How to Reinforce the Floor for Spiral Stairs

To install spiral stairs, you must first cut holes through the floors that the stairs will pass through. For holes that are a large enough size for the staircase, you’ll need to cut through the joists in the floors, which will weaken the floor structure. To avoid a failure in the flooring around the stairs, reinforcing the floors is necessary. Additional bracing beneath the floors will provide you with the reinforcement you need.

Tape measure
Chalk
Straightedge
Circular saw
Pry bar
2-by-4 inch planks
Wood screws
Drill
Screw bits
Reciprocating saw
2-by-8 inch planks
Framing nailer
Framing nails
Joist hangers

Instructions

- 1
  Mark the location on your floor for the stairway hole with chalk and a straightedge. The hole size depends upon the stairway so measure the dimensions of the stairway to determine the size of hole to cut.
- 2
  Cut the hole in the subfloor with a circular saw set to the subfloor depth. You do not want to cut through the floor joists at this time. Remove the subfloor from the hole to reveal the joists. Pry up the cut section of subfloor with a pry bar if necessary.
- 3
  Go beneath the floor to the location of the hole. Measure the distance between the subfloor with the hole in it and the floor beneath with a tape measure. Cut 2-by-4 inch planks to match the measurements. Cut a plank for each of the floor joists that the hole in the floor overlaps.
- 4
  Prop up the 2-by-4 planks beneath the subfloor against the joists to serve as temporary reinforcements. Screw the planks to the joists just outside the hole’s location with wood screws and a drill equipped with a screw bit.
- 5
  Cut the joists 3 inches from the edge of the hole with a reciprocating saw.
- 6
  Determine the length of the space between the cut joists along the hole. Cut four 2-by-8 inch planks to match the measured length. Place two planks on either side of the hole against the joists so that they’re flush with the hole’s side. Use a framing nailer to drive nails through the double planks and into the ends of the cut joists to reinforce the floor.
- 7
  Measure the length of the room and cut four more 2-by-8 inch planks to match the room's length. Place two of the boards against the joists that run closest to the hole to create an additional double joist on the opposite side of the board to the hole. Use the framing nailer to drive the boards into place at the existing joist to provide additional support and space the nails every 10 inches along the length of the boards.
- 8
  Place joist hangers on the ends of the cut joists against the double joists you just installed. Nail them into place with finish nails to add a bit more support to the cut joists. Remove the 2-by-4 inch temporary supports.
- 9
  Repeat the reinforcement process for all other floors that the stairway will pass through.

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