Flat
roof types
Modified
Bitumen: When selecting a product for a new roof membrane
system, there are many products from which to choose. One of the
best products, for flat roofs, is a modified bitumen membrane.
Originally developed and installed in Europe, modified bitumen has
enjoyed widespread success. Modifieds are now one of the fastest
growing segments of the commercial roofing market.
The modified membranes are reinforced with one or more plies of
fiberglas, polyester, or fiberglass/polyester composite. These sheets
are encapsulated in polymer asphalt. The sheets are then either
surfaced with ceramic granules (just like asphalt shingles) or remain
unsurfaced. The result is a roof system that combines the benefits
of multi-layer built-up roofs (hot asphalt and fiberglass systems)
with the installation advantage of a single-ply membrane.
The two most proven and common types of modified bitumen membranes
are SBS and APP.
SBS (Styrene-butadiene-styrene) formulations perform
well in cold weather applications, possess superior elongation and
recovery properties, and are quite versatile in their application.
They can be installed using hot asphalt, torched, cold adhesives,
or self-adhered. Mineral granules are generally used in surfacing
this type of membrane, as resistance to ultraviolet light is somewhat
limited. The newest type is the self-adhered product. This product
looks to be the better application as it takes away the danger of
the roof being set on fire due to the torch. Many insurance companies
no longer will cover roofing contractors who use a torch on the
roof.
APP (Atactic polypropylene) products are more suited
to application in hot weather, are resistant to weathering and aging,
and are more resistant to ultraviolet radiation than are the SBS
membranes. The method of application is generally limited to torching.
These membranes are generally manufactured without a surfacing,
although granule surfaced products are available. We recommend that
protective coatings be installed and maintained on the unsurfaced
sheets. We do not recommend the use of this product anymore as the
use of a torch on a roof is a risk that is not worth taking for
most homeowners.
Neither APP or SBS membranes are well suited to application in ponded
conditions. Furthermore, the effectiveness of these membranes is
limited in recover situations where moisture in the original roof
system exists. For this reason, we recommend that roofs be torn
off before a new roof is installed.
Metal
roofs
The two main systems are named after the method of joining panels
together: "standing seam roofing" has a raised seam and
"batten roofing" employs a wider cover cap. The standing
seam we use is either “double locked” or we use a “snap
lock” system. The “double lock” is what is typically
seen on a lower sloped roof, such as those on flat roofs in the
District. The “snap lock” is installed on roofs that
have a steeper slope. The difference between the two is that the
“double locked” seam is much more water tight than the
snap locked seam. (see figure)
The forming of the standing seam can be a tedious process and the
quality can be vary. For this reason, we have purchased state of
the art computerized equipment that allows us to pre-form our panels
in our fabrication facilities. This equipment allows us to provide
a consistent quality and saves on the labor costs, which we can
pass on to the homeowner.
Several types of metal are used for roofing: steel, stainless steel,
aluminum, copper and zinc alloys.
Steel, which is heavier than aluminum, is also much more difficult
to form than aluminum. This can result in an increase in labor costs,
depending upon the roof application. The steel sheets come with
a fluorocarbon coating called Kynar. These coatings come in a myriad
of colors.
Stainless steel is a very expensive roofing material, however, it
won't rust or corrode.
Terne coated steel is very common in the District.
This coating is easy to solder and has excellent paint adhesion.
This finish - a mixture of 80% lead and 20% tin--must be painted
after installation.
Aluminum, extremely lightweight, is the material
used most commonly in residential metal roofing. It won't rust,
but must be painted or coated for appearance. Coatings are similar
to those used on steel. Since they are factory applied, maintenance
is minimal. The lightweight makes it easy to work with and form
in common applications.
Copper, rooted in centuries of use, will not rust,
has no "finish" to scratch or peel, is soft enough to
easily tool, and weathers naturally to a beautiful verdigris patina.
Unfortunately, it's very expensive. However, when you factor the
cost of painting over the years, it may just be less expensive than
terne coated steel.
Properly installed, a metal roof should last as long as the house,
completely sealing out water, surviving high winds and easily shedding
snow. Metal is resistant to fire, mildew, insects and rot. We are
so convinced of the superiority of our metal roofing systems, we
back up our products with a lifetime limited warranty and will even
transfer the warranty to one future owner.
EPDM
– a synthetic rubber ( comprised of ethylene, propylene and
a diene) material, is installed in overlapping sheets to form a
membrane on top of a roof. The sheets are available in 45 mil (thickness
of a dime) or 60 mil (thickness of a quarter). The sheets are sold
in solid sheets up to 50’ wide and 100’ long. This makes
this product good for very large roofs, but no so good for small
roofs (such as in Washington DC) because of all of the penetrations.
The sheets are bonded together with solvent-based adhesives formulated
with volatile organic compounds. There are now solvent-free, pre-formed
tapes that are available. The current trend is to use these tapes
as there have been concerns over the effect of VOCs on the environment.
One of the major problems associated with EPDM is a defect known
as “shrinkage”. This is not to be confused with problems
resulting from expansion and contraction of the membrane. The material
can actually shrink, most notably around wall flashings. This is
a major problem especially on homes in Washington DC, where most
leaks occur along the wall flashings.
