ASTM C964-20

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: C964 − 19 C964 − 20
Standard Guide for
Lock-Strip Gasket Glazing
This standard is issued under the fixed designation C964; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope
1.1 This guide covers the use of lock-strip gaskets in compliance with Specification C542 in walls of buildings not over 15°
from a vertical plane. The prime performance considerations are weathertightness against air and water infiltration, and structural
integrity under wind loads. Included are terminology, design considerations, and fabrication tolerances when using lock-strip
gaskets in glazing applications.
1.2 The values stated in SI units are to be regarded as the standard. The inch-pound units in parentheses are for information only.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
C542 Specification for Lock-Strip Gaskets
C716 Specification for Installing Lock-Strip Gaskets and Infill Glazing Materials
C864 Specification for Dense Elastomeric Compression Seal Gaskets, Setting Blocks, and Spacers
C963 Specification for Packaging, Identification, Shipment, and Storage of Lock-Strip Gaskets
C1036 Specification for Flat Glass
E283 Test Method for Determining Rate of Air Leakage Through Exterior Windows, Skylights, Curtain Walls, and Doors Under
Specified Pressure Differences Across the Specimen
E330 Test Method for Structural Performance of Exterior Windows, Doors, Skylights and Curtain Walls by Uniform Static Air
Pressure Difference
E331 Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air
Pressure Difference
3. Significance and Use
3.1 This guide provides information and guidelines for the design of lock-strip gasket glazing systems. For related standards,
see Specifications C542, C716, and C963.
4. Comparison to Other Standards
4.1 The committee with jurisdiction over this standard is not aware of any comparable standards published by other
organizations.
This guide is under the jurisdiction of ASTM Committee C24 on Building Seals and Sealants and is the direct responsibility of Subcommittee C24.73 on Compression
Seal and Lock Strip Gaskets.
Current edition approved Aug. 1, 2019March 1, 2020. Published September 2019April 2020. Originally approved in 1981. Last previous edition approved in 20122019
as C964 – 07 (2012).C964 – 19. DOI: 10.1520/C0964-19.10.1520/C0964-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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DESIGN CONSIDERATIONS
5. General
5.1 Structural integrity and watertightness of a gasket glazing system is dependent on interaction of the several components
involved. These systems should be carefully designed and built.
6. Components
6.1 The major components of lock-strip gasket glazing and paneling systems are:
6.1.1 The supporting frame of metal, concrete, or other structural building materials,
6.1.2 Lock-strip gasket, serving as an elastomeric mechanical seal and as a retainer for panel or glass, and
6.1.3 Glass or panel infill.
6.1.4 The design of these components and their accessories are interrelated and the total system must be compatible.
7. Supporting Frames
7.1 Supporting frames are made of many materials, of which the more common are aluminum, steel, and concrete.
7.1.1 Metal—Die marks, ridges, offsets, and scratches in metal frames in contact with the gasket lips that could cause leakage
should be avoided. Metal in contact with any part of the gasket should have sharp edges and burrs removed to avoid the possibility
of damage to the gaskets that could result in structural failure through tear propagation. Weathering steel frames used in gasket
installations should be coated to prevent corrosion on the surfaces covered by the gasket to a line not less than 3.2 mm ( ⁄8 in.)
beyond the lip edge when installed.
7.1.2 Concrete—Gasket lips in contact with protrusions, crazing, form marks, and offsets on concrete surfaces could cause
leakage and glass breakage and such irregularities should be avoided. Concrete frames for lock-strip gaskets should be jointless
and are more suitable when precast, as the tolerances and smooth surfaces required are too exacting for cast-in-place concrete.
Special forms and meticulous casting procedures are required for optimum performance.
7.1.2.1 Corner Angles—Corner angles in the plane of the glass should be held to 62° tolerance to properly receive the gasket
lips. See Fig. 1.
7.1.2.2 Reglets—It is essential that the recess in the concrete be accurately cast so as to properly receive the spline of the gasket.
