After design the first thing to consider is the wood. We chose cedar decking for the slats on the top of the bench because cedar is naturally rot resistant and always a good idea for outdoor use. It’s also more finished than a lot of lumber because it’s meant for decking. This means that the surface is very smooth for cedar and it has nice rounded edges. Perfect for sitting on!
For the structural supports we went with Douglas fir. Though Doug fir doesn’t weather quite as well as cedar, it is a very sturdy wood and significantly less expensive than the cedar beams. We also don’t carry cedar 4x12s in stock, so if you plan to go the extra mile and use cedar throughout the bench make sure that you give yourself several days for the 4x12s to be ordered.
Step 1: Make your Cuts
Time to make cuts: Using a chop saw cut the 4x4 Douglas fir into two lengths of 59 ½”. Measure and cut the cedar decking into seventeen 14 ½” slats. You can actually get all of these cuts out a three 8-footers, but the extra two feet reduces stress by giving you a little wiggle room to make mistakes or to skip over blemishes in the wood.
Once the slats are cut, line them up on the 4x4s to get a visual for how it will line up. Because of the nature of life and wood no two benches will ever line up exactly the same way, so this is your chance to check for problems. Mark out the middle of the 4x4s and line up your middle slat. Now using 1/8” shims carefully measure out from the center to see where your final board will line up. You may find that the slats end too far out, or with some of the 4x4 still sticking out. The latter can easily be fixed by cutting the support shorter, but the former may be a little tricky. If there’s not enough room to add another slat you may need to increase or decrease the gaps between the boards to even it out. Hopefully our problem solving can save you time and heartache!
Now here come the tricky cuts: The 4x12. You can actually ask for the yardman at Dunn Lumber to make the initial cut into two 18 inch pieces. We recommend asking that they start by trimming the mill cut end to make sure it’s square because we ended up with a wobbly leg. Once you have your two legs cut it’s time to take out the notches. There are a number of ways to do this. Festool’s jigsaw is actually powerful enough to cut through a 4x12, but a regular jigsaw won’t have enough power. So if you have a Festool or if you’re looking for an excuse to buy one, go for it! Your other options are doing it completely by hand, or using a combination circular saw and handsaw. We used the the latter technique.
With a carpenter’s square mark out the width of the 4x4 on the face of the leg. Because of the ranging moisture content in wood, one 4x4 will never be the exact same size as another, so measure specifically for yours. Flip over the 4x12 and mark the notch on the other side squaring off the corners. Still using the square line up the marks on the front and back and connect them on the top and side of the 4x12. You should now have lines on all four sides for each notch.
With the 4x12 clamped to the table follow the lines carefully with the saw, double-checking each side as you go. It’s pretty straightforward if you’re using a handsaw or a jigsaw, but let us explain how we used the circular saw. When using a circular saw the first step is to extend the lines beyond what is needed so that you have something to follow even when you come to the end of your cut. Following the lines on the face of the board cut the saw through the wood until it just touches the perpendicular line. It’s better to end just shy than to go too far in this case. Rotate the 4x12 and make the same cut on the perpendicular line. Flip the board over and make the same cuts on the opposite side. This will cause you to end up with a notch that is cut all the way through near the edges of the board and not at all where the two lines meet. At this point you can grab your handsaw and carefully finish off the cuts. This process may be a little tedious, but it requires a little less out of your arm than the manual approach.
Step 2: Sand your Pieces
Now that all your cuts have been made the pieces need to be sanded. The decking is already pretty smooth and really only requires softening out the cut edges with some 150 grit sandpaper. The legs and supporting beams we also kept pretty rough. We used a sander with 100 grit paper to remove dirt, blemishes, stamp marks, and the remaining pencil lines from cutting the notches. We didn’t sand any finer, but of course you’re welcome to make it as smooth as you wish.
Step 3: Clean your Wood
After sanding comes cleaning. You can remove the sawdust with a vacuum, or a rag and some mineral spirits or paint thinner. This preps the wood for the wood conditioner, which in turn preps it for the stain. A little confusing and not absolutely necessary, but the conditioner helps the wood to stain evenly and have a professional look. Lay out a drop cloth to protect the surface that you’re working on. Apply the conditioner with the grain according to the directions on the can. Make sure to wipe off after application. We used a disposable foam brush to spread the conditioner. Wait the allotted time, as per the manufacturer's directions, for the conditioner to penetrate before starting to stain.
Step 4: Stain your Wood
For the stain we brushed on a darker color to the Douglas fir and a natural tone for cedar slats. This serves two purposes: 1) the Douglas fir and cedar won’t stain exactly the same and this conceals the fact that we’re using two different woods, and 2) it adds contrast and interest to the design of the bench. Apply the stain according to the directions on the can, and wait to dry. Remember that the stain helps to seal and protect the wood so it’s important to stain the bottom of the slats, inside the notches, and on the bottoms of the 4x12s even if you won’t see it.
Step 5: Assemble your Bench
Once the stain is dry you can assemble your bench. Start by standing up the legs and setting the 4x4s in place. We gave the 4x4 beams 3 ½ inches of overhang on either end of the bench, but this is totally a preference of taste. When you've positioned the 4x4s clamp them in place. We also lined up the slats again to see where they would sit on top of the 4x4s, this way we avoided the screws for the 4x4s getting in the way of the screws securing the slats. Using a 1/8” drill bit, pre-drill two holes into both ends of the 4x4. Repeat on the opposite beam. With the T30x1” driver bit and the 6” screws secure the 4x4 to the 4x12.
Step 6: Secure your Slats in Place
Line up your middle slat and secure in place with two trim screws in each beam. This box of star head screws actually comes with its own driver bit. Using the shims as spacers work your way out from the middle securing each slat as you go. You can use a straight edge to keep the ends even, or just feel it out with your hands. We eyeballed the placement of the screws to keep them centered, but you can also measure out the specific points on a 3x5 card and use it as a diagram to drill through.
When you get to the 4x4s you’ll notice that the lumbertite screws are pushing up the slats and making the surface of the bench uneven. This is a pretty easy fix. Place a piece of scrap wood on top of the slat and tap firmly on it with a hammer. This will indent the bottom of the slat with the location of the screw head. Clamp the board upside down onto a table and use a chisel to gouge out a little section for the head to fit. To make this easier you can draw a box about the marking with a utility knife and use these cuts to start chiseling.
Once all your slats are attached your bench is finished!