Building joints move in three different ways that affect the sealant in the joint: expansion, contraction, and shear.

Building joints move in three different ways that affect the sealant in the joint: expansion, contraction, and shear.

Modern commercial structures rely heavily on joint sealants to prevent water damage to buildings and their contents. While residential buildings use water-shedding techniques such as sloped roofs, lap siding, and overlapping flashings, many commercial designs don't; if a joint sealant fails, there is little or no barrier to leakage. Unfortunately, in today's building environment, there are many points in the design and construction process where bad judgment or bad behavior results in sealant failure. Read the following for tips on how best to avoid these situations.

Common Uses

Joint sealers are used to close open joints to keep water and air out (both exterior and interior); for appearance and cleanability (in interior surfaces where water resistance is not an issue); and to reduce sound transmission through cracks (usually interior and internal to composite assemblies). If none of the above considerations is applicable, joint sealers are probably not necessary. Although there are many types of joint sealers, this review covers joint sealants only—pourable or gunnable material of mastic consistency that sticks to each side of a joint.

Exterior. Most modern homogeneous rigid exterior substrates are purposely jointed, to allow movement without damage to the material. The two principal causes of movement are thermal expansion and contraction and seismic movement.

Some substrates, such as traditional shingling, can be overlapped to allow rainwater to run off while also allowing movement—these usually won't need sealing. Others incorporate the seal into the product design, such as metal panels with edge joints designed to prevent water infiltration.

Other combinations of exterior materials are simply different and, as a result, seldom form a watertight joint without the addition of a sealer.

The principal exterior substrates that are sealed are:

  • exterior wall joints (masonry, concrete, plaster/stucco, EIFS, for example);
  • door and window frames;
  • concrete paving joints;
  • metal flashings;
  • roof joints; and
  • seismic movement joints.

Interior. Joints indoors don't usually go through the thermal fluctuations that exteriors do, but they are also often jointed for other reasons. Gypsum board and plaster assemblies, for instance, often require control joints to prevent cracking. Interior joints are usually sealed to keep dirt out and make them look better. The principal interior substrates that are sealed are:

  • gypsum board;
  • plaster;
  • floor control and expansion joints; and
  • kitchen and bathroom wet joints.

To Avoid Failures

  • Choose the correct design solution. The life span of even the best sealer materials available is finite—usually less than the expected building life span. If failure of the joint seal would be very costly, a water-shedding solution might be a better solution.
  • Estimate the actual amount of movement correctly. Consider the width of the joint, the distance between joints, and the thermal range. ASTM C 1472 can help. Joint movement is of three types:
  • expansion and contraction (the joint gets wider or narrower);
  • shear or lap movement (the faces of the joint slide past each other, and the sealer undergoes twisting and stretching but no compression); and
  • expansion, contraction, and lap shear, all at once. (See diagrams.)
  • Choose a sealant product that will withstand the movement expected. Most joint sealer mistakes relate to movement— misjudging the actual amount of movement or selecting a product that won't withstand movement. Movement capability is the relevant product characteristic.
  • Choose a sealant that will withstand the environmental conditions. The second most common cause of product failure is degradation by water and weather (including indoor wet areas).
  • Specify the sealant product correctly. The two most common ways to specify sealants are, one, by listing the manufacturer and brand name(s) of acceptable products, and, two, by specifying characteristics by description and/or by reference to voluntary standards. If both techniques are used for the same product, be sure that they are not contradictory.
  • Specify the scope of sealant work completely. Implementation mistakes occur most commonly because of a failure to completely identify the joints to be sealed and the product(s) to be used for each.

    The extent of sealing work is not always apparent from the drawings. So, it is commonly necessary to describe the extent of the sealing work in words. This may be placed on the drawings, as notes or a schedule, or may be included in the specification.

    Regardless of methodology, the important point is that the description fully describe the extent of the work by identifying all the joints to be sealed.

  • Specify execution correctly. Most joint sealers require expert installation, without which failure is likely. Require installers to follow the manufacturer's installation instructions and specify reputable manufacturers who provide detailed instructions (do-it-yourself products don't usually come with detailed instructions). Take care not to introduce conflicts by specifying execution that contradicts manufacturers' instructions.

    Specifying Sealants

    Joint sealants are usually specified in a single section describing products, execution, and administrative requirements:

  • 07920—Joint Sealants (MasterFormat 1995)
  • 07 92 00—Joint Sealants (MasterFormat 2004)

    Concrete pavement joint sealants are sometimes specified in a separate section:

  • 02750—Paving Specialties
  • 32 13 73—Concrete Paving Joint Sealants

    When more than one type of sealant is specified, the drawing notes or a schedule must identify which ones are to be used in which locations. Use terminology that will explicitly “tie” the product in the schedule or on the drawings to the product in the specification.

    Some people like to give each specified sealant a Type A–, Type B–style designation. This technique has the advantage of allowing the exact type of sealer to be changed by altering the specification, without any need to change the drawing/schedule notation.

    Some statements that might appear in a sealant schedule:

  • “Control joints in brick veneer: Sealant Type A.”
  • “Joints between concrete columns and brick veneer: Sealant Type A.”
  • “Joints between window and door frames and brick veneer: Sealant Type A.”
  • “Control joints in interior gypsum board: Acrylic latex sealant.”
  • “Joints between kitchen and bath counter backsplash and wall: White silicone sealant.”

    In some cases, the sealant is to be furnished and installed by the installer of the product that is to be sealed. For instance, the window specification might require the window installer to complete the installation by sealing around the window.

    In that case, the sealant and the sealing work can be specified in the window section. Alternatively, the sealant product and installation may be cross-referenced from the window section to the joint sealers section.

    Whenever there are a lot of instances like this on the project, it's more convenient to cross-reference to eliminate repetitive language.

    Although installation requirements for sealants should be specified, it is relatively safe to rely on a statement to “install in accordance with manufacturer's instructions.” Most manufacturers will not stand behind their product if it is not installed in accordance with their instructions and recommendations.

    ASTM C 1193 covers typical applications in great detail as help to the specifier, but referencing it as a specification requirement is basically useless; there are too many options to be able to enforce any of them. See the web link at the article's end for other references, including a free guide from the U.S. government.