TWO
TYPES
OF BITUMEN |
The two generic types of bitumen
used in the roofing industry are asphalt
and coal-tar. Asphalt bitumen is derived
from distillation of crude oil and begins
as the oxidized residue from the bottom
of the petroleum cracking unit. Coal-tar
bitumen is a by-product of the production
of coke from coal.
|
| WELDING
AGENT |
Many believe that roofing bitumens
are the glue which cements the roofing
felts together. This is only partially
correct. What actually occurs is that
the heated bitumen melts and fuses with
the saturated bitumen in the roofing
felts. This creates a permanent weld
of each ply of roofing felt, assuming
of course, proper bitumen temperature
has been achieved. By understanding
this function, you can readily see how
wet or dirty felts result in separation,
delamination and blistering, and the
importance of this step in built-up
roofing. Additionally, sufficient application
temperature is critical to effecting
a permanent weld of each ply.
|
| WATERPROOFING |
The oils in both asphalt and coal-tar
bitumen provide the waterproofing of
the completed built-up roof assembly.
Note:
Per square foot, a five-ply built-up
roof assembly contains a higher percentage
of oils in its thickness than does
a three-ply built-up roof assembly.
Since oils dictate the service life
of a built-up roof, the three-ply
built-up roof assembly has a relatively
short service life of seven to ten
years before replacement.
|
| CONSIDERATIONS |
A primary consideration in a new built-up
roof application is the type of bitumen
to use. This decision should be based
upon desired durability, the amount
of roof slope, adverse fallout and comparative
costs.
|
ASPHALT
BITUMEN |
Type III asphalt is the most commonly
used roofing bitumen and is generally
the most economical. Type I asphalt
is referred to in the industry as dead
level and the applied cost can run from
5-10% above Type III. Note:
Asphalt bitumen cannot accept ponding
water and will rapidly deteriorate
under ponding water conditions. The
water molecules attempt to float the
lighter asphalt molecules.
|
COAL-TAR
PITCH
AND BITUMEN |
Coal-tar pitch bitumen is probably
the most desirable roofing bitumen for
use on a flat or almost flat roof, due
to superior waterproofing and weathering
characteristics. However, you can expect
the cost a tar built-up roof to range
from 10-20% higher than a comparable
asphalt built-up system. Coal-tar pitch
bitumen is unaffected by ponding water
and generally has a service life of
15 to 20 years. It is ill advised to
use ASTM Type III coal-tar bitumen based
upon numerous failures. While coal-tar
bitumen was less carcinogenic than coal-tar
pitch and produced less odor, in many
applications it became prematurely brittle
and caused failure of the roofing system.
|
ROOF
SLOPE AND
SOFTENING POINT |
The amount of slope in a roof deck
is important due to the softening point
of the various bitumen types and their
resultant flow characteristics.
|
RECOMMENDED
SLOPE |
The following chart outlines the bitumens
in use today and the range of softening
points versus slope:
| BITUMEN |
SOFTENING
POINT |
RECOMMENDED
MAXIMUM SLOPE |
| Asphalt Type I |
135-151 |
Up to 1/2" per foot |
| Asphalt Type II |
158-176 |
1/2" to 1-1/2" per
foot |
| Asphalt Type III |
185-205 |
1" to 3" per foot |
| Coal-tar Bitumen |
133-147 |
Up to 1/2" per foot |
|
EQUIVISCOUS
TEMPERATURE
(EVT) |
All roofing bitumens have a specific
temperature at which they achieve the
optimum viscosity for mopping onto the
roof. This temperature is called the
equiviscous temperature (EVT). Asphalt
bitumen manufacturers are currently
printing the EVT on their containers.
The practical working range for all
bitumen types is EVT ± 25°
F.
|
MAXIMUM
HEATING
TEMPERATURE
FOR BITUMENS |
Type I
Dead Level 475°
F, Maximum
Asphalt
Type II
Flat 500°
F, Maximum
Asphalt
Type III
Steep 525°
F, Maximum
Asphalt
Type IV
Extra Steep 525°
F, Maximum
Asphalt
Type I & III
Coal- tar 425°
F, Maximum
Bitumen
|
DO
I HAVE
A PROBLEM? |
When inspecting your built-up roof,
keep the following in mind:
- The age and number of plies have a direct correlation to service life. Generally only by examining a core from the roof can continued service life be accurately predicted.
- If your built-up roof is asphalt bitumen, closely examine the flood coat in the areas of ponding water. Damaging ultraviolet deterioration of asphalt is accelerated in ponding conditions.
Contact us to discuss your findings,
or should you desire, we can stop by
and give you a professional evaluation
after inspecting your roof.
|
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