This can be accomplished with a plastic reglet that has a removable weakness membrane as shown in Fig. 1. The removable
membrane maintains the proper recess shape and keeps concrete out of the reglet while being cast. The removable membrane can
be T-shaped, when desirable, with the stem projecting from the reglet to provide a more convenient means of attachment to the
formwork of the concrete panel. After casting, the weakness membrane is easily removed. Plastic reglets are available with flanges
extending beyond the gasket lips, providing smooth contact surfaces. An advantage of the plastic flange is the provision of a
smooth rigid surface for contact with the gasket lip. The plastic flanges are butted together at the corners requiring a joint which
should be properly aligned and sealed. The exposed plastic flange should be solidly cast into the concrete without any voids or
honeycombing at the leading edge of the flange because water could enter the interface between the flange and the concrete into
which it is cast. An advantage of the flangeless reglet is that the exposed joint between the flange and the concrete as well as the
A Sharp arris (no radius) required
B Nominal angle ±2° tolerance
C Smooth surface required
D 6.4 mm ( ⁄4 in.) minimum
E Removable weakness membrane
F Flange provides smooth surface at lip
FIG. 1 Reglet-type Gasket in Concrete
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corner butt joints are eliminated and the gasket lips make direct contact with the concrete frame. With this concept it is essential
to have a continuous smooth surface free of voids or honeycombing for the gasket lips to seal against because water could bypass
the gasket lip and enter under it. Also important is to have a sharp arris at the corners of the concrete frame so that the corners
of the gasket lip can properly contact and seal against the concrete. When plastic reglets are used, joints in them could cause leaks
unless sealed. When the gasket lips are in direct contact with the concrete, meticulous casting procedures and close surveillance
are required to assure a proper finish along the contacting surface.
7.1.2.3 Frame Lug—It is difficult to achieve watertightness with a gasket gripping the lug of a concrete frame as shown in Fig.
2. Casting the lug to the 0.8-mm (6 ⁄32-in.) tolerance required is unrealistic when dealing with concrete. Also, casting it without
a tapered draft for ease of form removal results in complicated form work. A tapered draft provides poor control over gasket lip
pressure and results in a reduction of pressure when excessive edge clearance permits the gasket lips to slip to the narrower part
of the lug. Unless the gasket gripping the lug of a concrete frame has enough mass, insufficient lip pressure against the concrete
frame and leakage could result because of the relatively large lug width.
7.1.3 Joints—Ideally, the best type of frame over which to seat the gasket is one without joints. However, the realities of
construction should be recognized and dealt with. Watertightness between the lock-strip gasket and frame depends on unbroken
pressurized contact. Joints in metal, unless welded and ground flush and smooth, make this concept difficult to achieve. Members
on either side of a butt joint should be installed as true to plane as possible. If the design relies upon sealed metal-to-metal joints,
the small void between the gasket lip and metal should also be sealed with a supplementary sealant. A recommended safeguard
is to have a built-in drainage system within the frame. In this way, any water penetrating the frame joints or gasket to frame joints
will be directed back to the outdoors. An aid towards minimizing the possibility of water penetration between the gasket and frame
at static (fixed, nonmoving) metal joints in single openings may be seen in Fig. 3. The direction of the joint is horizontal between
the horizontal and vertical members at the top of the frame, and vertical between the horizontal and vertical members at the bottom
of the frame.
7.1.4 Frame-to-Gasket Lips Clearance—Because lip pressure is critical in resisting the passage of water, the design of the
supporting frame must allow at least 3.2-mm ( ⁄8-in.) clearance between the installed gasket lip and any projecting flanges or fillets.
This allows the lips to exert unrestricted pressure against the frame as shown in Fig. 4. Where the frame lug and projecting flange
form a fillet, the recommended clearance should not include the convex portion.
8. Gaskets and Accessories
8.1 To accommodate the wide variety of glass and panel thicknesses available as well as allow for mounting to various types
of framing members, a wide variety of gasket cross sections are produced by the extrusion manufacturing process. The technique
of extruding varies among the manufacturers, and has a limiting factor on the complexity of cross-section designs produced.
8.1.1 Gasket Types—Lock-strip gaskets are typically identified by their general cross-section configuration. The most common
are H-type and reglet type. Other special and proprietary interlocking types have been developed as a result of modifications to
the basic types, usually because of provisions for mounting or mating to special framing members. Gasket sections are generally
of two types: the perimeter section and the muntin section.
8.1.1.1 H-Type—The basic H-type gasket, its installation, and nomenclature are illustrated in Fig. 5. After the gasket is installed
over the frame and the glass or panel infill installed in the gasket, the lock-strip, which is of higher durometer, is forced into a
groove in the gasket. A resultant compressive force is transferred to the lips which apply pressure to the frame and glass. Sufficient
lip pressure against smooth surfaces creates an effective weathertight seal. A wide selection of H-type gaskets are available that
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accommodate glass, panels, and frame lug thicknesses ranging from 1.6 to 32 mm ( ⁄16 to 1 ⁄4 in.). Gaskets accommodating
thicknesses greater than 32 mm (1 ⁄4 in.) are also available. Thick panels should not be mounted on relatively thin lugs as the
weight of the glass or panel cannot be supported properly. The best performance can be expected where the lug thickness equals
or exceeds the thickness of the glass or panel. There are exceptions to this recommendation which are dependent upon other factors,
such as lightweight panels, extremely small openings, or situations where total performance is not required. Acceptable deviations
require engineering analysis, consultation with the gasket manufacturer, and testing.
