Dimensional Tolerances in Construction and for Surface Accessibility

2 - Specifications

Specifications provide an ideal place to prevent many of the problems associated with construction tolerances. Specifications allow the requirements to be stated as succinctly or as elaborately as required. The four items that should be communicated as stated in the introduction to this guide can clearly be defined in the specifications with corresponding dimensions shown on the drawings. These include: (1) tolerance requirements (what is wanted), (2) the standards used, (3) how compliance will be verified, and (4) what the result of noncompliance will be.

If specific tolerances or requirements are not stated in the specifications (or drawings) it is generally held by the courts that industry standards apply. However, many industries do not have tolerance standards or the problematic element may be part of a larger assembly for which there are no standard tolerances, the placement of a toilet, for example. When tolerances do not exist and there are no clear standards, disputes arise and the courts may decide the issue, with the accompanying cost and time implications for the design professional. If there are conflicts between drawings and specifications, specification requirements generally override the drawings.

Current practice
Nearly all architects use the 3 - part format, developed by the Construction Specifications Institute, for writing individual specification sections. The 3 - part format has places to include all requirements related to tolerances and measurement protocols. One or more of them should be used as needed to describe the requirements of a project. These include the following.

• References to industry standards: Part 1, References

• Required test reports (third - party verification) and similar documents: Part 1, Submittals

• Mockup requirements: Part 1, Quality assurance

• Regulatory requirements and pre - installation meetings: Part 1, Quality assurance

• Shop fabrication of elements: Part 2, Fabrication tolerances

• Special techniques and interface with other work: Part 3, Construction requirements

• Final, installed tolerances: Part 3, Site tolerances

Many manufacturers and trade groups have guide specifications that include their product's tolerances. In addition, master specifications, such as SPECTEXT® and MasterSpec® include tolerances in many of their sections.

2.1 Best practices for stating tolerances in the specifications

2.1.1 Clearly state the required installed tolerances for critical construction elements. In most cases, it is best to refer to industry standard tolerances or other industry documents when they exist. For example, ACI 117, Specifications for Tolerances for Concrete Construction and Materials and Commentary can be included without having to list the hundreds of individual tolerances given in the standard. If tolerances stricter than those given in an industry standard are required for a specific project, these should be stated, with the recognition that tighter tolerances may increase construction cost or time or both.

2.1.2 State the required tolerances for elements for which there are no industry standards. These may also include elements for which there is an industry tolerance for one aspect of the element but not others. For example, a product may have a manufacturing tolerance and an installed positional tolerance but not have an orientation tolerance for plumb.

2.1.3 State requirements for critical accumulated tolerances. These are instances where individual products and installation procedures may conform to industry standard tolerances but the final installed element may not meet regulatory, functional, or aesthetic requirements.

2.2 Best practices for stating standards

2.2.1.

 List all applicable industry standards that define tolerances and measurement protocols, if any.

2.2.2 Set requirements for independent testing agencies that may be required to perform measurement for compliance. Specify the measurement tools that should be used to determine compliance with the tolerances.

2.3 Best practices for stating how compliance will be verified

2.3.1 Define the measurement protocols to be used to check for compliance with the tolerances and standards. This may include where measurements are made, how many should be made, and the number or percentage of measurements that must fall within the limits to be considered acceptable. For example, specifications may include limits on the number of local variations (dips and humps) on an accessible ramp to a maximum of 10 percent slope for no more than 20 percent of the measurements taken. If industry standards exist for measurement protocols, these should be referenced.

2.3.2 Define the measurement tools that should be used. The acceptable tools should be selected based on the accuracy required and a reasonable balance between the accuracy of the tool and the time, cost, and experience required for measurement. For example, floor profilers can provide a very accurate measurement of surface flatness but are expensive and require training to use. For exterior surfaces, a surveyor's transit or digital inclinometer may provide acceptable measurement for the same task.

2.4 Best practices for stating the result of noncompliance

2.4.1 Based on the methods for measuring compliance, define what remedial actions are acceptable when construction elements exceed tolerances. These may include total replacement, partial replacement, adjustment, moving, filling, patching, or other operations as appropriate for the construction element.

2.4.2 When defining acceptable methods of correction for a non - complying element, give reasonable consideration to how the remedial action may affect construction time, cost, adjacent construction, and appearance. If possible, give the contractor options for how the element may be brought into compliance.