Carpentry projects must be square, level, and plumb to be successful. Square means corners are 90 degrees. Level is always gauged by a device, such as a carpenter's level, but level does not always mean parallel to the ground. Plumb is vertical, most accurately gauged by a plumb bob. Levels can show when posts are plumb.
Assuring that your elements are square, level, and plumb increases the structural integrity of your project. Make sure your squaring and leveling tools are in good shape, and protect them from damage at the work site. If you suspect a tool is inaccurate, check it against a known good one. If you can't repair or adjust it, purchase a new one.
Position the framing square on an inside or outside edge of the joint and look along the lengths of both the tongue (short side) and the blade (long side). The tongue and blade should fit flush against the surfaces along their entire lengths. If you see light anywhere along either edge of the square, reposition the boards, pulling or pushing one or both of them until the framing square fits snugly. Don't be satisfied if only the corner of the square is tight. The corner is not true unless the square fits snugly along the entire lengths of the tongue and blade.
Use a carpenter's level that is 4 feet long to level and plumb your construction whenever you have room for its length. Shorter levels may be affected by warps or waves in the boards themselves and might not prove as accurate. Boards are level or plumb when the bubble is centered in the appropriate vial.
Technology has improved leveling devices so much that they make the task virtually goofproof. For a modest investment you can purchase an automated water level—it beeps when the water is stabilized in the tube. Or you can buy a laser level that indicates level and projects a visible level line across long distance. A few years ago tools like these were only for the pros. Today's prices put them within reach of the average homeowner.
To level on-site, just extend the length of your level with straight boards. Get in the habit of checking each piece as you install it and use the widest board possible—narrower boards (like 2x4s) may flex and give you a false reading. Center the level on the board to minimize the effect of flexing or crowning.
When you need to level objects or surfaces within 6 to 8 feet of each other, a carpenter's level set on a straight board will do. But you can make almost any leveling task easier and more accurate with a water level (available at many hardware stores). Essentially two pieces of clear plastic tubing that fasten to the ends of a hose filled with water, this tool relies on the principle that water will seek its own level over any distance. Hold the ends of the level against both surfaces, and mark each board at the water line.
Posts have to be plumb in two directions, and you can use a carpenter's level on both sides to plumb them. Make this job easier by strapping on a post level. It's specifically designed to plumb posts in both directions at once.
Before making a crosscut, make sure the end of the board you measure from is square—otherwise one edge of the board will be longer than the other. Check the end with a layout square or combination square on narrow stock; hook a framing square on the outside edge of wider boards. Mark the end and cut it square if necessary.
Corners of your layout have to be square or the deck will be out of square. On small sites, such as a concrete pad at the bottom of stairs or a small deck extension for a barbecue, you can lay out square corners with a 4x8 sheet of plywood. For larger sites use the 3-4-5 triangle based on the Pythagorean theorem: The square of the length of the longest side of a right-angle triangle equals the sum of the squares of the lengths of the other two sides. The longest side of a triangle with sides 3 and 4 feet long is 5 feet long.
To check a corner angle, mark a point 3 feet from the corner along one side and another point 4 feet from the corner along the other side. Measure the straight distance between the points; if it's 5 feet, the corner is square.