How To Replace a Planter Box Rail Cap
"MoistureShield's Vantage decking boards serve as a great alternative to treated lumber for this garden rail cap."
Today, we're going to show you how we went about replacing the two-by-six “rail cap” that tops a planter bed.
The raised bed we worked on was made of four-by-six lumber with a treated two-by-six topping the walls. The two-by-six looked great when it was installed, but over time, the gaps between boards grew and some of the boards began to split and cup. It’s common for treated lumber to do this when exposed to direct sunlight, so in hindsight, it wasn’t the best material choice. Structurally, the rail cap was fine—it just didn’t look quite so nice anymore.
So—how to replace it, and what to replace it with?
Let’s tackle the second question first. For this project, we used a manufactured decking product made by MoistureShield. There are a lot of brands out there to choose from, but MoistureShield fit the bill here perfectly. Why? For one thing, their products come in a two-by-six size, whereas many other brands only offer thinner boards.
Another reason was color: Many manufactured boards these days are “capped,” meaning they have a fibrous core that’s covered by a hard shell, or “cap” (kind of like an M&M candy). With a MoistureShield board, the color runs all the way through the board material consistently. Capped boards work great for decking, but color and texture were more important for this project.
A third reason for choosing MoistureShield was our project’s proximity to soil and water. MoistureShield won’t soak up water (even if submerged!), so humidity, standing water, and sprinklers won’t cause the product to swell. And, as with any manufactured product, there’s no chance of rot, decay, or insect damage (on the rare occasion one of these problems does arise, MoistureShield offers a straightforward lifetime warranty). You’ll also never need to stain or seal the product, making it low-maintenance all around.
Now let’s jump into how to replace the rail cap, including some tricks and tips for a great result, whether your rail cap tops the balusters of a railing system or caps a planter bed.
Step 1: Measure for replacement boards
You’ll first want to use your tape measure to measure the linear length of your rail cap. Our old rail cap measures 46 feet, so we purchased two (2) 20-foot boards and one (1) 10-foot board.
We opted for a color called “Bridle,” but there are several colors to choose from. If you’re not sure which color you need, you can find small sample pieces at your local Dunn Lumber store.
Step 2: Remove old boards
Grab your drill/driver to remove the existing rail cap board. Our boards were simply screwed down, so this step was an easy matter of backing out the screws and lifting away the boards.
Step 3: Cut side boards
Manufactured decking does not come precision-cut, so make a fresh and trim off at least ⅛” from your side boards (in our case, the 10’ boards) using a speed square and a saw so you start with a neat, squarely cut end. MoistureShield recommends using a saw blade with no more than 20 teeth.
Measure and cut both pieces, cutting the ends at 45-degree angles (for the corners).
We determined our existing side boards were identical in length, so we measured the long side of one board and marked that length on our 10-foot board. We then drew a 45-degree angle with a speed square and cut the board at a 45-degree angle, using the speed square as a guide. Since we already had the 45-degree angle from the first board, we simply made a square cut at the proper mark.
(Another nice thing about MoistureShield's boards is both sides are identical (there’s not one “good” side), so you don’t have to worry as much about cutting your angle the right way since you can flip the board over.)
Set the side boards in place once they're cut to your liking, but don’t fasten them down yet.
Step 4: Check installation instructions
Regardless of the brand of manufactured lumber you’re using, it’s a good idea to visit the manufacturer’s website and familiarize yourself with installation tips. Things you’ll want to make yourself aware of include: What type of saw blade to use when cutting, how to gap joints, and set-back for screw placement from the edge and ends of the board.
MoistureShield’s instructions were very informative on all these things and more, with great diagrams in addition to written instructions.
Step 5: Cut and install front boards
At this point, you should have about a 38’ length to cover with two boards. You’ll need 45º angles at the far ends, with a butt joint where the two boards meet in between.
Trim off one 20’ board at a 45º angle just as you did in the step above. Lay it along the length of the planter box in just the right place.
Using the short side piece and the long front piece, set the boards in place with a nice straight corner joint and a consistent reveal/overhang. (We took a scrap of lumber and drew a line 1 ½” across it to allow us to have a consistent reveal down the whole front of the planter bed.) Then, screw one end of the long board in place, and then the other (we’ll screw down the middle section later). We used a depth-setter tool for these screws, which we’ll explain in the next step.
Then, measuring from the square end of the first long board down to the corner of the other side board, figure the length needed and cut the other 20’ board to that length, with a 45º angle at the end. Now’s the time to measure twice and cut once—measuring and cutting correctly are critical to ensure the long side of the board will stretch all the way to the far outside corner.
Once cut, set this board in place with the proper reveal. MoistureShield recommends a ⅛” gap at the joint where two boards meet; this is to allow expansion of the boards in hot weather.
The temperature was very hot the day we did this project, and the boards were already expanded. Because of this, we used a 1/16” spacer at the butt joint and then screwed both ends in place.
Step 6: Install screws
Now it’s time to face-screw the boards in place (that is, drive the screws through the face of the two-by-six into the timber below). MoistureShield recommends using screws 2 ¾” long or longer.
For our rail cap, we used Screw Products’ C-Deck screws because they’re specifically designed for manufactured decking and there are many screw head colors to choose from. (The screw head will show against the surface of the MoistureShield board). You can use the handy color chart on their website to find a color match for your decking board.
MoistureShield recommends keeping screws ¾” from the ends or edges of boards. A jig with pencil marks is helpful here to ensure consistency in your screw placement (for aesthetic reasons, not structural).
It’s important to note that we used a depth-setter tool for this step. This tool disengages the driver at just the right time so the head of the screw is level with the top of the board—not standing proud, not sunk in too deep.
Down the length of both front boards, sink two screws every 16”, using your jig to make sure you have the right reveal. The board has some give, so if the reveal is off, some tugging or pushing will get it into the right place before you drive the screws home.
Step 7: Install side boards
When the front boards are secured in place, line up and install the side boards. The idea here is to line up the joint just right with a perfect 45º angle. The shorter side piece can be positioned just so, and then secured in place with screws.
If the two pieces don’t line up just right, you can make another pass with the circular saw on the short piece, but bevel the cut ever so slightly (less than 5º). Cut so the bevel goes downward and back toward the far end of the board. Having this beveled end means you won’t be butting two vertical edges together, making a straight, clean joint easier to achieve.
After making any needed adjustments and getting your corner to your liking, fasten the side boards down with a pair of screws about every 16”.
We ran into a small snag at this point. The wood below the cap was secured with a rod of steel that stuck up slightly above the surface; this meant the two-by-six couldn’t lay perfectly flat and the joint was uneven.
To solve this, we carved away a bit of material on the underside of the two-by-six so the board would lay flat. You can do this with a chisel, or by making shallow holes with a drill—or in our case, by carving a section away with the blade of our circular saw.
And that's all there is to it! For more garden DIY projects, check out our stainless steel garden trellis and this easy garden tent for protecting your plants from the cold.
