Consistently changing systems and new materials
will be discussed and introduced. Problems
and solutions will be looked at, and cost
feasibility studies will be outlined and
presented to you in future NEWSLETTERS.
To begin, let's look at and understand
the often specified ROOF SYSTEM known as
the BUILT-UP ROOF...
UNDERSTANDING YOUR
BUILT-UP ROOF
To properly maintain your built-up roof,
first an understanding of how your built-up
roof was originally designed must be accomplished.
Several basic material and structural component
design changes have taken place since the
origin of built-up roofing.
BUILDING
STRUCTURE |
First, construction design has changed
dramatically; rather than solid "Rock
of Gibralter" strength, we have
a "lightweight" philosophy
- just as strong perhaps, but built
with a design flexibility factor.
The roof actually moves on the supports;
very seldom today do you find poured
reinforced concrete, rather light 22,
24, or 26-gauge steel decks, lightweight
concrete, gypsum and in some instances
plywood. Bays are wider, introducing
more stress to the roof system. When
we look at square footage, 15,000 to
30,000 square feet were the sizes of
the "Rock of Gibralter" design,
not the 100,000 to 300,000 square feet
as we find today.
|
INSULATION
AND ITS
EFFECTS |
Second, by adding insulation we have
decreased the heat flow through to the
roofing system, causing a greater thermal
shock to the bitumen. This thermal shock
induces considerable movement of the
roof mat, and if you've allowed the
mat to become brittle and inflexible,
it cannot take such movement and invariably
will crack. Let's look at an actual
example of thermal shock on an insulated
roof with the ambient air temperature
at 100° F and the surface temperature
of the black roof mat at 162° F.
After a rainstorm the surface temperature
dropped 72° F within two hours.
After the rain stopped and the sun reappeared,
within one hour the surface temperature
quickly rose to 155° F.
|
QUALITY
OF
TODAY'S FELTS |
Third, with the advent of fiberglass
felts, the traditional organic felts
are seldom used. Fiberglass felts offer
higher tensile strength than traditional
five-ply organic roofs of yesterday.
Felts themselves are NOT waterproof,
it is the bitumen between each ply that
provides the waterproofing and by reducing
the number of plies by 40% (5-ply down
to 3-ply), we have reduced the amount
of bitumen by 40% as well. Felts are
the tensile strength of the roof mat,
and add nothing to the waterproofing.
|
| BITUMEN |
As pointed out above, today's specifications
call for less bitumen, up to 40% less
in most cases, as compared to specifications
of 20 years ago.
Fourth, the petroleum industry, due
to the rising prices of heating oil
and demands of the petro chemical industry
for more petroleum by-products, has
ever increasingly left less and less
residual waterproofing oil in today's
bitumen.
|
| DESIGN |
Design of the roofing system is critical.
As mentioned above, built-up roofs used
to have five layers of 15# felt and
bitumen in addition to the top pour
of bitumen known as the flood coat.
This gave a total weight of felt of
75 pounds (15 pounds times five layers).
Today, in order to achieve the same
weight of felts, one 15# layer and two
30# layers are applied. However, even
though there is the same weight of felts
for strength, two layers of bitumen
moppings are omitted which means there
is not as much waterproofing coverage
as was found when laying five layers
of felts. In addition, asphalt bitumen
is being used in areas of no slope to
low slope where coal-tar pitch should
be used. Unlike asphalt, coal-tar pitch
is unaffected by ponding water. |
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