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



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@...

From: "rikki nitzkin" rnitzkin@...
Reply-To: strawbale@...
Date: Wed, 30 Mar 2005 11:43:14 +0000
To: strawbale@...
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-dipp?ng, 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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