Pythagoras knew it.
Building something square is essential. While it’s something you concentrate on in the early stages of a project, it will pay off later in tons of intermediate and finishing stages.
Now, naturally square doesn’t mean that all sides are of equal length. Square means, for example, that the four outside angles of a building or deck are 90 degrees — or add up to 90 degrees in the case of beveled or rounded corners.
If a floor is not framed squarely, eventually the subfloor sections won’t line up perfectly with the joists. The finish floor will have problems, too. Then there’s the roof. Even if your walls are perfectly plumb, if they’re built on a foundation (and/or floor) that’s not square, roof sheathing and — most noticeable — shingles will eventually appear as if they’re wandering off on an angle.
Walls not built square will, well, lean sideways. That means the next wall will lean, too.
Here are some ways you can ensure your projects are square.
To determine how square your foundation is, or the precise placement of your joist headers, measure diagonally from opposite corners. When the distance is equal, the line between the four points is square.
If you’re building a wall on the ground, and the above method shows you’re out of square, whack it a few times at the offending corner. (With a big wall, a sledge hammer helps.) Once you do have it square, nail braces across it, or nail on your sheathing to keep it that way.
The sheathing itself can help. Since we presume it’s square, use it to align the studs and the plates on square.
Pythagoras is also a big help. With his help, we know that if you measure 3 units along one wall, and four units along another, the distance from these two points across the floor will be 5 units. The bigger the units, the greater your chances of accuracy. So in the case of squaring up walls as you erect them, use as much of the walls as you can. If you have a couple of eight-foot-long walls, measure six feet along one, eight along the other, and the distance between the points should be 10 feet.