[Strawbale] Parapetted SB walls (was Re: 5 perms / no ventilation; and top-of-wall )

[Rob Tom]

On Fri, 16 Mar 2007 15:40:19 -0400, David Neeley <dbneeley at gmail.com>  
wrote:

> A few years ago, a discussion was held about a Canadian house with such  
> a wall
> --and, as might be expected, it developed a severe leak problem at the  
> base of theparapets...aided, as I recall, by the use of cement plaster  
> so much in vogue in Canada at the time.

Thanks for that David.

I hadn't heard about the leaks that developed in the Canadian parapetted  
SB house but my guess is that it's the house in Oakville Ontario circa  
2005 ?  (There's a copy of a newspaper article on that building in the  
SB-r-Us FILES section:

                        http://shorterlink.com/?3XSD4A     )

If it is that house, and if the leaks are in fact at the base of the  
parapets then I'm not so sure that the leaks had anything to do with the  
choice of plaster. (ie Lime or earthen wouldn't have changed the result).

All I know of the Oakville SB house is what is in the newspaper piece  
(above) and I do remember noticing what appeared to be rain leaders (aka  
"downspouts") emerging from the soffits (ie the undersides of the eaves  
overhangs) adjacent to the walls and thinking to myself "Uh oh, that looks  
like trouble".

It is possible to make parapetted walls here in Canada and not have leaks  
at the parapet/roof junction.

The problem with parapetted _SB_ walls is that parapetted designs don't  
typically provide any sort of overhang protection to keep the plaster dry  
(and SB beaneath the plaster), a problem which it appears the designer(s?)  
(and builders?) of the Oakville SB house tried to address by providing  
some nominal overhangs.

But if the pipes in the photo are in fact rain leaders, their presence at  
the eaves would seem to indicate that the roof was sloped towards the  
parapets (otherwise the rain leaders wouldn't be doing anything) so that  
any accumulations of water (and re-frozen water as a result of snow/ice  
melt) would be directly over the SB walls.

That in itself may not be a major problem if the base of the parapets were  
flashed properly and the rain leaders do the job they were intended to do  
but in Ontario Canada in winter, exposed rain leaders tend to become a  
solid icicle from the roof to the ground (ie no flow of water) and quite  
often burst apart as a result of the freezing.

That's the reason why parapetted roofs here in Canada are usually designed  
to slope to drain to somewhere at the middle of the roof and the rain  
leader is inside of the building where it can be kept warm so that it  
remains operational even in winter.

In the desert SW where the "Santa Fe " style of SBH originated, I think  
that the roofs are often drained by scuppers that penetrate the parapet  
above an exterior wall. Not an ideal condition but workable I suppose for  
that climate.

The other thing about parapets in Ontario is that they can create a still  
zone adjacent to the parapets where snow/ice can accumulate in the shade  
and if the parapets are at the extremities of an overhang (as with the  
Oakville SB house), then there'd be a condition that is identical to the  
situation that results in ice dams, ice dams being the cause of major roof  
leaks, even on roofs with significant slopes.

If the leaky parapetted SB house was not the Oakville house, then pretend  
I didn't write any of the above.

=== * ===
Rob Tom
Kanata, Ontario, Canada
<A r c h i L o g i c   at  c h a f f y a h o o   dot   c a >
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