Wheelchair ramp: 7.40 m of path is not enough
A community hall needs a wheelchair ramp to its front door. The door threshold is 540 mm above the concrete path. The hall committee has 7.40 m of straight path available from the footpath to the door, and the ramp must finish with a 1.20 m flat landing at the top, directly in front of the door. The New Zealand accessible design standard requires a maximum gradient of 1:12 for a ramp of this height, and a level landing at the top. The ramp surface will be timber decking 1.20 m wide, and the committee has a quote of $96 per square metre of ramp surface (measured along the slope). The treasurer says, '540 mm rise needs 540 × 12 = 6.48 m, so 7.40 m is plenty.' The builder is not so sure.
1. Check whether the ramp can meet the 1:12 gradient with the space available. Show the required run, the available run, and the actual gradient. State how far short the path is, or how much the threshold would need to be lowered.
Read the worked answer
Available horizontal run for the ramp = total path − top landing = 7.40 − 1.20 = 6.20 m.
Required run at 1:12 = rise × 12 = 540 mm × 12 = 6,480 mm = 6.48 m.
6.20 m < 6.48 m, so the ramp does not fit at the required gradient. Actual gradient = 540 ÷ 6,200 = 0.0871, which is 1:11.5, steeper than 1:12. The treasurer has ignored the landing in the 7.40 m; 7.40 m is only 'plenty' if the landing is forgotten.
To comply, the committee needs 6.48 + 1.20 = 7.68 m of total path, so it is 280 mm short. Alternatively, the threshold would need to be lowered to 6,200 ÷ 12 = 516.7 mm, a reduction of 540 − 516.7 = 23.3 mm.
2. Calculate the slope length and the area of decking needed for a compliant ramp, then find the cost at $96 per square metre. Use the compliant dimensions.
Read the worked answer
Using the compliant run of 6.48 m and rise 0.54 m, slope length = √(6.48² + 0.54²) = √(41.9904 + 0.2916) = √42.282 = 6.5025 m.
Area of decking = 6.5025 × 1.20 = 7.803 m².
Cost = 7.803 × $96 = $749.09, so about $749 (plus waste, fixings and labour if not included).
Using the short 6.20 m run would give slope length √(6.20² + 0.54²) = 6.223 m, area 7.468 m² and cost $716.93, but that ramp would not meet the 1:12 standard, so the $749 figure is the relevant one for a compliant design.
3. The committee must choose a fix. Compare the options of extending the path or lowering the threshold, and recommend one with reasons. Include the numbers that support your recommendation.
Read the worked answer
The committee has two realistic fixes:
- Extend the path by 280 mm so the ramp run is 6.48 m. This keeps the door at its current height but may affect the footpath, drainage or boundary. It also adds a little decking cost.
- Lower the threshold by about 24 mm (to 516.7 mm or less), which reduces the required run to 6.20 m. This is a smaller change but may involve altering the door, floor level or weathertightness.
A third option, a steeper ramp, is not acceptable because it would breach the 1:12 maximum and be unsafe or non-compliant.
Recommendation: check whether the path can be extended by 280 mm first, because it preserves the building and is a straightforward construction change. If not, lower the threshold. In either case, confirm the final gradient and landing with the building consent process before ordering materials. The treasurer's arithmetic is right about 6.48 m but wrong to ignore the 1.20 m landing.