To estimate the painted area of corrugated asbestos cement sheets, we need to determine the percentage increase in area above the painted area. Let's break down the steps to find the correct answer:

1. Understand the problem:
- We are dealing with corrugated asbestos cement sheets.
- We need to estimate the painted area.
- We need to find the percentage increase in the area above the painted area.

2. Determine the painted area:
- Corrugated asbestos cement sheets have a specific shape and dimensions.
- The painted area refers to the portion of the sheet that is covered with paint.
- We can calculate the painted area by multiplying the length and width of the sheet.

3. Calculate the total area:
- The total area of the sheet includes both the painted area and the area above it.
- To determine the total area, we need to add the area above the painted area to the painted area itself.
- The area above the painted area can be calculated by subtracting the painted area from the total area.

4. Find the percentage increase:
- The percentage increase is the ratio of the increase in area above the painted area to the painted area, expressed as a percentage.
- We can calculate the percentage increase by dividing the increase in area by the painted area and then multiplying by 100.

5. Determine the correct answer:
- Comparing the given options, we find that option 'C' (20%) is the correct answer.

In summary, to estimate the painted area of corrugated asbestos cement sheets, we need to find the percentage increase in the area above the painted area. The correct answer is option 'C' (20%).