[Strawbale]Re: holding down the roof (was: Look Ma', no hands!)

[Derek Roff]

In some cases, a gale-force wind (>55 kph/34 mph) can apply more lift to 
the roof of a building than the combined downward forces of both live and 
dead loads for that same roof.  In other words, winds can put more force 
into trying to lift a roof off, than the weight of the roof and its 
heaviest snow load.  In these winds (especially when there is no live 
load), the roof will fly away, unless restrained by a tie-down system of 
some sort.

And there always is a tie-down system of some sort.  The question is 
whether the system is sufficient for the building design and local 
conditions.  When owner-builders start getting creative, and leaving out 
traditional building components and strategies, they vastly increase the 
risk of failure due to unexpected (by them) factors.  Gale force winds 
(less than half the velocity of hurricane force winds) occur fairly 
frequently in almost every location on earth.

Many owner-builders are unaware of this risk, and most people severely 
underestimate the magnitude of the wind forces on the roof.  Obviously, 
there are many variables in local conditions and building details. 
However, reliable roof attachment is always important.  Common construction 
techniques can provide the necessary attachment, while obscuring the 
function.  For example, most bale compression systems used by 
owner-builders also do a good job of holding down the roof.  However, the 
latter function is often ignored or underrated.

I am curious about the roof attachment for the buildings described in 
Rikki's message.  It may be that there is some roof tie-down system that is 
not obvious.  Or perhaps the buildings are at risk.  Earthen plaster by 
itself is not sufficient for holding the roof down.

Derek

(PS- Please forward my comments to the European Strawbale list.  Since I am 
not a member, I doubt that my posting will go through.)

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...

> From: "rikki nitzkin" <rnitzkin at hotmail...>
> Reply-To: strawbale at amper....muni.cz
> Date: Wed, 30 Mar 2005 11:43:14 +0000
> To: strawbale at amper....muni.cz
> Subject: [Strawbale]part 2: Look Ma', no hands!
>
> I always thought that to build a solid, load-bearing SB house it was
> necessary to include a tie-down system and a roof-plate.
>
> On a recent trip investigating SB houses in Spain, I found out that this 
is
> not true.
>
> I saw eight SB structures with know tie-down system, five of which had no
> roof-plate, and all were perfectly solid.  The oldest of these houses is 4
> years old, and hasn´t moved at all.  The biggest is two stories high (29m2
> per floor).
>
> This two-story house was built using a unique technique: the man plastered
> each course of bales as it went up.  The first three courses could be laid
> and plastered at once, then about one course a day, so that the straw/clay
> plaster had time to set.  He built the whole house, alone, in less than 
two
> months.  The walls have not compressed at all.  The beams for the second
> story are placed over very small (2x2) strips of wood laid on top of the
> bales--no tie-down. The roof beams the same, just plastered all around 
with
> a heavy straw/clay plaster.  The roof is very light-weight.
>
> This reminds me of Tom Rijven´s system of bale-dippìng, but one step
> farther.
>
> Does anyone know of any other homes (load-bearing) built without tie-down
> systems or roof-plates?
> How have they held up?  If it really works (in these houses it seems to) 
it
> could save a lot of time and money in building . . .
>
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