DECK -
INSULATION - FELTS - BITUMEN - DESIGN
Previously we discussed the five MAJOR material
and structural component design changes that
have occurred since the origin of built-up
roofing.
| ROOF
DECK |
Wood tongue-and-groove and 5"
to 6" thick structural poured-in-place
concrete were the traditional substrates
that the built-up roofing was originally
designed for. Neither of these two decks
required insulation for the built-up
roof to perform. Both were rigid-type
construction, and deck movement was
not considered in the design. Typically
four types of decks are commonly used
in the design of new facilities today.
These are as follows:
- Steel decks of 22- and 24-gauge are supported by steel bar joists that are sloped to drain.
Metal/steel decks require insulation between the steel and the BUR to prevent moisture condensation on the underside of the roof mat. This specification can result in ridgelining or windowpaning of the roof mat. To prevent this occurrence, two layers of insulation are specified, and joints staggered. In meeting Factory Mutual Wind Uplift Requirements, the first layer of insulation must be mechanically fastened to the steel deck with one fastener per two square feet or greater.
- Metal deck with lightweight concrete insulating fill.
This deck is also supported by steel bar joists; however, they are all deal level with supporting steel all at the same elevation. The slope-to-drain is achieved by tapering the lightweight fill, from a minimum thickness of 1-1/2" to a height that will obtain a minimum of 1/4" in 12 slope. Most deck assemblies are under designed to 1/8" in 12 to save dollars. In specifying slope-to-drain, always design 1/4" or greater in 12 to slope. This type deck assembly does not require roof insulation; however, it is often found incorrectly specified directly over the lightweight fill. The correct method of design is to install a venting base ply directly over the lightweight fill, and then install insulation over the venting base ply if you require more R-Value, or install the BUR directly over the venting base ply if no additional R-Value is required.
- Poured-in-place gypsum over form board.
The inverted tee (or bulb tee) holds the 3 x 4 form board panels, and is welded to the steel bar joist. The bar joist determines the slope-to-drain, and the gypsum (with scrim wire) is installed over the form board at the constant thickness of 2-1/2". This type deck does not require roof insulation, and the design of the roof assembly can be the same as for the metal with lightweight fill deck.
The reason a venting base ply is used on the above two deck types is due to the fill material; i.e., lightweight concrete fill and gypsum are installed in a wet slurry and contain a tremendous amount of water/moisture that never gets out in its entirety before being roofed over.
- Wood fiber with cementitious binder, commonly called by the manufacturer's trade name of Tectum.
The panels of Tectum are supported by bulb tees welded to the steel bar joist. Panels come in 2-1/2" or 3-1/2" thickness, and slope-to-drain is achieved by the steel bar joist. This material is often specified in gymnasiums, because it also is a very effective sound deadener. This type deck does not require insulation; however, if additional R-Value is required, insulation can be designed into the roof assembly. Consisting of wood fibers bound together with a cementitious binder, this type deck is easily damaged by water/moisture infiltration.
|
| INSULATION |
The primary purpose of insulation
is to restrict heat flow, and is measured
by its "R" or "C"
value. The types of insulation commercially
in use today include perlite, fiberglass,
polyisocyanurate, and extruded and expanded
polystyrene, to name a few.
If additional insulation is required
to raise the R-value, it would be
less costly and more efficient to
install it above a dropped ceiling
in all areas where this type of insulation
is feasible. Insulating above a dropped
ceiling costs approximately 50% less
than insulating under a roof membrane.
More thermal efficiency is achieved
from insulation above a dropped ceiling,
because the insulation is not exposed
to such a wide variation in temperature.
|
| FELTS |
The primary function of felts, whether
organic or fiberglass, is to provide
tensile strength to the roof mat.
Allow an organic felt to get wet
during application, or via moisture
infiltration after, and tensile strength
is lost. Remember, felts can get wet
after they are laid as well as before.
The result is splits 6, 8, and 10
feet long due to the loss of tensile
strength.
Fiberglass felts have a greater strength
than organic felts, and are not damaged
by moisture infiltration to the extent
the organic felt is. However, these
are often laid with voids between
plies. This is due to two factors.
First, workmen stepping on the fiberglass
felt while the bitumen is still liquid
cause voids by pushing the bitumen
away from this area. Second, it seems
that unless you stand on the roof
and beat the applicator over the head
with a stick, they will not back-squeegee
the fiberglass felt as it is being
laid. It is interesting to note that
all major manufacturer's specifications
call for back-squeegee of the fiberglass
felts as they are being laid; however,
every contractor will tell you this
is not necessary.
|
| BITUMEN |
Let's look at the two different types
of bitumen, where they are derived from,
and some facts you should know before
specifying.
Asphalt bitumen comes from the petroleum
industry. Its specific gravity is
lighter than water; it contains fillers,
one of which is clay (the reason the
manufacturer will not warranty a roof
with ponding water); it does not contain
any solvents (this allows it to be
melted in a kettle); and it is manufactured
in four different flow points (Type
I - 135° F, Type II - 158°
F, Type III - 185° F, Type IV
- 210° F), with Type III being
the most commonly specified of all.
Coal-tar bitumen comes from the cooking
of coal. Its specific gravity is heavier
than water; it does not contain any
fillers; it will not break down under
ponding water; it does not contain
any solvents (this allows it to be
melted in a kettle); and it is manufactured
in three different flow points (Type
I - 140° F, Type II - 126°
F, and Type III - 147° F), with
Type I being the most commonly specified
of all.
NOTE: Coal-tar bitumen contains creosote,
and for this reason and this reason
only, asphalt cannot go over coal-tar,
but coal-tar bitumen can go over asphalt.
The creosote is continually evaporating
into the air; asphalt bitumen being
placed over coal-tar would not allow
the continual escape of creosote from
the coal-tar bitumen, and a violent
reaction would take place. Remember,
there are no solvents in either coal-tar
or asphalt bitumen that are manufactured
to be melted down in a kettle.
|
| DESIGN |
Economics dictates the structure of
the building; however, if we keep in
mind what the built-up roof was originally
designed for, we can successfully install
it on today's construction.
Specifiers often forget expansion
joints, proper height of flashings,
proper height of equipment, minimum
number of plies and protection of
flashings to name a few. These are
all basic principles and rules you
must follow when designing a built-up
roof. We have all seen specifications
that do not abide by the rules which
results in failure or dramatically
shortened roof life. By simply keeping
these five basic components in mind
when designing a reroof, you can achieve
longer roof life span than the national
average.
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