sub-tropical high- 
humidity wind-driven-rainy (1.5 - 2m p/a) sort of mess we live in.

It may prove a good starting point for strawbale eaves that other  
climatic areas can work from.

Even so I would tend to suggest an absolute minimum roof overhang to  
exposed wall height of 1:4 or 600 mm (2 ft) over a 2.4 m (8ft) high  
wall, or 900 (3ft )over a 3.6 m (12ft) high wall - including at gable  
ends.

Two storey walls may need an intermediate skirt roof as per the  
Japanese example given.

Ground clearance to prevent splash up also needs to be dealt with -  
good boots and a good hat never went astray.

Enough for tonight

Graeme
Graeme North Architects
49 Matthew Road
RD1
Warkworth
tel/fax +64 (0)9 4259305

graeme at ecodesign.co.nz
www.ecodesign.co.nz

On 26/08/2010, at 4:42 PM, martin hammer wrote:

> 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 North’s 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,
>>>
>>>  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 <http://www.ecodesign.co.nz>
>>>>
>>>>
>>>>
>>>>  On 23/08/2010, at 5:32 PM, martin hammer wrote:
>>
>>
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