How To Build Stairs 2

Use the stairwell length formula to calculate the stairwell opening. See figure 44B. Stairwell length = Unit Run divided by Unit Rise times the sum of the headroom and floor frame plus 75 mm.

Calculate the unit rise first. The total rise is 2100 mm. The desired rise is 180 mm. Calculate the unit rise. 2100 divide by 180 = 11.67. It is rounded off to 12. 2100 divided by 12 = 175, the unit rise. 175 mm is an acceptable rise because it falls between the building codes upper and lower limits of 200 mm and 125 mm. The unit rise will be 175 mm.

Calculate the unit run. 175 + 175 = 350. 550 - 350 = 200. 700 - 350 = 350. The unit run limits in the building code are 355 and 210. Select a unit run between (and including) 210 and 350. The unit run will be 250 mm.

The headroom is 2000 mm. According to the1998 B.C. Building Code Section 9.8.3.4. Page 281, headroom in dwelling units must not be less than 1.95 metres and 2.05 metres in other buildings.

The floor frame thickness is the sum the floor joist vertical measure of 255 mm and the finish and subflooring measure of 30 mm. The floor frame thickness is 285 mm.

Stairwell Length = 250 mm divided by 175 mm times (2000mm + 285 mm) + 75 mm = 3339 mm.