--- Ken Follett <[log in to unmask]> wrote:

> > the sealant will trap any water which gets in
>
> It is my understanding and observation, or possibly
> simply a dumb confusion,
> that thin section stone used in modern cladding in
> some cases will be
> suspended in such a manner that a joint filler
> between the stones is pretty
> much irrelevant and that water that penetrates into
> the structure behind the
> stone cladding will meet an intended impermiable
> barrier, if one has been
> installed, and that the water will run down, via
> gravity, the facade at a
> line interior to the stone cladding.

CORRECT!

These days, thin stone cladding is rarely constructed
in a manner where the stone is stacked so that dead
load is transferred down through the stone (like brick
masonry).  Instead, each panel is independently
supported by mechanical anchors, so that differential
movement between the stone panel and the backup
structure can occur.  There have been many (and
continue to be many) failures of stone cladding on
high-rise (and not-so-high rise) buildings where
insufficient (or no) movement capacity has been
provided and consequently the stone becomes deciduous
(falls, not just in Fall).

In this situation, mortar will crack and fall out very
quickly (and doesn't allow the movement anyway).
Therefore, must use flexible material like sealant.

Typically, many designers assume that the stone is
waterproof and the sealant will provide a face seal.
Bad decision.  Personally, I like to specify a
waterproof barrier behind the stone, and ideally,
leave the joints open so that water can drain through
the cavity.  This does mean however that you have to
be very careful to avoid rubbish in the cavity, and to
ensure that water passing over concrete does not pick
up soluble material and deposit it on the soffits or
the glass of the windows below.

It is also going to make conservation of late 20thC
buildings really interesting!!  Can you imagine trying
to get authentic 1990s composition silicone or
polyurethane sealants manufactured in the year 2090
when nobody remembers the composition, let alone has
access to appropriate manufacturing equipment????

> On Michael's column project I've not noticed
> discussion here as to thermal
> movement of the stone in repsect of the desired
> durability characteristics of
> a mortar. Massive stone becomes a thermal sink
> (moreso than with a thin
> veneer) and as the sun passes a column there are
> daily cycles of stresses in
> thermal movement of the stone. In effect I think
> that a mortar would
> experience a grinding action.

Hence my suggestion earlier that the outer part of the
joint should have something flexible.  I really like
the suggestion by somebody to use a lead bedding.
That sounds amazingly neat.  Problem might be water
migration and consequent staining.  Might not be an
issue.

I also like Michael's proposed solution ... setting
mortar, putty rope as green seal, and pointing mortar
... question is what that pointing mortar should be so
that it doesn't fall out!

Cheers

david

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