Eliminating Openings In Building Assemblies

Every building is full of openings. A shingled roof has an opening under each shingle. A wall has cracks around windows and doors, and joints between the units of material from which the wall is made. Additional cracks and holes may form as the building ages and deteriorates.

We can attempt to eliminate all these openings by using preformed gaskets and sealants. As a sole strategy this is unreliable. Gaskets may not seal securely if they are the wrong size or resiliency, or if the surfaces they touch are rough or unclean. Sealants may fail to adhere properly if the materials to which «they are applied are not scrupulously clean and properly primed, or if the installer does not compress the sealant fully into the seam. Both sealants and gaskets can deteriorate from weathering and from the flexing and stretching they may undergo as the building ages. A building skin that relics on sealants and gaskets alone for watertightness will leak sooner or later. Furthermore, even a small defect in a sealant or gasket that is exposed to the weather can leak very large amounts of water, just as a small hole in a bathtub can create a very large puddle. Small defects in devices for keeping water away from openings or neutralizing the forces that can move the water are seldom as catastrophic.

Sealants and preformed gaskets are extremely useful, however, as components of an overall strategy for making a building skin watertight. Therefore, it is important to know-how to detail sealant joints and gasket joints correctly, and how to incorporate them into more complex schemes for controlling water penetration. The detail pattern that relates to eliminating openings in building assemblies is

Sealant Joints and Gaskets (page 31)


There are a number of effective ways to keep water away from openings. Often it is useful just to be able to keep most water away from an opening—-to reduce the volume of water that must be dealt with at the opening itself In many cases we can easily and securely keep all water away from an opening.

The detail patterns that relate to keeping water away from openings in building assemblies are

Wash (page 7) Overlap (page 12) Overhang and Drip (page 14) Drain and Weep (page 17) Cold Roof (page 19) Foundation Drainage (page 21)


There are five forces that can move water through an opening in a wall or a roof: (1) gravity, (2) surface tension, (3) capillar)' action, (4) momentum, and (5) air pressure differentials. In most cases, it is surprisingly easy to detail a building assembly so that all five of these forces are neutralized, and the most secure strategies for keeping water out of a building are based on this approach.

We have already encountered the detail patterns for neutralizing two of these forces, because these same patterns are useful in keeping water away from openings in buildings. The force of gravity is neutralized by

Wash (page 7) Overlap (page 12)

Surface tension, a force that causes water to cling to the underside of a surface where it can run through into an opening, is neutralized by

Overhang and Drip (page 11)

The patterns for neutralizing the other three forces are

Capillary Break (page 22) Labyrinth (page 24) Rain screen Assembly (page 25) Upstand (page 29)

The capillary break neutralizes capillary action. The labyrinth neutralizes momentum, and the rainscreen assembly and the upstand neutralize air pressure differentials. By combining these seven patterns in each exterior joint of a building, we can make a building entirely waterproof.

A wash is a slope given to a horizontal surface to drain water awav from vul-

j nerable areas of a building. In general, ever)' external horizontal surface of a building should have a wash.

1. A window or door sill, whether made of stone, concrete, wood, or metal, always has a wash to keep water from accumulating next to the door or sash. A minimum slope for this type of wash is about 1:10 or 1:12 (1" per foot). A steeper slope drains water faster and is more secure, because the more quickly water is removed from a surface, the less time ii has to leak through. It is also more difficult for wind to drive water up a steeper slope.

2. The wash on this concrete chimney cap keeps water away from the vulnerable crack between the clav flue

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