Truncated Domes
ADA 705.1 Truncated Dome Dimensions Spec (2026)
Cojo
May 7, 2026
6 min read
ADA Standards 705.1 sets the geometry requirements for every truncated dome installed under federal law: base diameter 0.9 to 1.4 inches, top diameter 0.45 to 0.9 inches and 50 to 65 percent of the base diameter, height 0.2 inches, center-to-center spacing 1.6 to 2.4 inches, and base-to-base spacing of at least 0.65 inches. Any dome that misses any of those five dimensions is non-compliant on day one regardless of contrast, placement, or material selection.
This guide unpacks each ADA 705.1 sub-requirement, shows how to verify it at install with a caliper and scale rule, and lists the common errors that cause an inspector to reject a dome surface.
ADA Standards for Accessible Design, Section 705.1, defines the geometry of the truncated-dome surface used as a detectable warning. The standard is incorporated by reference into 36 CFR Part 1191 and applies to every public-use detectable warning under U.S. federal law.
The dome base measures 0.9 to 1.4 inches in diameter. The base is measured at the walking surface, where the dome meets the substrate. A dome with a 0.85-inch base or a 1.5-inch base is non-compliant.
The dome top measures 0.45 to 0.9 inches in diameter, AND must be 50 to 65 percent of the base diameter. The top-to-base ratio is the part most commonly missed in spec verification. A dome with a 1.4-inch base must have a top between 0.7 and 0.91 inches to satisfy both the absolute and proportional requirements.
Each dome rises 0.2 inches above the walking surface. Height is measured from the substrate to the highest point of the dome. A dome that is 0.15 or 0.25 inches tall is non-compliant.
Dome centers are 1.6 to 2.4 inches apart, measured in the dominant direction of pedestrian travel. The same spacing applies in the perpendicular direction.
The bases of adjacent domes must be at least 0.65 inches apart. This requirement prevents the bases from merging or running into each other on densely-spaced tile patterns.
A two-person crew can verify ADA 705.1 dimensions on a typical 24-by-48-inch curb cut in under 10 minutes with three tools.
A 6-inch dial or digital caliper measures dome base, dome top, and base-to-base distance directly. Use the inside-jaw measurement for base-to-base, the outside-jaw measurement for base diameter, and a depth probe for top diameter where the calipers cannot fit.
A depth gauge or a flat scale rule across the dome row measures dome height. Set the rule across two adjacent dome tops, then measure the gap between the rule and the substrate at the dome center. The gap should equal 0.2 inches when the dome top is 0.2 inches above the substrate.
A standard scale rule measures center-to-center spacing. Use the etched centerline of each dome as the reference. Most installers verify a 5 by 5 dome grid (5 in each direction) and accept the result if all 25 measurements fall in spec.
Verify at least 6 domes per panel and at least 1 row in each direction. ADA inspectors increasingly verify a full grid, so the install crew should plan for full-grid verification on public-bid jobs.
These are the dimensional failures that show up most often on Cojo retrofit jobs.
A panel with a 1.4-inch base and a 1.0-inch top fails the 50 to 65 percent ratio (1.0 / 1.4 = 71 percent). The absolute dimensions can be inside the absolute range, but the ratio still fails. Manufacturers occasionally publish panels in this configuration; the spec sheet number does not always match the molded reality.
A new panel at 0.2 inches that wears to 0.15 inches at year 8 fails ADA 705.1 height. ADA 705.5 wear performance is what catches this on inspection. Composite panels in high-traffic environments are the most likely failure mode.
Molded panels can have spacing variation of 0.05 to 0.15 inch across the surface due to mold flex during cure. A panel with center-to-center spacing of 1.55 inches in some rows and 2.45 inches in others fails ADA 705.1 in both directions of variation.
Densely-spaced budget panels sometimes have base-to-base spacing under 0.65 inches. Verify with a caliper. A panel that fails this requirement cannot be made compliant by any installation technique.
Even a perfectly molded panel fails ADA 705.1 height if the substrate is out of plane. A panel installed over a substrate that dishes 0.1 inches across the panel width will measure 0.2 inches at high points and 0.3 inches at low points. Substrate levelness is part of ADA 705.1 compliance even though the standard is silent on substrate.
Reputable manufacturers publish a spec sheet with the following information:
| Spec line | Required value |
|---|---|
| Dome base diameter | 0.9 to 1.4 inches (specific value per product) |
| Dome top diameter | 0.45 to 0.9 inches and 50 to 65 percent of base |
| Dome height | 0.2 inches |
| Center-to-center spacing | 1.6 to 2.4 inches (specific value per product) |
| Base-to-base spacing | 0.65 inches minimum |
This article reflects ADA Standards for Accessible Design as of 2026-05-07 and product spec sheets current at publication. Always verify current dimensions, contrast thresholds, and placement requirements with your local jurisdiction and the U.S. Access Board before issuing a final spec. Federal guidance under 36 CFR Part 1191 controls when state or local rules conflict. Manufacturer spec sheets are evidence of design intent, not proof of installed compliance — caliper verification at install is required.
On a Salem state-government retrofit in March 2026 we received a shipment of polymer-concrete panels from a manufacturer's distributor. Quick caliper check on the second pallet revealed center-to-center spacing of 1.55 inches in 4 of 12 rows on a sample panel, which fails ADA 705.1. We pulled the entire pallet, returned it under the manufacturer warranty, and shipped a replacement that cleared the caliper test. Pre-install verification saved roughly $4,800 in re-install costs and a week of inspection delay. Field verification is not optional.
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