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Design
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Moisture Control | Roof
Decks | Rigid Insulation | Roof
Membranes
Rigid Board Insulation
Tapered Insulation Design Factors
1. Existing Substrate Condition: The designer shall evaluate the deck
for conditions that may affect the draining surface of the roof system,
and then specify a tapered insulation system with sufficient slope so
as to accommodate the following:
a. Existing and potential deck deflections, including irregularities in
the deck.
b. Actual slope of the roof deck.
c. Slope of roof assembly for future roof system installations.
2. Drainage Design: The designer shall specify the removal of water from
the roof assembly through drains, scuppers, and gutters; and shall specify
the quantity, size, and location of these components in a drainage plan
that has been calculated to accommodate anticipated water volume capacities,
and that complies with local codes.
3. Tapered Insulation Perimeters & Penetrations: Tapered insulation
shall be designed to allow for the correct clearance of flashing reinforcing
sheets and counterflashings at perimeters and penetrations. Curbs shall
be designed for the correct height above the tapered insulation so as
to allow for the minimum flashing height tolerance of 8”. The
height of adjacent walls and openings in walls, such as windowsills, doorsills,
thruwall flashings, and weep holes shall be high enough to allow for minimum
flashing requirements. Insulation thickness at intersections between individually
drained roof areas shall match to allow for the smooth transition of the
roof system membrane. Area dividers shall be installed to facilitate transitions.
If a constant fascia profile is desired along a low roof edge, the tapered
insulation shall be designed with a uniform thickness along that edge.
Parapet walls shall be constructed to sufficient height so as to accommodate
and allow for the additional thickness of the tapered insulation system.
4. Compatibility with the Roof System Membrane: Tapered insulation shall
be compatible with the roof system membrane. If there is a question as
to compatibility, the insulation manufacturer and Fields shall be consulted
for information about such compatibility.
5. Slope Pattern: The designer shall calculate and specify the required
roof slope. More than one pattern may need to be specified in combination
to achieve positive drainage for the entire roof assembly. Selection of
the tapered layout pattern shall be based upon the geometry of the area,
the required slope within a specific area, and the relationship of the
pattern to other adjoining areas. For a square or rectangular roof area
with perimeter gutter drainage, a one way or two way slope pattern is
a good design.
For a square roof area with an interior drain, a four way slope pattern, directed toward the interior drain, will provide uniform perimeter thickness and simplify edge detailing. A four way slope pattern also provides uniform slope over all fout intersecting areas.
A two way slope pattern provides results similar to the four way slope pattern. However, saddles must be designed for the intersecting valley formed by the two way slope to correctly channel water to the interior drains.
For a similar rectangular roof area with two interior drains, a two way
slope pattern incorporating a saddle and crickets made from tapered insulation
must be designed.
Valley centerlines shall be 45° from the direction of slope, making
valley centerlines 90° apart. Designing layout patterns with 45°
valley lines makes it easier to cut the board stock from one edge or corner
to the opposing corner as well as providing the maximum slope in the valley.
Any departure from the simple 45° cut concept can create challenges
during layout and increase on-site installation time.
When a valley cannot be installed at a 45° angle to the slope, calculate
the tapered insulation heights at both sides of the valley, and adjust
the slope so that the heights of the tapered insulation at each side of
the valley match.
When a ridge occurs, such as between adjoining tapered patterns, the designer
shall specify the thickness of the two adjoining patterns so that a common,
uniform thickness occurs at the ridgeline. To achieve consistent ridge
thickness, the ridgeline shall be spaced equidistant between the drainage
points. The ridgeline shall be perpendicular to a line that connects the
two drainage points.