Wood beams used for structural frames or supporting posts are often notched to make strong joints with other beams. Cut notches into the ends or the middle, depending on where the beams join. Half-lap joints are a common way to join posts to beams for deck rails or beams to posts on arbors and pergolas. In all cases, a snug fit is the hallmark of a strong joint. Loose or sloppy-fitting joints lack strength and may fail over time.
Measure the width of the two beams. Mark the width of the first, overlapping, beam on the second beam with a square, drawing both lines across the width. Turn the second beam on edge and transfer the width lines to the edge. Do this for both sides. Repeat this process on the other beam if it will be notched.
Determine the depth of the cut -- cut away no more than half of a beam's thickness. If two beams are of equal thickness, notch halfway through both beams to make the beams flush with each other at the joint. Mark the cut depth between the marks on the sides. Cut away less material to preserve more of the beam's strength, which is important if the joint will not have other support.
Set the cut depth on the circular saw to the cut depth marked on the side of the beam. Cut across the beam just inside both marks and preserve the lines -- you can always widen the notch if necessary but you can't make a notch narrower. Make multiple cuts approximately 1/4 inch apart for the entire width of the notch. Complete this process for both beams if they will both be notched.
Chisel away remaining material and make the inside faces of the notches smooth. Do this for both beams if you are notching both. Test fit the joint and make adjustments using the circular saw to widen a notch and the chisel to make adjustments to the inside of the notch face.
Ideally, the chisel should be the same width as the notch, but often this is impractical. Select 2-inch-wide or 3-inch-wide chisels for smooth notch faces. If the look or fit doesn't matter, don't notch the beams flush with each other; this removes a lot of material and strength from the beams. A deck post usually requires a notch only on the post and doesn't need a corresponding notch on the beam.
Never notch an existing structural house beam to make room for plumbing, duct work or other systems since it will compromise the structural integrity of the beam. Wear appropriate safety gear while using power tools, including safety glasses, a dust mask and ear protection. Do not wear loose clothing. Secure long hair. Keeps fingers, hands and body parts away from spinning or moving blades. Read and understand your power tool's instructions and specific warnings.