[Greenbuilding] glass panels as porch roof?

[LarenCorie]

From: "Michael O'Brien" <obrien at hevanet.com>

> I would like to encourage you to set the glass panels in
> gaskets or flexible blocks that would hold securely, yet
> allow the glass to expand and contract with temperature
> changes.

Hi Michael;

Your priorities are right on. However, in this case, there
is a simpler way to mount the glass that is easier, more
durable, and also a lot more water tight.  That is to lay
the glass on top of the rafters (ideally beams, so they
have adequate width for lap and movement) with a
stop, only at the bottom.    If there is foam insulation
available, about the thickness of the glass compressed
(or slightly thinner), cur it to bring the rest of the area
up to level with the plane of the face of the glass. Leave
a very small gap between the foam and glass edges.
Only minimal is needed, for movement, since the foam
will flex. Once the whole plane (roof around the glazing,
in this case) is level, cut strips of Ice & Water Shield, 
or equivalent, to cover all non-skylight areas. It should
overlap the glass as much as possible, without being
visible from below/inside (or just a nice straight edge,
barely visible).        Get a smooth piece of wood (or
comparable) and use it to burnish the I&W shield
where it bonds to the glass, to make sure that all
the air bubbles, and crease lines are rubbed out.
Battens can then be installed over the edges of
the glass (keep screws and nails away from the
glass edges). Wood battens tend to cup, so I
recommend batten material that will not, such
as fiber cement boards.

   Detailing for the lower end of the glass is significantly
trickier than described above. It is highly advantageous
to avoid creating a gutter that would accumulate leaves,
and other roof dirt/debris.    Ideally, you want the glass
to slightly overhang the wall, or most outward structure
(even a fairly conventional eave) so that water running
off it, will fall outside the structure.   I have often used
an aluminum angle with a short leg, about the same 
length as the thickness of the glass. No cap or batten
covers the glass along this edge. Just fill any gaps 
with a quality silicone sealant.     Water and debris
should freely slide off the edge of the glass roof.
I did quite a few of these, back in the early 1980s,
with design details to control overheating, and even
used it to heat the domestic hot water.       If it has
potential for Summer overheating, why not use that
heat?  Under such a roof can be a pretty good place
for a batch water heater. 

> With big panels it's a significant issue, because if the
> glass expands in a frame that can't flex and causes
> pressure on along its edges, it might fracture.

That is why it makes no sense to create rabbets to set
the glass down into a place where it could get pinched.

> If you look at a cross-section of a commercial glazing
> system you can see elastic gaskets between the glass
> and frame.

   Standard glazing tape does a very poor job at sealing
site-built glass roofs. The problem is that, because it only
seals the tiny distance between the glass face and a stop
or batten, it lacks the ability to bridge gaps that increase
by more than about 1/16", whereas the taping system
I described, can remain weatherproof, even if the
glass, were to moves an inch or more.

 -Laren Corie-
 Natural Solar Building Design and
 Solar Heating/Natural Cooling/Energy
 Efficiency Consultation Since 1975
 www.ThermalAttic.com  (many new
 photos and pages, coming soon)

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