Things You'll Need
Large wooden stakes
2x6 boards about 30 percent longer than the height of your porch eaves, enough of them to put two every five feet around the porch.
2-inch wood screws
3-inch wood screws
Replacing damaged or rotted support columns on your porch is hard enough work without having to wonder if the roof is going to collapse on you. Don't take chances. By properly supporting the porch roof with temporary but reliable bracing, you can go about your project without worry. The bracing will take an afternoon, and give you as many stress-free working days as you need.
Measure the height from the ground to the eaves under your porch. Then measure on the ground out in front of the porch from one corner, taking the tape measure to half of whatever the height of your eaves was. Put a stake there in the grass. Repeat from the other end of the porch.
Measure between the two stakes. If the length is 10 feet or less, measure out from the center of the length, using the same number as before, and put a stake there. If the span is more than 10 feet but less than 15 feet, divide the length by three and put two evenly spaced stakes at the division lines. If you have an extremely long porch, measure out and put down stakes every five feet or less.
Measure out from either side of both corners of the porch, at the same number, and put a stake there.
At each stake, use your spade to dig a hole eight to ten inches deep, with a flat vertical wall on the side furthest from the porch.
Have your assistant get on the ladder and hold the tape measure under the eaves where they meet the horizontal support beam that the columns sit under. Take the other end of the tape to the bottom outside corner of the hole.
With your circular saw, cut twice as many 2x6s as there are stakes, at the length of the measurement you just took. On one end of each 2x6, use your tri-square to mark a line from the top corner of the 2x6 to a mark 2-3/4 inches in from the bottom edge. The mark should look like an elongated triangle on the end of the board. Cut each board at that mark.
Lay one board on top of another, matching the cut angles. Connect them using pairs of 2-inch screws sunk into the face of the top board, with a pair of screws every two feet. Repeat for all the pairs of 2x6s.
Stand your first brace against the porch, with the narrow edges of the joined 2x6s facing up and the cut angle facing toward the porch in a vertical position. Put the uncut bottom into the hole and drive a stake in front of it to help hold it in place.
Tuck the angled side of the brace under the eaves, with the cut part pressing vertically against the support beam that meets the columns. Screw the brace into the beam with 3-inch screws, driving them into the side of the board at an angle. Repeat for each brace, until they're all in place. Make sure they're tight. You can now proceed to take out the columns.
Wear eye protection when using your circular saw.