Thermoplastic - The most common thermoplastic roof
membranes are PVC and TPO. Vinyl is the shortened term for “polyvinyl
chloride.” Vinyl, or PVC as it is more commonly referred,
is a molecule comprised of carbon, hydrogen and chlorine (taken
from salt). Vinyl membranes for roof applications were first developed
in Germany and then arrived in North America. The performance history
of TPOs in this country is not quite as long, although at least
one TPO has been on the market for almost 10 years. However, it
is still a fact that all TPOs are not created equal. Recent research
by the National Roofing Contractors Association (NRCA) found that
there are significant differences between TPO membranes currently
being sold in the U.S. The effects of some of these differences
are still unknown at this time. NRCA believes other differences
will have notable effects on the usability and long-term field performance
of TPO membranes. Since these types of roofs have either had a history
of problems or not enough of a track record, we are closely watching
the performance of these products before we install them.
Built-Up Roofs are roofs consisting of multiple
plies of roof felts laminated together with bitumen. Built-up roof
material usually consists of bitumen-saturated fiberglass felt.
A surfacing is generally applied and can be asphalt, aggregate (gravel
or slag), emulsion or a granule-surfaced cap sheet. While this is
often regarded as one of the best roofing systems for a flat roof,
the installation cost is high due to the danger of the material.
While we still offer built-up roofs, we do not usually recommend
them for residential properties in the District as the access to
the roofs make it a difficult and expensive option.
Specialty
Roofs
Turrets – A turret
is a small tower that projects from the wall of a building such
as a castle. A turret projects from the edge of the building, rather
than continuing to the ground. In Washington, we usually see either
round or square shaped turrets. The square shaped turrets are easier
to roof and can be roofed with a wider option of materials. The
round turrets usually only look good if they are roofed with natural
slate.
Built-in gutters –These are usually found
(on the homes in Washington) along the front of the mansard roof.
They are usually located directly below the turrets as well. Built-in
gutters are very popular among architects. Properly installed, built-in
gutters can provide a lifetime of trouble-free service. However,
built-in gutters expose a building to higher risk of water intrusion
than any exposed-type gutter. When leaks occur in built-in gutters
the water is likely to intrude directly into the building. Because
out of sight usually translates to “out of mind”, we
only install built-in gutters using materials that we can solder.
We also try to use only materials that do not need maintenance such
as copper or (preferably) lead-coated copper. Downspouts should
be located in a manner that minimizes or eliminates the need for
expansion joints. Continuous cleat and drip edge are recommended
along the front, while water proof membranes should extend at least
24 inches behind wall lines in ice dam areas.
Slate
– Slate is one of the most aesthetically pleasing and durable
of all roofing materials. Installed properly, slate roofs require
little maintenance and will last 60 to 125 years or longer. Some
slates have been known to last over 200 years. Found on virtually
every type of building, slate roofs are perhaps most often associated
with institutional and government buildings, where longevity is
an especially important consideration in material choices. Slate
is available in a variety of colors. The most common are grey, bluegrey,
black, various shades of green, deep purple, brick red, and mottled
varieties. Generally, the slates of Maine, Virginia, and the Peach
Bottom district of York County, Pennsylvania are deep blueblack
in color. The slates of Lehigh and Northampton Counties, Pennsylvania,
are grayish-black in color. Green, red, purple, and mottled slates
come from the New York-Vermont district. The slate producing region
of New York, which centers around Granville and Middle Granville,
is particularly important because it contains one of the few commercial
deposits of red slate in the world.
Slates are also classified as fading or unfading. Fading slates
change to new shades or may streak within a short time after exposure
to the atmosphere. For example, the "weathering green"
or "sea green" slates of New York and Vermont are grayish
green when freshly quarried. Upon exposure, from 20% to 60% of the
slates typically weather to soft tones of orange-brown, buff, and
gray while the others retain their original shade.
Slates designated as unfading maintain their original colors for
many years.
The fact that the slates are fading or unfading provides no indication
of the durability of slate. Rather, time has shown that the Vermont
and New York slates will last about 125 years; Buckingham Virginia
slates 175 years or more; and Pennsylvania SoftVein slates in excess
of 60 years; Pennsylvania HardVein slates and Peach Bottom slates,
neither of which is still quarried, had life spans of roughly 100
and at least 200 years respectively. The life spans provided should
be used only as a general guide in determining whether or not an
existing slate roof is nearing the end of its serviceable life.
In recent years, slates from China, Africa, Spain and other countries
have begun to be imported into the United States. The use of imported
slates should probably be limited to new construction since their
colors and textures often do not match those of U.S. slate.
Clay tile – The clay tiles that are prevalent
in this area are the flat shingle tile, the Spanish Tile, and Mission
Tile.
The flat shingle tile replicates the materials used on medieval
European and early American homes, ranging from rugged tree bark,
rustic natural stone, to the highly sophisticated polished surfaces
of quarry tile.
Spanish Tile, one of the most popular field tile,
features a simple one-piece barrel design that is classic yet aesthetically
versatile. The origin of this tile goes back to the pan and cover
barrel tiles found on European buildings and old western missions.
Mission Tiles are a two-piece tile comprised of
curved pans and covers installed at regular or random exposures
for design creativity.
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