Skip to main content

ARCH 2614/5614 Lecture notes

Jonathan Ochshorn

contact | office hours | homepage | current index for ARCH 2614/5614 | lecture index for ARCH 2614/5614

Sealant joints

sealant being applied

Sealant joints allow movement, but also supply continuity where materials are not only joined but need to connect in a continuous manner. Three main types:

  1. low-range (caulks) with 5% movement capability, for minor cracks

  2. medium-range (acrylics, butyl rubber) with 5-10% movement capability, e.g., for cladding joints that are non-movement joints

  3. high-range (silicones, polyurethanes, polysulfides) with 25-100% movement capability, durable for up to 20 years, used for movements joints in cladding.

In other words, not all joints are equal: some come about because buildings are assembled from pieces, but have no associated movement (or very little). At one extreme, for pieces bearing on each other in compression (like traditional masonry construction), the space between these pieces can be filled with mortar that hardens into a firm and brittle — i.e., non-flexible — mass. At the other extreme, where movement between pieces is variable — i.e., might place the joint into tension — a brittle mortar-like fill would not work, and a more flexible (and "stretchable") substance is required.

This difference in the behavior of joints subjected to compression only versus compression and tension (or tension only) explains in part why traditional masonry construction could be built without any control or expansion joints:

  1. All horizontal joints are stressed almost exclusively in compression.

  2. Because the bond between traditional mortar (lime-based) and the masonry units being joined is relatively weak, especially compared to modern mortar mixes, tension cracks at vertical joints (e.g., due to thermal contraction in cold weather) do not split the masonry units but instead create hairline cracks between the masonry units and the mortar.

  3. Individual masonry units are relatively small, so that any thermal or shrinkage stresses which would place vertical joints into tension result in extremely small cracks — using larger units would result in larger movement-induced cracks.

traditional masonry walls without sealant joints
Behavior of traditional masonry walls without sealant joints in cold climates: hairline cracks may form on the outside of vertical joints, but compressive stresses prevent cracks in horizontal joints.

Movement capability is defined as the amount of movement that the sealant is capable of, divided by its original width, and expressed as a percentage. Note that 100% movement capability in a sealant refers to its ability to expand only; such a sealant might have a 50% movement capability when it contracts. This is because nothing can contract 100% — that would be like having a 1 inch joint width that contracted to 0 inches.


We get:

Examples of movement capability:

Sealant joints typically consist of the sealant itself, and a backer (back-up) rod; the rod has a "bond breaker" to prevent adhesion of the sealant to the rod. The bond breaker is either applied as a separate element (like a piece of slippery tape) or is integral with the backer rod.

backer rods
Backer rods (left -- source) and backer rod installation (right -- source)

backer rods
Backer rods installed at the Schwartz Center for the Performing Arts at Cornell University (photo by Jonathan Ochshorn, August 2017)

The backer rod has three primary functions:

  1. It creates a back surface so that the sealant is only applied where it is needed.

  2. It defines the shape of the sealant so that it can properly expand and contract within the joint.

  3. It provides a surface for a bond breaker (often integral with the backer rod itself) to prevent adhesion of the sealant to the back surface. This is important so that the sealant can expand and contract properly.

Sealant installation video
The video linked above provides proprietary advice on how to properly place sealant between a horizontal and vertical surface (countertop). Note that for cladding panels, the objective is somewhat different, as the sealant must be placed between two parallel surfaces.

Sealant installation video (Milstein Hall, Cornell)
The video linked above is an unedited shot of the black silicone sealant being installed between glazing panels at Milstein Hall, Cornell University (May 6, 2011; J. Ochshorn)

Typical joints range in width from 1/4" to 1-2". The depth of the sealant joint is 1/2 of the width, except that it is limited to a minimum of 1/4" and a maximum of 1/2" (these two values are sometimes given as 1/8" and 3/8" respectively).

sealant joint

sealant joints
A = sealant; B = sealant width; C = sealant depth; D = joint filler (backup rod); E = tooling depth; F = joint-filler depth; G = sealant contact depth; H = sealant recess depth.

sealant joints
Untooled with no joint filler is no good (left); untooled with joint filler is no good (middle); tooled with joint filler (back-up rod) is correct.

sealant joints
Mode of failure when sealant joints are not installed correctly (right image).

