Cob: thatch

[Shannon C. Dealy]

On Wed, 12 Jan 2000, Ted Schluenderfritz wrote:

> Hullo,
> 
> another question. I'm debating whether to use thatch or not and would like some
> opinions...
> 
> For a 1000 sq ft building what am I likely to pay if I hire it done?

I don't know exactly, but the numbers I have generally heard put the price
of a good quality thatch job at around two to three times the price of
a standard asphalt shingle roof, which actually makes it about the same
price, because a good thatch job should last at least two to three times
as long.

> is it possible to learn enough at a week long workshop to thatch my roof without
> being fearful that my loft office will be flooded, infested and burnt to a
> crisp.

Not sure, the workshop I took was two weeks long (I haven't seen any one
week workshops), though I feel quite comfortable that I can do an
"adequate" job with my current skills.  By adequate, I mean that the roof
will hold together and not leak.  I have no illusions that it will look
as good as a professional job, or last as long, quality work requires
experience, and I don't have enough hands on experience at this time.

> I've read that one can fireproof thatch....is this using toxic chemicals?

The only "fireproofing" treatments that I have heard of for thatch use 
a salt compound (I don't remember the particular salts) of one flavor or
another.  These compounds tend to hold moisture in contact with the
thatch long after it rains, causing rot and greatly accelerating the
deterioration of the roof, reducing their life expectancy to something 
like five to ten years.  The best approach approach that I have heard of
is to block air flow from the interior of the house to the underside of
the thatch.  This gives a roof that is roughly comparable to cedar shakes
for fire resistance.  Flemming Abrahamsson (master thatcher from Denmark)
in one technique uses the rounded edges of logs as an integral part of his
roofs (what's left after the log has been cut into boards) to create a
barrier to air flow, with cob used to fill the gaps.  These wood "scraps"
also provide the attachment point for wires he uses in his modern
thatching technique.  Blocking the interior air flow is the most important
thing, because if you don't the whole house acts as a chimney, feeding air
to the burning roof.  With the interior air blocked, the tightly packed
thatch is somewhat like a tight straw bale, and will tend to slowly
smolder rather than burn.

> what about pests? I like animals well enough but I don't want them living in my
> roof.

Insects, birds, etc. can be something of a problem, depending on where you
live, but they tend to use it as a home, since the thatch does not have
any food value.  How you deal with them depends on how much it bothers
you, and how much of a problem they cause with the roof.

Shannon C. Dealy      |               DeaTech Research Inc.
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