Composite Decking is It Easy to Cut Curves
Many composite decking manufacturers – including Trex, the company that makes the decking that seems to be most popular with my clients – feature curved decks in their marketing materials. So naturally, my clients have acquired many design ideas by the time they call me. Most of the time, the curved features they see were created by heating and bending the deck boards, a labor-intensive process that usually requires an investment in heat blankets or an expensive special oven. If you're new to that game, however, and haven't quite figured out the temperature and timing, you can end up ruining and wasting a lot of decking, which – at nearly $100 for a 20-footer – can be costly.
As an alternative, I offer what I refer to as "cut radius" decking. Using a few basic tools and carpentry techniques, it's possible to cut extremely smooth curves. This in turn allows me to create rounded deck features without the steep price that comes with bending.
Laying Out the Curves
Most of the curved decks my company builds are close to grade and rarely require scaled drawings, especially when no permits are needed and a plan review isn't necessary. Instead of working from a plan or using a string to pull an arc, I like to play around with different shapes and find the size and configuration of the curves right on the ground using flexible ½-inch-diameter PVC irrigation pipe and trial templates cut from ¼-inch-thick MDF.
Even when there are drawings, I try to make what I call the "spring" – or size – of the curves based on what I think looks best. Because I'm particular, and because I like to involve the homeowner, this can take a little time as we work out the details.
Framing
When framing, I typically support the joists with a drop beam and run the cantilevered joists long. Then, depending on the joist layout and particular framing details, I can go ahead and lay out the curves directly on top of the joists and cut them to their final lengths. Before cutting any joists, however, I make a final MDF template. MDF is an easy material to cut and shape to a desired layout, and it makes for a smooth ride for saws and pencils. This results in more accurate cuts, with the proper bevel angle to match the curve I'm trying to form.
To make rim joists that are flexible enough to conform to the curves defined by my template, I make a series of relief cuts - or kerfs - in the back (or front) of standard PT 2-by stock. The spacing of the kerfs and their depth are dependent on the length and radius of the curve. There's no special math involved: I just play around with scrap lumber until I find the sweet spot. Making the size and spacing of the relief cuts as uniform as possible will create a smoother bend. I fasten the kerfed rim joist to the framing with HeadLok flat-head structural screws.
Then, for an even smoother transition, I apply a layer of either ½-inch or ¾-inch (depending on the radius) 0.60 ACQ PT plywood ripped to the width of the rim joist. Before installing the rim, I spray all the kerfs with Coppergreen preservative. Afterward, I flash the assembly with flexible self-adhesive flashing tape to keep water out.
Sometimes, I have to be a little more creative with the framing. For example, on one project my clients wanted a grade-level deck that would cantilever out over a pond without any visible framing underneath. To frame it, I installed a cantilevered beam in the center of the proposed radius; that beam then supports a pair of straight "spring line" rim joists and the curved outer rim joist. To maintain the curve, I packed the gap between the spring-line rim joists and the outer curved rim with blocking.
Depending on how tight the curve is, I also make relief cuts in the fascia boards used to trim the rim joist. The depth of cut and kerf spacing depend on the fascia material, as some composites are more flexible than others. To make the kerfs, I use a small, Mafell KSS 40 cordless cross-cutting saw, which has an integrated track for making quick, accurate cuts.
Templates
After the framing is complete and I've installed the composite fascia, I spread out enough pieces of MDF to cover the entire curve. Then I use a small block that's 5/8 inch larger than the fascia (the desired nosing on all of the decks that we build) or a SimpleScribe tool (handy for scribing various widths and reveals) to mark the outline of the curve onto the MDF. Before removing the marked MDF sheets from the framing, I make reference marks to make sure I'll know exactly where everything goes when I place the template over the decking.
When making the template, I cut carefully and as close to the line as possible without going over it accidentally, typically doing most of the work with a Rockwell RK3440K VersaCut mini circular saw. For tighter curves, I use a jigsaw. Then I carefully clean up and smooth out the template edge with a belt sander.
Next, I trace this template onto another piece (or pieces) of MDF, to preserve the pattern. Then, I scribe an offset line--using the distance from the blade to the outside edge of the shoe of the saw that I'll use to cut the decking--and cut along it to create another template. When I cut the decking, I'll lay this second template on top of the first to guide the base of the saw – typically a Rockwell RK3441K 4 ½-inch compact circular saw. Again, I'm careful to follow the cut line closely with my saw and smooth the edges afterward with a belt sander to get the curves as close as I can to perfect, making sure I don't remove too much, to maintain consistency in the nosing.
Cutting the Decking
Most of the time, I cut the decking using a Rockwell 4 ½-inch circular saw guided by my template. But for tight curves, I use a router. First, I rough-cut the fat off with a jigsaw or even the circular saw, so that I'm not trying to bite off too much at one time with the router.
Some guys are happy making multiple passes to trim a lot of material off the ends of the decking. In that case, an up-cut bit works fine; it will leave some frayed edges on the cap, but a few passes with a razor knife dragged across the edge of the decking tends to clean off the fuzz. I prefer to trim the decking in a single pass, though, so I carefully rough-cut the decking to within 1/8 inch of the cut line with my saw first. Then I can use a down-cut spiral bit, which evacuates the shavings better, and trim the decking with a glass-smooth finish in a single pass. I use a Whiteside solid carbide ½-inch-diameter downcut router bit with a guide bushing riding against the MDF template.
Because of the curve, we can't install a picture-frame border, so we paint the ends with color-matched paint. This is something we talk to the homeowner about upfront.
Source: https://www.jlconline.com/deck-builder/cutting-curves_o
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