FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

Volume II CONSTRUCTING THE BUILDING 13
When two flat wood surfaces are in contact in a connection, the contact surface tends to retain any water
directed to it. The wider the connection’s least dimension, the longer the water is retained and the higher
the likelihood of decay. Treated wood in this contact surface is more resistant to decay but only at an uncut
surface. The least dimension of the contact surface should be as small as possible. When the contact surfaces
are for structural bearing, only as much bearing surface as needed should be provided, considering both
perpendicular-to-grain and parallel-to-grain bearing design stresses. For example, deck boards on 2x joists
have a smaller contact surface least dimension than deck boards on 4x joists. A beam bolted alongside an
unnotched round wood pile has a small least dimension of the contact surface. Figure 13-10 illustrates the
least-dimension concept.
Poor durability performance has been observed in exposed sistered members. When sistered members
must be used in exposed conditions, they should be of ground-contact-rated treated wood, and the top
surface should be covered with a self-adhering modified bitumen (“peel and stick”) flashing membrane. This
material is available in rolls as narrow as 3 inches. The membranes seal around nail penetrations to keep
water out. In contrast, sheet-metal flashings over sistered members, when penetrated by nails, can channel
water into the space between the members.
Other methods of improving exposed structural frame durability include:
Using drip cuts to avoid horizontal water movement along the bottom surface of a member. Figure
13-11 shows this type of cut.
COASTAL CONSTRUCTION MANUAL
Figure 13-10.
Examples of minimizing
the least dimension of
wood contact surfaces
Figure 13-11.
Drip cut to minimize
horizontal water
movement along the
bottom surface of a wood
member
13-15