Dimensional Tolerances in Construction and for Surface Accessibility

1.2 Suggested tolerances

1.2.1 Walks and other non - ramp surfaces. When overall running slope for walks is measured according to Section 1.1.3 a recommended tolerance for running slope is +1%. When overall cross slope for sidewalks is measured according to 1.1.4 a recommended tolerance for cross slope is +0.5%.

1.2.2 When flatness of running slope for an accessible surface other than a ramp is measured according to Section 1.1.5 no more than 20% (rounded to the nearest whole number) of the measurements should exceed ±1/4 in. in 10 ft (±6 mm in 3 m). When flatness of cross slope for an accessible surface other than a ramp is measured according to Section 1.1.6 at least 80% (rounded to the nearest whole number) of the measurements should not exceed a 2% slope. The remaining measurements should not exceed a 2.5% slope.

1.2.3 Landings. Both measurements of ramp landings as described in Section 1.1.15 should not exceed a plus tolerance of 0.5%.

1.2.4 When local horizontal discontinuities and vertical alignments are measured according to Section 1.1.9 a recommended tolerance is ±1/8 in. (3 mm).

1.2.5 Ramps. When overall running slope and cross slope for accessible ramps are measured according to Sections 1.1.11 a recommended tolerance for these slopes is +0.5%.

In the ideal case, planning for a 7.5% running slope allows for construction inaccuracies while still maintaining the required 1:12 slope. However, when a design slope of 1:12 is indicated a tolerance of +0.5% is reasonable.

Many accessibility experts consider a 2% cross slope to be the maximum. However, there is conflicting research concerning the need to have a 2% maximum cross slope and that the actual maximum depends on user type (wheelchair, walker, cane, etc.), length of travel, and other variables. It seems reasonable to allow a +0.5% tolerance for ramp slopes and cross slopes.

1.2.6 When local variations (flatness) in running slope of ramps are measured according to 1.1.13 at least 80% (rounded to the nearest whole number) of the measurements should not exceed an 8.3% slope. The remaining measurements should not exceed a 10% slope.

Allowing a small percentage of localized slopes to exceed 8.3% is based on the allowable slopes in ADA/ABA - AG (2004) for existing buildings of 1:8 (12.5%) for maximum rises of 3 inches and 1:10 (10%) for maximum rises of 6 inches. The 1980 ANSI A117 standard also allowed this with the additional provision that an existing ramp slope of up to 1:8 could have a maximum run of 2 feet (0.6 m). Allowing 20% of local variations to slope up to 10% seems reasonable for a distance of one foot. This would mean that localized dips and high points in a 2 - foot distance would be about ¼ in. (6 mm) or a little less.

1.2.7 When local variations (flatness) for cross slope of ramps are measured according to 1.1.14 at least 80% (rounded to the nearest whole number) should not exceed a 2% slope. The remaining measurements should not exceed a 2.5% cross slope. When four or fewer measurements are made, only one should not exceed a 2.5% cross slope, while the others should not exceed a 2% slope.

1.2.8 Exterior stairs, cast - in - place. When cast - in - place exterior stairs are measured according to Section 1.1.17 the requirements of the local building code shall govern tolerances.