The key to understanding how the building envelope works is to realize that a waterproofing medium must exist on all vertical and horizontal surfaces and to realize that the entire system is inter-connected and over lapping at these interfaces. However, it is very important to note that the waterproofing envelope is very seldom continuous or uninterrupted at vertical and horizontal transitions. Good design dictates that flashing should be built as two pieces (base and counter flashing) to compensate for differential movement between vertical walls and horizontal decks. Differential movement occurs due to the different types of materials used to construct roof decks and walls, varying coefficients of thermal expansion, different modulus of elasticity and unequal loading.

What further complicates the matter is that there is movement not only between the various waterproofing systems incorporated in the building but also movement within the same waterproofing system.

Figure 3 shows the four basic types of movement which occur in a typical vertical wall control joint. They are compression, extension, longitudinal extension, and transverse extension. The amount of movement in a control joint is a function of the following:

• The type and location of the anchors used to secure the building's exterior skin
• The degree to which the exterior skin absorbs moisture
• The amount of structural loading caused by live loads, dead loads and wind loads
• The amount of shrinkage a building material will under go once installed

The above factors determine the size, location and number of joints which are needed in a wall system to resist wall movement. Esthetics should never determine where control joints are placed.

In addition to the size and location of control joints the construction of joints is critical to their long term performance. Figure 4 illustrates the concept of three point adhesion. Sealant joints must be allowed to move across their width or else stress concentrations will build up within the joint and rip it apart. Adhering the sealant on only the two opposing sides of a joint eliminates three point adhesion and allows the sealant to stretch and compress.

Back    Continue