NOTE 1—Insufficient mass at A and relative long distance from B to lock-strip minimizes potential for adequate lip pressure at B.
FIG. 2 Gasket Mounting on Concrete Lug
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FIG. 3 Single-opening Metal Frame Joints for Increased Watertightness
FIG. 4 Gasket Mounting Clearance
A Hinge H Glass or panel
B Lock-strip I Bite
C Lock-strip cavity J Edge clearance
D Lip (sealing edge) K Frame-to-glass dimension
E Channel recess L Frame lug
F Flange M Frame
G Web
FIG. 5 Basic H-Type Gasket, its Functional Principles and Nomenclature
8.1.1.2 Reglet Type—The reglet-type gasket is a patented type whose functional principles and nomenclature are illustrated in
Fig. 6. Reglet-type gaskets are designed with a spline that fits into a reglet. The seal against the frame is accomplished by forcing
the spline of the gasket into the reglet so that the fins on the side of the spline retain the gasket in the reglet and thus hold the sealing
lips of the gasket tightly against the frame surface. The seal against the glass or panel is accomplished by the insertion of the
lock-strip as with the H-type gasket. Most reglet-type gaskets are designed to fit into a reglet that is 19 mm ( ⁄4 in.) deep and 16
mm ( ⁄8 in.) wide. If the reglet is of lesser depth, the gasket will “bottom-out” and not provide a proper installation. If the reglet
is too wide, the gasket will not be held in place properly and thereby provide difficult glass or panel installation. If the reglet is
too narrow, the gasket will be difficult to install. Reglet-type gaskets are available that accommodate glass or panel thicknesses
1 1
from 1.6 through 32 mm ( ⁄16 through 1 ⁄4 in.). There is an important basic difference between the H-type and reglet-type gaskets
that should not be overlooked in field application. The lock-strip of both gaskets causes lip pressure against the frame and glass,
but with the reglet-type gasket, lip pressure is also affected by the depth of the gasket spline in the reglet. This is controlled by
the installer at the site as well as by the geometry of the gasket and reglet. Available are reglet-type gaskets that have projecting
offsets at the upper or lower part of the spline. These are designed to control the depth at which the spline is inserted into the reglet.
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A Hinge H Glass or panel
B Lock-strip I Bite
C Lock-strip cavity J Edge clearance
D Lip (sealing edge) K Frame-to-glass dimension
E Channel recess L Spline
F Flange M Reglet
G Web
FIG. 6 Reglet-type Gasket, its Functional Principles and Nomenclature
8.1.1.3 Special Interlocking and Proprietary Types—In addition to the basic H-type and reglet-type gaskets, there are various
special interlocking and proprietary type gaskets. Several of these are illustrated in Fig. 7. The basic principle of the interlocking
type is to achieve greater roll-off resistance of the gasket from the frame by mechanically interlocking the gasket to the frame.
8.1.2 Gasket Joints—Best sealing performance is achieved with a continuous gasket having factory-formed injection-molded
joints. In the use of discontinuous ladder-type gaskets or stick systems (assembled in the field, using cut lengths), achieving a
weathertight seal requires a field application of sealant or adhesive recommended by the gasket manufacturer. Geometric continuity
should be achieved at the juncture of extrusion and molded joint. Sharp offsets, the limits of which have not as yet been established,
can break the continuity of the lip seal and prevent or reduce water tightness.
