In major home improvement or other carpentry-intensive projects, determining if something is square or in-plane is critical to the overall integrity of what you are building. A relatively small error in these relative measurements can make a huge negative difference in the finished product, so that is why it is important to know how to test them. Below is how a do-it-yourselfer can use a few common hand tools to determine if something is square or in-plane:
Determining square with the use of a tape measure and pencil
One of the most basic carpentry techniques is building a frame using dimensional lumber. Each corner of the frame must be 90 degrees, or the deviation from a right angle will be magnified many times over, especially if the project is large or complex. Determining if it is square can be easily done with a tape measure and pencil by applying the mathematical principles first explained by the ancient Greek mathematician, Pythagoras.
Begin by laying out the pieces of dimensional lumber to be joined, so the corner between the two pieces makes a visually-correct right angle. Join the pieces with a single 10d nail in the corner, but be sure not to fasten the corner too firmly at this point.
Once fastened together, begin by placing the end of a tape measure at the corner you made and extend it alongside one leg of the frame; measure to 3 feet and make a small pencil mark on the outside edge of the leg. Repeat this same process alongside the other leg of the frame you made, but measure to 4 feet this time and make a mark accordingly.
Next, place the end of the tape measure at one mark you made and extend the tape out across the corner of the frame until it crosses the opposite mark. If you have assembled the frame with a perfectly square corner of 90 degrees, then the measurement will indicate 5 feet exactly between the two marks. If there is a discrepancy of more or less than 5 feet, then bend the two legs of the frame until the measurement is exactly 5 feet; once you achieve that, attach a crosspiece of 1-inch by ½-inch stake material to each leg of the frame to hold it in position. The crosspiece can be removed once the frame has been made rigid as the construction process continues.
Ensuring two end-connected pieces of lumber are in-plane
Abutting the ends of dimensional boards is a necessary step in some construction work. For example, if you need to add two boards end-to-end to extend the working length, then abutment is necessary. However, the entire length should be in-plane to ensure working accuracy. Making a determination regarding whether two abutted boards are in-plane can be done with the use of two framing squares.
Begin by laying the boards on a large, level working surface and abutting the ends to each other. Next, place framing squares, with legs that are at least 2 feet in length, at each end of the boards so the two vertical sides are facing each other. From the side, the framing squares will take on the appearance of two letter "L's" back-to-back.
If the boards are in-plane, the backsides of the framing squares will touch each other down their entire length. If there is any gap between the framing squares, then this indicates a situation where the boards are not in-plane with each other. A gap at the bottom of the squares indicates the boards are bowed upward at each end, while a gap at the top of the squares indicates the centerpoint is higher than each end.
To correct a pair of boards with high ends, insert shims underneath the center of the boards and continue checking for in-plane until the problem is corrected. Reverse the procedure for a pair of boards with low ends by stacking shims underneath end of the board. Once the boards are in-plane with one another, use a strip of 1-inch by ½-inch stake material and a couple of 10d nails to fasten them together temporarily.
If you don't have the tools to carry out these directions, then check out a company like Bourget Bros Building Materials.