I’ll give you the straightforward answer first: the best way is to bolt the bench to the deck’s structural framing—the joists or solid blocking under the deck boards—using exterior-grade lag screws or structural screws with stainless washers. If you only screw into the deck boards, the bench will probably feel fine at first, but it can loosen over time, especially with wood movement and weather.
That said, the exact method depends on whether your bench has pre-drilled mounting holes.
If your metal bench has mounting holes, position the bench where you want it. Before drilling, try to locate the deck joists beneath. Most decks have joists 12 or 16 inches on center. A stud finder with deep scan or measuring from known screw lines can help. Try to align at least two bench mounting holes over joists. If you can’t hit joists, add blocking below the deck from underneath between two joists. This gives you solid material to anchor into.
Mark the holes with a pencil or tape, then move the bench and drill pilot holes. For hardwood decking or joists, pilot holes are non-negotiable to avoid splitting. A bit slightly smaller than the screw shank is right.
Hardware matters. Use 1/4-inch or 5/16-inch stainless steel or hot-dipped galvanized lag screws, 2.5 to 3 inches long, with flat washers and lock washers. Stainless is best near coastal areas or treated lumber. Avoid cheap zinc hardware that rusts and stains the wood.
Before driving each screw, put a small bead of polyurethane sealant or exterior silicone around the hole. This prevents water from sitting in the wood around the screw and slows rot. Then drive the screw through the bench foot, through the washer, and into the pilot hole. Tighten until the foot is snug and the bench doesn’t rock. Don’t over-tighten, or you can bend the metal feet.
If your bench does not have mounting holes, use galvanized L-brackets or angle brackets. Place a bracket on each leg or frame rail. Screw one side into the bench leg using a self-tapping stainless metal screw, and anchor the other side into the deck with a structural deck screw. You can also use heavy-duty galvanized conduit straps around tubular legs if that fits the bench style better.
The same rule applies to composite decking: anchor into joists or blocking, not just the composite board. Composite expands and contracts more than wood, so screws through only the board can work loose. Use screws rated for composite and stainless brackets.
A few extra tips for a solid install: if the bench is in a windy or exposed spot, anchor all feet or at least four points. Consider adding rubber or plastic isolation pads between the metal feet and the deck to prevent moisture trapping and staining. You can still bolt through them. For theft-prone areas, use tamper-resistant or one-way security screws instead of standard hex or Phillips heads. Recheck the bolts after the first month and at the start of each season. Wood swells, metal flexes, and hardware can loosen.
So my honest recommendation: don’t rely on adhesive, zip ties, or short wood screws. Use lag or structural screws into joists or added blocking, with stainless washers and sealant. That’s the longest-lasting, safest way to secure a metal outdoor bench to a wooden deck without damaging the deck or the bench.