Good to Know: Pressure-treated lumber is likely to have a high-moisture content when first purchased. If your boards feel wet and heavy, set them aside for a week or more so they have time to dry out a little. Stack them with spacers to allow air circulation, and keep them in your garage or shop, or at least out of direct sunlight. If you cut wet lumber and assemble a project with it, the parts are more likely to warp or to crack where fasteners restrict the movement (shrinking) of the wood as it dries.
After the lumber has had a chance to dry some, sort the boards according to the parts you need to cut from them. Note that the 2 x 4 and 2 x 6 stock for the frame and stretcher parts gets ripped slightly narrower, to 3 and 5 inches, respectively; this ensures a uniform width and clean edges for these parts. For parts produced in larger quantities, such as the bench slats (C) and leg slats (D), it is important that the pieces are a consistent length, so rig up an auxiliary fence and a stop block on your miter saw or table saw to cut the parts to final length. Do the same for the frame braces (B); there are only three of them, but for the assembled frame to be straight and square they must all be exactly the same length.
Cut the frame joinery and assemble the frame
Using the project diagram and cutting list as reference, cut pieces to desired lengths. The only joinery details required for this project are the notches cut in the frame rails (A). The open notches at the ends are called rabbets, and the closed notch in the center is called a dado. Cut them as shown, using a table saw and dado blade, if available. If not, use a portable circular saw with a guide board to make multiple scoring cuts, then clean out the waste wood with a chisel. Next, drill holes through the notches as shown for the screws that will help hold the frame together. Countersink the holes on the outside faces of the rails.
Set the first frame rail flat with the notched face up. Glue and screw the three braces into the rabbets and dado as shown, then fit the remaining rail onto the upper ends of the braces and secure it the same way. Then set the frame flat so it rests on the rail edges.
Add the bench slats
Use a square to make a preliminary check on the corners of the frame, ensuring they are accurate 90-degree corners. Adjust as necssary, then confirm your results by measuring the frame diagonally from corner to corner. If the measurements are different, push on the corners that gave the longer reading and then measure again. When both dimensions are equal, the frame is square.
To install the bench slats (C), drill holes for screws as indicated in the Fastener Detail (pdf), and start the installation with the two at the ends. (These have the screw holes drilled different from the rest, to align with the frame braces.) The outer edge of each slat should be offset 3/4 inches past the frame brace as shown, and its ends should extend 1 inch past the frame rails. Align these two slats carefully so their outer edges are 59-1/2 inches apart and perfectly parallel, then clamp them in place and secure with 1-5/8-inch screws.
Next, cut a 1/2-inch-wide spacer from scrap stock at least 16 inches long (so it will straddle the frame rails). Test the spacer by holding it alongside one of the bench slats (C) and checking their combined width; it must be exactly 4 inches or the slats won't fit properly when they're all installed. If it's not, re-cut the spacers until the dimensions are exact.
Now you can start fitting the rest of the bench slats. Set the spacer alongside one of the end slats already attached and fit another slat next to it. Align the ends and drive two screws (one near each end) to hold the slat in place. Move to the other end of the bench and repeat the sequence for the second slat on that end, then alternate from one end to the other as you attach the remaining slats. As you work toward the center, check periodically to ensure that the unfilled area measures the same distance along each side of the bench; the dimension should always be a multiple of 4, plus 1/2 inch. When the final bench slat goes in at the center, it should have the same 1/2 inch gap on either side; if less, you can rip the slat slightly narrower to compensate.
After all the bench slats are in place and checked for alignment, drive screws through the remaining holes to secure them.
Install the leg slats
It's easier to install the bench's leg slats (D) if you work with the assembly upside-down. Drill screw holes in these slats as shown, then clamp the end slats (the four with the offset screw holes) into place according to the Construction View illustration. Note that the three end slats on the front and rear rails of the bench are aligned with the bench slats at the ends. Use the 1-5/8-inch screws to attach these legs, then use the spacer to position the remaining legs slats as shown and secure them with screws. There's no need to glue them in place, and this screws-only technique allows easy replacement if any parts get damaged.
Install the stretcher and leg cleats
With the bench still upside-down, cut a pair of 6-1/2-inch wide spacers from scrap plywood and nest them on the edge of the rear frame rail, resting against the leg slats. Set the stretcher (E) on these spacers and drive screws through the leg slats into the stretcher. Repeat this method for the two leg cleats that attach to the legs at the front and end of the bench.
Check that all screw heads are countersunk flush with the surrounding wood surface, and use a sanding block to ease any sharp corners or edges. Apply an exterior stain or the finish of your choice and let dry.