8.1.2.1 Corners—Continuous and adequate lip pressure provided by the gasket against frame and glass is a key factor in the
design of the gasket for watertightness. A pressure of 7 N/linear cm (4 lbf/linear in.) has been determined to be the minimum that
will satisfy this requirement. However, of utmost importance is the requirement that this pressure be continuous and uniform at
every point along the lip of the gasket. The present lip pressure test in Specification C542 is a test for average lip pressure over
the entire test specimen length. When applied to the extruded portion of the gasket, it can reasonably be assumed that the pressure
would be continuous and uniform at every point, provided the gasket lips are not damaged. This is not necessarily true of the
molded corners. Gaskets of various corner designs can pass the lip pressure test, but not all have the capability of sealing out the
passage of water. This may be the case with a corner having square lips on the frame side where, because of the longer diagonal
distance to the lock-strip, little or no lip pressure may be obtained at the apex. It should not be assumed that passage of the lip
pressure test in Specification C542 provides assurance that the gasket is adequate for resistance to the passage of water. Gasket
corner designs having square lip seals are not as common as previously. Gaskets having approximately 6.4 mm ( ⁄4 in.) radius at
the external corner lips are now available. With such a design, a more uniform edge distance is maintained from the lip edge to
the lock-strip as shown in Fig. 8. In this way, lip pressure is not generally reduced around the corner as with a square lip because
of the appreciably longer moment arm. Generally, the round lip is concealed by an external noncontacting square lip for appearance
but the seal is provided by the contacting round lip. The angle of the gasket molded corner should conform within 5° to the corner
angle of the frame. Molded corner angles of less than 45° should be avoided as the insertion of an extremely acute corner angle
is impossible without damage to the molded corner or panel unit.
8.1.2.2 Tees and Crosses—Tees permit the juncture of the perimeter member of a gasket to a muntin member. Crosses allow
for division of a glazed area horizontally and vertically. Crosses and tees can be injection molded as well as corners. The corner
angle conformation tolerance of 65° is also a requirement for tees and crosses.
8.1.2.3 Butted Joints—Where feasible, long sections of gaskets should be joined end to end by factory-injection molding. Where
field-butted joints are required, an application of sealant or adhesive recommended by the gasket manufacturer should be used in
the joint under compression to achieve weathertightness.
8.1.3 Lock-Strips—The purpose of lock-strips is to apply pressure to the gasket sealing lips, causing the lips to grip and seal
against both the frame and the glass or panel. Lock-strips may be separate from, or an integral part of, the gasket proper. The
separate lock-strip should be 10 points harder in durometer (A scale) than the gasket itself. The additional hardness of the lock-strip
resists deformation under compression and maintains the designed gasket lip pressure for longer periods than would a lock-strip
of equal size of a lower durometer.
8.1.4 Gasket Systems—A gasket system is produced when perimeter gaskets and muntin gaskets are assembled and designed
to mate with corresponding frame members as a total unit. Gasket systems are referred to as (a) supported, in which all the muntin
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AB —Horizontal members
CFG —Vertical member for lateral support
D —Horizontal or vertical perimeter member
EH —Horizontal or vertical members
FIG. 7 Special Interlocking and Proprietary Type Gaskets
FIG. 8 Gasket Corner Design Features
members are metal supported gasket members; or (b) unsupported, in which vertical muntin gaskets are unsupported by metal
members. Supported systems should be used where optimum performance is required.
8.1.4.1 H-Type System—An H-type system uses an H-type gasket for the perimeter as well as for the muntins. This system
permits using a glass panel and a spandrel panel, or a glass panel and a panel containing an operating window insert, in the same
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system. The gasket manufacturer should be consulted prior to designing such a system, to ensure that perimeter and muntin gaskets
are compatible and can be joined together to produce a favorable system.
8.1.4.2 Reglet-Type System—A reglet-type system typically includes a reglet-type gasket for the perimeter and a supported or
unsupported H-type gasket for the muntins. The reglet type systems require a greater control over the opening dimensions,
particularly if the opening is of concrete. The tolerances published by the gasket manufacturer for reglet-type systems should be
followed.
8.1.4.3 Stick System—A stick system incorporates gaskets that are straight lengths of extrusions cut to size and joined during
installation in the field. Manufacturer’s recommendations should be followed regarding the installation techniques to be used as
well as the type and location of adhesives or sealants to be used for the joints.
8.1.4.4 Ladder Assembly System—A ladder assembly system is produced by vulcanizing muntin gasket extrusions to perimeter
gasket extrusions with the use of an injection-molded Tee-joint. These may be vertical or horizontal ladder assembly systems as
shown in Fig. 9. Production experience shows that a ladder gasket assembly larger than 20 ft (6.1 m) in length becomes too difficult
to fabricate or handle. Field installation problems are also encountered. Where large areas are to be glazed using the ladder gasket
assembly, the assemblies are produced in conveniently sized sections, and joined in the field. Joining is accomplished by butting
under compression (“crowding”) the free ends of the gaskets together. A weathertight seal can be accomplished by the injection
of an appropriate adhesive or sealant recommended by the gasket manufacturer in the butted gasket joint after the unit is glazed
and lock-strip inserted. In horizontal ladder systems where the vertical gasket member is unsupported the vertical gasket is used
only as a weather seal and does not provide any significant structural support to the vertical glass edges. For vertical ladder systems
the horizontal gasket members must be supported (see 10.1.10.4).
