As with the rest of our healthy house, no wood was used in
the roof framing, or in the roof itself. Normally, a roof is built
up in a layered fashion. The bottom layer is usually plywood that
has been nailed to the underlying structure. Then comes a layer
of felt. This is often coated with tar or some other petroleum-based
product. It is this layer that normally makes the roof waterproof.
On top of the tar comes wood shakes, tile, or even metal roofing
like what we used.
The material I selected is a standing seam metal roof. Although
the manufacturer certifies it for use without a plywood underlayment
and without a felt waterproofing layer, it is not often installed
that way. However, we insisted, and the installer complied. We
were also able to keep the warranty intact.
Without a felt layer, we had to make sure that all seams were
carefully caulked. The installer originally proposed a very toxic
flexible adhesive compound that we ended up rejecting. Instead,
we used a white 100% silicone that dried with a very low odor.
It had a much stronger adhesive capability than another clear
100% silicone material we were also using. The roof is attached
to the top ridge, and is held down with clips along its length.
The end near the eaves is caulked, but not solidly attached. This
allows the roofing to expand and contract as it heats up and cools
down, without buckling or warping.
Because the roof was much more complex (and more attractive)
than most commercial buildings, which is where most metal roofing
is used in California, again some special techniques had to be
developed during the installation process. Where the roof hits
the light tower was one problem area, because of the complex angles
and junctions involved.
We also had to have some hat channel material fabricated for
the roof. The hat channel was used as purlins that ran across
the rafters. The clips that hold the roofing down are attached
to the purlins.
Without the felt layer, we decided to take some extra precautions
to make sure that condensation would not develop on the under
side of the roof. We used eight large attic vents, placed up near
the ridge on the back side of the house so that they aren't visible
from the street in front (see the photo below). There is also
a large, fully vented soffit area. So air is able to move from
under the eaves, through the attic, and out the vents. The purlins
also provide about a half inch space through which air can move
from one rafter bay to another. Lots of air movement will help
ensure that no condensation develops on the underside of the roof.
In a normal roof, this condensation wouldn't be a problem, because
it would just run down the surface of the felt. It our case, it
could drip and eventually cause mold problems.
Besides the vents, roof jacks were also installed for the plumbing
vents and the vents for the dryer, the range hood and the ventilation
system. A vent was installed in the garage for a 500 cfm fan.
The fan is attached to a timer, so that after we park the car,
the timer can be activated to pull some of the toxic fumes out
of the garage.
We were told that the roof would take a month to install. It
ended up taking three months.
You might be interested in the following book on roof
framing from Amazon.com: