A: As you discovered, most plastic lumber sags easily. Manufacturers warn that real wood, not plastic lumber, should be used for the support structure on deck boards, and they advise spacing the supports closer together, often 12 inches. From the pictures you sent, it appears that your bench is four to six feet long, far exceeding that recommendation.
That said, there are manufacturers that produce garden benches very similar to yours that have plastic lumber seats and are four to six and even eight feet long. How do they do it?
In some cases, there are three leg supports — one on each end, as on your bench, and one in the middle. The product description may tout the triple armrests as a way to divide the seating space, but there is also a clear engineering benefit: Having that extra set of legs in the middle helps keep the seat from sagging.
In other cases, manufactured plastic-wood benches have legs only on the ends, but they have a subtle, different type of bracing in one or more places mid-span: a metal tie that connects the seat slats to the back of the bench. It’s like the bracing you installed but longer, so the metal bends upward and is also screwed to the back of the bench’s back. This allows the bench back to share whatever load is pressing on the seat. Even if the bench back is also plastic lumber, the boards there are oriented so their widest dimension runs up and down, allowing them to shoulder more weight than the seat boards, which have their skinniest dimension doing the work.
There’s probably no way for you to obtain a third leg to insert in the middle of your bench, but you might be able to adapt the idea of tying the seat to the bench back, using the kind of heavy-duty strapping that builders depend on to tie parts of a structure together. Home Depot sells Simpson Strong Tie 18-gauge strapping 1¼ inches wide by 36 inches long for $3.25. You can customize the length with a hacksaw, and you shouldn’t need to drill through it to attach screws because the strapping comes with perforations. Paint it black before you install it.
Another option is to strengthen the seat slats by attaching strips of metal underneath. For this, you could use slotted angle (also painted black before you install it). A piece 4 feet long and 1½ inches wide on each side costs $10.98 at Home Depot. When you screw one side of the angle to the bottom of the seat, the other side of the angle acts as a stiffener. If necessary, you can dramatically increase the stiffness by doubling up pieces of slotted angle. Align a pair of pieces so one side of each piece butts against one side of the other, and bolt them together. The pieces should form a T when viewed from the ends. When you screw the angle to the seats, choose elongated openings, rather than round ones, if possible. Use screws that are narrower than the perforations, with washers so the screw heads don’t poke through. Leaving a gap where the screws go through the metal accommodates the plastic, which will expand in hot weather and contract in cold weather at a different rate than the metal will.
And another solution, of course, is to replace the plastic lumber. If you opt for wood, use oak or a hardwood decking type, such as ipe. Cedar and other softwoods are likely to sag over a long span, though not as much as plastic lumber.
Or, if you want to stick with plastic lumber, you could replace the pieces you bought with plastic lumber fortified with fiberglass strands to make it far stiffer than standard plastic lumber. Find sources online by searching for “structural plastic lumber.”
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