FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

Volume II Designing the builDing envelope 11
Enhanced Underlayment Option 3. Option 3 is the typical underlayment scheme (i.e., a single layer
of #15 felt tacked to the sheathing, as shown in Figure 11-25) with the added enhancement of selfadhering
modifed bitumen tape. Tis option provides limited protection against water infltration if
the shingles blow of. However, this option provides more protection than the typical underlayment
scheme. Option 3 is illustrated in Fact Sheet 7.2 in FEMA P-499.
Figure 11-28 shows a house that used Option 3. Te self-adhering modifed bitumen tape at the sheathing
joints was intended to be a third line of defense against water leakage (with the shingles the frst line and the
felt the second line). However, as shown in the inset at Figure 11-28, the tape did not provide a watertight
seal. A post-storm investigation revealed application problems with the tape. Staples (arrow, inset) were
used to attach the tape because bonding problems were experienced during application. Apparently, the
applicator did not realize the tape was intended to prevent water from leaking through the sheathing joints.
With the tape in an unbonded and wrinkled condition, it was incapable of fulflling its intended purpose.
Self-adhering modifed bitumen sheet and tape normally bond quite well to sheathing. Bonding problems
are commonly attributed to dust on the sheathing, wet sheathing, or a surfacing (wax) on the sheathing that
interfered with the bonding.
In addition to taping the sheathing joints in the feld of the roof, the hip and ridge lines should also be taped
unless there is a continuous ridge vent, and the underlayment should be lapped over the hip and ridge. By
doing so, leakage will be avoided if the hip or ridge shingles blow of (see Figure 11-29). See Section 11.6 for
recommendations regarding leakage avoidance at ridge vents.
Figure 11-28.
House that used enhanced underlayment Option 3 with taped sheathing joints (arrow). The self-adhering
modifed bitumen tape (inset) was stapled because of bonding problems. Estimated wind speed: 110 mph.
Hurricane Ike (Texas, 2008)
SoURCE: IBHS, USED WITH PERMISSIoN
COASTAL CONSTRUCTION MANUAL
11-29