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

[Chris Mowatt]

Rikki said:
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."

Derek said: "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."

I say:
For a conventional duo-pitched gable roof, I believe this is only possible
if the slope or pitch of the roof is less than 12:12 (45 degrees). For roof
pitches greater than this there will be a downward force on both roof
pitches. Problems only really start with a roof pitch lower than 9:12 (37
degrees), where an uplift or suction force will be exerted on both windward
and leeward roof pitches. The relevant wind pressure coefficients are:
  Gabled Frames (V:H)
  Roof Slope           Windward Side  Leeward Side
   <9:12  (37 degrees)     -7              -7
   9:12 to 12:12           +4              -7
   >12:12 (45 degrees)     +7              +7
                                  (Source:UBC 1995)

It is possible for a roof with a pitch in the range 9:12 to 12:12 to
generate a net uplift force greater than the downward forces made up of any
dead loads plus any other live loads, but this depends on the level of
exposure and the velocity of the wind. If a site is protected by trees,
neighbouring buildings or the geography of the land then the velocity of the
wind may never approach the levels required to produce sufficient uplift to
counter the downward loads. If the site is exposed and or elevated an
engineer should be employed to calculate the roof tie-down requirements
necessary to combat any theoretical roof induced uplift forces.

Perhaps the buildings Rikki saw in Spain had roof pitches that were 12:12 or
above, or were protected from wind, or the roofs were of sufficiently
massive construction for their weight to more than match any possible uplift
force (although this sounds unlikely since Rikki describes the two-storey
one as light-weight) or maybe they do not experience strong winds in these
locales.

As Derek says, "the question is whether the system is sufficient for the
building design and local
conditions."

Also, the roof structure has to be sufficiently strong and integrated to
stay in one piece when subjected to unusually fierce winds. Localised forces
can be sufficiently strong to break the fixings of some part of a roof and
change the aerodynamics dramatically. On 28 January 2002, I watched as 100
mph winds destroyed our residential caravan (trailer to most of you). First
it lifted a small area of roof on the windward edge. In successive gusts the
wind peeled this back and lifted it so as to act as a lever on the main roof
structure. The roof structure started to lift as a whole at which point it
became like a sail and the next gust turned the whole caravan/trailer upside
down.

However, I'm not sure I agree with Derek when he says, "reliable roof
attachment is always important". I think it is only important where your
chosen roof design and local conditions make it necessary. Most bale
compression systems do a good job of holding down the roof, but I doubt this
is a necessary function in all cases.

One analysis I've seen (I forget where), advocated a floating roof design,
where the free-standing roof structure was slackly tethered to the building
structure. The idea being that most of energy in a gust of wind can be
absorbed in the lifting effort required to lift the weight of the roof
structure so the fixings for the tethers do not need to be as strong as if
the roof was rigidly fixed to the building - where they would have to resist
the full uplift force. I believe the amount of movement allowed was very
small.

I'm also not sure that I agree when Derek writes, "Earthen plaster by itself
is not sufficient for holding the roof down". Firstly, as outlined above, it
may not be necessary to hold the roof down. Secondly, it is the method of
attachment that determines whether earthen plaster will be sufficient.
Earthen plaster is a brittle material that has low tensile strength and so
is not good at resisting axial tensile forces, such as the pull exerted on
fixings connected to a roof structure being subjected to wind-induced lift
and or shear. On the other hand, earthen plaster in a strawbale wall is a
homogenous material reinforced with cellulose fibres. Any tensile force
exerted on an earthen plaster will be evenly dissipated to the rest of the
wall system. The trick is to ensure the earthen plaster is reinforced with
sufficient tensile material at the point of attachment. Even better is to
ensure that there are no specific points of attachment, for example by
having a continuous webbing or mesh provide the attachment between the roof
structure and the earthen render wall system, such that there are no
localised weak points.

I believe that the only purpose pre-compression of bales serves is to make
the wall stable enough to render/plaster/stucco. If this can be achieved by
a plaster-as-you-go approach then great. Tom Rijven´s system of
bale-dippìng, Bob Merril's matrix method or Rikki's Spanish variant all
sound like practical ways of achieving a wall stable enough to
plaster/render/stucco without the rigmarole of bale pre-compression. If a
roof needs tie-downs then these can easily be added later, as a mesh, web or
cable, perhaps embedded in the final coat of plaster/render/stucco.

Just because pre-compression systems can provide a roof tie-down method
doesn't mean they should or that use of one justifies the other.


Chris Mowatt