8.1.4.5 Grid Assembly System—A grid assembly system is one in which horizontal and vertical muntin gaskets intersect within
the perimeter gaskets as shown in Fig. 9. Intersecting muntin gaskets are assembled by a molded joint. All grid systems should
be of the supported type.
8.1.5 Setting Blocks—The purpose of setting blocks is to provide positive support, but prevent direct contact between the bottom
of the glass or panel and the web of the sill gasket member. A certain amount of vertical edge clearance is required in order to install
the gasket without tearing the upper corners of the gasket. If the glass or panel were to be installed directly onto the web of the
sill gasket member, too much edge clearance would be left at the head member. With setting blocks, the glass or panel can be
lowered down to the web of the sill member for easier glass insertion at the head and then raised within the gasket enclosure for
insertion of setting blocks to provide the desired edge clearance space at the sill member before the lock-strip is installed. Factors
to consider in the use of setting blocks are (a) compatibility of materials, (b) location and quantity, (c) avoidance of support at the
gasket corners, (d) maximum pressure permitted on the gasket-bearing surface under the setting blocks, and (e) geometry of the
setting blocks with regard to the gasket, to the gasket cross section, and glass or panel width.
8.1.5.1 Material—The setting block material should be an elastomer of durometer (A scale) of 85 6 5, of rectangular cross
section, dimensionally stable, causing no interactive deterioration of it, the gasket, or the glass or panel infill and in compliance
with Specification C864.
8.1.5.2 Location—A continuous strip or two setting blocks at the quarter points are permissible, provided the maximum pressure
described in 8.1.5.3 is not exceeded. The continuous strip should be cut short of the width by 102 mm (4 in.).
8.1.5.3 Width—The width of setting blocks should be 1.27 mm (0.05 in.) less than the nominal glass or panel thickness. Less
width could result in improper support for the glass or panel. Greater width could result in a loss of weathertightness as illustrated
in Fig. 10.
8.1.5.4 Depth—The depth of setting blocks should be such as to vertically center the panel in the gasket opening thus equalizing
the clearances on top and bottom. However, edge clearance should not be increased to the extent that required push-out capacity
is jeopardized (see 10.1.1).
FIG. 9 Gasket Systems
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FIG. 10 Effect on Gasket Caused by Too Wide or Overloaded Setting Blocks
8.1.5.5 Length—The total cumulative length of the setting blocks should be a minimum of L as defined herein, and a maximum
of the opening width less 102 mm (4 in.), so as to ensure a minimum expansion of the gasket web at the pressure points.
L 5 W/PT
where:
L = total cumulative length of setting blocks, mm (in.),
W = weight of glass or panel, kg (lb),
T = thickness of glass or panel, mm (in.), and
P = bearing pressure of glass or panel on setting blocks, max = 103 kPa (15 psi).
8.1.6 Edge Spacers—When installing unsupported vertical ladder assembly gasket systems, installation of a continuous rubber
edge spacer is required between the head of each glass or panel and the web of the gasket muntin above. This procedure maintains
the muntin center line at its proper elevation during installation and to prevent the possibility of future settling of stacked lites. The
configuration and durometer (A scale) of the edge spacers should be as recommended by the manufacturer of the gaskets but no
less than that of the gasket.
8.1.7 Weepholes—Weepholes in the sill portion of the gasket can be used to drain away water that has entered between the
gasket lips and glass during driving rain and gusting winds. Glass manufacturers require that weepholes be used with insulating,
wired, and laminated glass, as water trapped in the gasket channel can have deterimental effects on such units. Weepholes in single
glazing is optional.
1 3
8.1.7.1 Size and Location—Weepholes, when used, should be 6.4 to 9.4 mm ( ⁄4 to ⁄8 in.) in diameter and at least three per
opening with one at the center and one at each end between setting block and corner. Whenever possible, provide the weepholes
in the web of the gasket so that the water can drain down into the frame and then be channeled outward, in preference to draining
outward through the flange of the gasket, as shown in Fig. 11. Weepholes in the flange permit unwanted water to drain outward
but can conceivably also permit water to enter from one side of the system to the other under certain conditions of pressure
differential.
...