Sealing the Building Envelope

By Dennis Kulesza
Metropolitan Restoration and Waterproofing Corp.
70 Von Hillern Street
Boston, MA 02125

Facility management has become a complicated discipline in today’s changing world. Concerns, such as, strategic planning, ergonomics, bench marking, accessibility, post occupancy building evaluations, partnering and down sizing continue to devour a facility manager’s time. It is little wonder that building leaks do not rate preferential treatment. Inevitably, though, keeping a building safe from water infiltration becomes an important part of every facility manager’s duties.

Water infiltration causes major problems to a structure. Water damages a building first cosmetically then structurally. It is important to realize is that by the time a stain shows up on the interior of your building most likely irreversible damage has been caused to the exterior. Water soaked roof insulation will never dry out. The effective “R” value of the insulation is now destroyed. Trapped moisture in insulation can also decay a roof deck and will cause roofs to fail prematurely. Water entering walls will rust steel relieving angles and carrying beams which support the structure. Moisture penetrating reinforced concrete structures carry chloride ions which will rust reinforcing bars causing them to expand in size resulting in spauling concrete. As one begins to understand the mechanics of water infiltration one begins to understand the importance of keeping a building watertight.

The best way to prevent water from entering a structure is to implement a scheduled maintenance program which incorporates routine visual inspections. Table 1 & 2 are checklists typically used when inspecting roofs and walls respectively.

Roofs should be routinely inspected in the spring and fall. Inspecting a roof in the fall will allow you to ready your roof for the winter. Fall is a good time to check drains and gutters to see that they are free of debris. Clearing debris from drains is a simple task during mild weather but it becomes a difficult if not impossible once temperatures drop below freezing. Inspecting a roof in the spring helps identify damage caused to the roof over the winter. Make repairs before spring rains begin to prevent water from causing permanent damage to your roof. Cursory inspections should also be done after wind storms and after any work is done to your roof or roof top equipment.

Walls are typically more durable than roofs and usually require inspection once a year. A close look at Tables 1 & 2 reveals that many of the items on the roof check list also appear on the wall check list. An example is coping and counter flashings. This occurs because both the roof system and wall system depend on these building components to keep water out. These components make the transition between systems and are actually shared by the roof and the walls. To help illustrate and explain the relationship between roof, wall, and horizontal waterproofing systems within the same structure the concept of the “Building Envelope” has been developed.

The building envelope concept maintains that in order to effectively keep water out of a structure one must design and construct continuity between the vertical and horizontal components of the building’s waterproofing systems. Accordingly one must consider walls when dealing with roofs and vise versa.

Examine figure 1, Typical Roof Cross Section, and note that the roof membrane is only part of the system which keeps water out of a building. Perimeter flashings make the transition between the roof and the side walls. Thru-wall flashings make the transition between the roof and intersecting walls such as is found at penthouses and similar structures.

Figure 2, Cavity Wall Cross Section, indicates that the building envelope concept must be followed through in the design of walls. Thru-wall flashings must be incorporated in cavity wall systems above relieving angles and at deck intersections.

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.

A bond breaker material should be used when constructing a joint to prevent three point adhesion. Joints should be designed and located so that the limit of movement in any direction does not exceed 25% of its width. Manufacturers of sealant joint material have expertise to offer when either designing new or replacing old sealant joints. I strongly recommended that you consult sealant manufacturers when designing and constructing sealant joints.

Promenade and terrace deck waterproofing systems are designed and installed very similar to roof systems. The major difference is waterproofing systems must be able to support foot or vehicular traffic. The two most common type of membranes used for promenade or deck waterproofing are liquid applied or loose laid sheet membrane.

The advantages of a liquid applied system are:

  • Material is spread over the entire deck as a liquid and dries to form a monolithic surface- no seams.
  • The finished surface can come in direct contact with pedestrian or vehicular traffic.
  • Membrane is fully adhered to substrate resulting in minimal water migration between deck and membrane making leaks easy to find.
  • Easy to maintain since membrane is expose.

The disadvantages to liquid applied membranes are:

  • Expensive and time consuming deck preparation in retrofit applications. Deck must be clean with no contaminants for membrane to properly bond.
  • Substrate cracking is detrimental to the system as it can reflect through the membrane.

Sheet membrane advantages are:

  • Preparation of the substrate is not as critical since membrane is usually separated from deck by a separation course.
  • Deck contamination and deck cracking is not as crucial.

Sheet membrane disadvantages are:

  • Cannot directly support pedestrian of vehicular traffic – must use protection course.
  • Maintenance and repairs are costly. Protection course must be removed to expose membrane.
  • Leaks are difficult to find since water can travel between loose laid membrane and substrate.

In summary water can have devastating effects on a structure. Keeping water out of a building begins in the design stage. Good design must be complimented by quality workmanship and the use of the right construction materials for the given application. The rest is up to the facility manager. The manager must have a good maintenance system in place which incorporates routine inspections. The water tight integrity of the building envelope will only be as strong as the weakest link in the construction and maintenance chain. Facility managers play a critical role in keeping water out of a facility. They are the maintenance link in the chain to a watertight structure.