The image below (Kennedy-Roberts hall, Cornell, Gwathmey-Siegel Architects) shows the location of sealant joints in the brick veneer. Typically, the joints correspond to (1) shelf angles at the floor levels and (2) column lines, resulting in a pattern of rectangular "panels" of brick, each of which can move separately.
Kennedy-Roberts Hall, Cornell, sealant joints
Kennedy-Roberts Hall, Cornell, sealant joints
Kennedy-Roberts Hall, Cornell, sealant joints
Gwathmey-Siegel Architects, Kennedy-Roberts Hall, Cornell showing sealant joints at floor levels and column lines (top and middle with joints highlighted with dark lines) and brick damage at joint (bottom)

Sealant joint example

sealant joint example

Find the necessary joint width, W, for the horizontal sealant joint shown above.

Assume the following:

The solution is best found by constructing a table with columns for maximum joint contraction (i.e., brick expansion), and maximum joint expansion (brick contraction). Note that 144" refers to the panel height, i.e., 12 feet.

  Max. joint contraction Max. joint expansion
thermal expansion (144")(0.0000036)(90-45) = 0.023" (144")(0.0000036)(65-0) = 0.034"
moisture expansion (144")(0.0005) = 0.072" (144")(0) = 0"
structural deflection 0.125" 0.125"
subtotal: all joint movement 0.220" 0.159"
required sealant width multiplier for movement:
100 / MC = 100/25 = 4
4 x 0.220" = 0.880 4 x 0.159" = 0.636
additional sealant width for tolerance 0.1875" 0.1875"
Total sealant joint width 0.880 + 0.1875 = 1.068" 0.636 + 0.1875 = 0.824"

The answer is 1.068" or, rounding to the nearest 1/8" increment, approximately 1". A section through that horizontal joint is shown in the sketch below:

sealant joint example
Schematic section through sealant joint (flashing with drip edge and weep holes not shown)

Of course, the sealant joint width would be a bit less if the moisture expansion of the brick was taken as 0.0002 or 0.0003. This judgment can be made based, in part, on the amount of time that the brick has been out of the kiln.

** Coefficients of thermal expansion: Scroll down to bottom of page for table of common values. For the example used here: 3.6 x 10-6 = 0.0000036.

** Moisture expansion in brick: "Based on past research, long term moisture expansion of brick can be estimated at between 0.0002 and 0.0009. A design value of 0.0003 should be used when designing composite masonry walls. A design value of 0.0005 should be used in veneer walls where an upper bound of movement is estimated."

Links to technical notes on brick construction can be found here.

Hybrid sealants

Hybrid sealant joints
Hybrid sealant joints
Hybrid sealants with non-invasive anchoring useful for joints wider than 1 inch: they fold rather than stretch. Primarily silicone.

I.M. Pei's Moakley U.S. Courthouse on Fan Pier in Boston

My first experiment with a Flip camcorder, Oct. 2007: sealant joints in Boston (video shot by R. Ochshorn; extemporaneous narration by J. Ochshorn). Video link.

Common coefficients of thermal expansion for building materials

Materialin. per in. per degree F x 10-6
  • Brick
  • Concrete block (dense)
  • Concrete block (expanded shale)
3.6 - 4.0
  • With gravel aggregate
  • With lightweight aggregate
  • Gypsum
  • Portland cement
  • Granite
  • Limestone
  • Marble
3.7 - 7.3
  • Aluminum
  • Brass
  • Bronze
  • Cast iron
  • Copper
  • Lead
  • Stainless steel
  • Structural steel
Wood, parallel to grain
  • Fir
  • Pine
  • Oak
Wood, perpendicular to grain
  • Fir
  • Pine
  • Oak

Table adapted from David Ballast, Architect's Handbook of Construction Detailing