Why a focus on unit size?
What exactly counts as “masonry”? The very nature of masonry construction is to bond and connect smaller individual units to form a larger element or structure. If the “units” are something small like individual grains of sand for instance, then the result would more closely resemble a stucco or parging. On the other hand, if the “units” are several metres in size then they would function more as individual panels . When do the units become too small, or too large such that they can no longer be considered masonry work? This is something that is clearly defined through the suite of masonry related CSA standards.
Masonry units that are considered to meet the definition of “Unit Masonry” are allowed to have a maximum height and length, of 200 mm and 400 mm, respectively, and a minimum thickness of 75 mm. Masonry units with dimensions greater than, or thickness lesser than those permissible dimensions may still be used in masonry construction; however, the suitability of these larger units for unit masonry construction methods, tolerances, and load transfer to structural backing must be confirmed through independent analysis and testing.
Part of our Masonry Specification Series
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Disclaimer
The information contained here is intended to serve as educational content for designers, specifiers, or contractors. It is not to be relied upon for formal technical advice, as masonry projects may have details and considerations that are unique to a particular project and may be beyond the scope of the content of this page.
Recommendations
If masonry units exceeding the dimension limitations in the CSA Masonry Standards (e.g. nominally 600 mm or 800 mm long units) are specified for a project, it is advisable to request information from the unit manufacturer on independent testing to confirm:
- The unit tolerances and physical properties meet the requirements of CSA A165-14; and
- The masonry construction methods and tolerances contained within CSA A371-14 can be achieved; and
- The loads can be transferred to the structural backing, including connector spacing and installation methods and differential movement details to meet CSA S304-14 and CSA A370-14.
It is also recommended to specify the construction and acceptance of mock-ups or samples panels to evaluate bond pattern, mortar, workmanship and construction tolerances, and general appearance.
Discussion
CSA S304-14, CSA A371-14, CSA A370-14, CSA A179-14, CSA A165.1-14, and CSA A82-14 limit the dimensions of units that are considered to be “unit masonry.” Furthermore, the main body of provisions in the design standard, CSA S304, and construction standard, CSA A371, are only applicable to unit masonry (minimum 75 mm thick, maximum 200 mm in height, and maximum 400 mm long). In the example below, a veneer unit is being specified that exceeds the dimensional limitation for unit length of 400 mm. The potentially problematic aspect of this specification is that a reference to CSA A371-14 is specified for the construction tolerances.
Example of potentially problematic specification:
2.03 MATERIALS
.1 Manufactured stone concrete masonry units: to CSA A165 Series-14
.1 Ground face stone units, with rectangular solid, modular units.
.2 Dimensions:
1. Manufactured widths: 90 mm.
2. Manufactured lengths: 590 mm
3. Manufactured heights: 190 mm.
3.03 CONSTRUCTION TOLERANCES
.1 Manufactured stone unit masonry tolerances in accordance with CSA A371-14.
The following discussion highlights the potential issues with applying CSA standards and construction practices to a masonry unit that does not fit within the dimensional limitations. In this case, the material standard is referenced for the concrete masonry units: CSA A165 Series 14, and the construction standard is referenced for tolerances: CSA A371-14.
Limitations on Unit Dimension
Design requirements for masonry veneers can be found in CSA S304-14 Design of Masonry Structures, Clause 9. Within the requirements, the Standard provides limits on both the unit material and unit dimensions (Clause 9.1.2) for Unit Masonry Veneer.
9.1.2 Unit material and dimension limitations
Unit masonry veneer shall be construction using clay (shale) masonry units, calcium silicate (sand-lime) masonry units, or concrete masonry units; the individual units shall be limited in height to not more than 200 mm, limited in length to not more than 400 mm, and limited in thickness to not less than 75 mm.
Note: Masonry units exceeding the specified maximum size limits may be considered to satisfy the requirements for unit masonry veneer, provided that independent testing confirms suitability for unit masonry construction methods, tolerances, and load transfer to structural backing.
The size limitations of masonry units have been harmonized throughout all of the material, construction, and design CSA Masonry Standards. Units within those size limitations are suitable for unit masonry construction methods and tolerances due to the extensive research and field experience that has been conducted on units within this size range as well as decades of satisfactory performance of structures featuring these materials. Units exceeding these size limitations are not covered by the standards and therefore the minimum requirements, performance, and tolerances may not be applicable.
Manufacturers are responsible for providing adequate testing to ensure that units which exceed the dimensional limits to unit masonry may still be designed and installed using masonry CSA standards. Test data, analysis, and relevant specifications shall be provided to the designer and masonry contractor to confirm unit suitability for unit masonry construction methods, tolerances, and load transfer to structural backing. This article provides some of the possible issues that should be considered for testing but does not represent a comprehensive list.
Construction Tolerances
Frequently, specifications indicating the use of masonry units larger than the definition of unit masonry also call for the application of tolerances contained in CSA A371-14, that may not be possible to meet with the larger masonry units. In particular, designers often comment on the alignment between adjacent units, noticeable under coincident lighting (e.g. from a surface-mounted light fixture), casting unwanted long shadows and giving the appearance of poor workmanship.
Masonry CSA standards’ requirements and tolerances may be applicable to units exceeding the dimension limitations if the manufacturer of the masonry units has completed independent testing for this unit to confirm items including:
- The unit tolerances and physical properties meet the requirements of CSA A165-14; and
- The masonry construction methods and tolerances contained within CSA A371-14 can be achieved; and
- The loads can be transferred to the structural backing, including connector spacing and installation methods and differential movement details.
In all cases of masonry construction, it is recommended that designers should specify the construction of a “mock up” wall or “sample panel”, as a segment of a masonry assemblage or a stand-alone panel (CSA A371-14, Annex A) before the start of general masonry construction. This should be done in addition to any testing provided by the manufacturer.
Non-diffuse light shining onto a masonry wall
Designers should be aware that while coincident lighting from a surface-mounted light fixture or sconce may be used to enhance the architectural features of rough or textured units, accenting the shadows created by the 3-dimensional geometry of the surface of the rough masonry wall (e.g. rough stone walls), this same effect may unexpectedly occur with coincident lighting on a smooth masonry wall surface. In such cases, minor construction imperfections (even those that may be within agreed construction tolerances), especially when long masonry units are used, may cast unwanted long shadows and give the erroneous visual appearance of unacceptably poor workmanship. If the resulting shadows on the masonry veneer cause any aesthetic concerns, it is suggested that a change to the lighting will achieve the desired aesthetic while maintaining cost effectiveness and durability for the intended building function.
Load transfer to structural backing
The final consideration for veneer units would be the location and connections of the veneer to the back-up wall and the design of masonry ties. The spacing of ties selected based on their engineering design must consider the new unit dimensions and corresponding modular spacing available for placement of ties. Units that are taller or longer than unit masonry may require special considerations when designing tie systems that have non-standard spacing, increased stiffness or strength, and may require some type of mechanical fastening to the unit. Dimension cut stone, as an example, is a type of masonry unit that may exceed the dimensional limits for unit masonry that have special provisions provided by CSA S304 Appendix A. These provisions provide designers with a good background to how larger units may be accommodated.
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Part of our Masonry Specification Series
Offering recommendations accompanied with background explanatory material to explain how these recommendations were formed. Click here to see the full series.
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As part of our overall effort to educate the design community and our contractor members, CMDC has compiled a collection of articles meant to address common areas of misunderstanding or confusion that have resulted in issues in past projects.