Finishing a Basement 4 - Laying Out Walls
The concrete walls in your basement are not going to be plumb; they will likely have bulges in them and the corners probably aren't going to be 90-degrees. Your challenge will be to lay-out and frame of your basement walls so that you will create with plumb walls and 90-degree corners.
In most houses, the main support beam is straight, which allows it to serve as your reference point. Drop a plumb bob from the side of the beam and mark the floor directly under the bob. Now measure from the mark to the concrete walls in a number of places to determine where the wall is closest to the mark.
Working inward from that closest spot, measure the width of your framing lumber (a 2x4-inch board is 3.5-inches" wide) and add an extra inch. That spot will be the position of the back of your bottom plate. Leave extra space (at least 1-inch) behind your finished walls to allow air flow.
Follow the same process then add 1-inch plus the width of the framing lumber to determine the location of the walls that run at 90-degrees and mark them. Use a chalk line to join the mark where the walls will actually be positioned.
To ensure that your corners are true 90-degrees, measure back 3-feet from where the two chalk lines intersect and make a mark. At an approximate 90-degree angle, measure 4-feet and make a mark on the other chalk line. If you have a true 90-degree corner, the distance from the 3-foot mark on the chalk line to the 4-foot mark on the other line will be exactly 5-feet.
If it's not, move the measure until the long measurement is exactly 5 feet to form a perfect 90-degree corner. Use the same process to lay out all four corners in your room and your building job will be greatly simplified.