Honestly, if the crack is dry and the wood is sound, you can usually repair that slat without replacing it. I’ve done this on park benches several times, and the repair holds up if you use a weatherproof adhesive, clamp properly, and seal the wood afterward.
First, inspect the crack. If the wood is soft, dark, or crumbles when you poke it with a screwdriver, that is rot rather than a simple split. Rot needs to be removed or the slat needs local replacement. If it is a clean split or a hairline crack, keep going.
Remove the slat if possible. Most street bench slats are held with carriage bolts or structural screws. Taking the board off makes cleaning and clamping much easier. If you cannot remove it, work in place and tape off the metal brackets.
Clean the crack thoroughly. Use a stiff brush, a pick, compressed air, or a vacuum to get dirt, moss, and old finish out of the split. Wipe the area with denatured alcohol or mineral spirits and let it dry completely. Epoxy and glue do not bond well to dirt or moisture.
Choose the right adhesive. For outdoor bench slats, a two-part exterior epoxy is the best option because it fills gaps, resists moisture, and does not shrink like standard wood glue. If the crack is very tight and you can clamp it closed, a high-quality exterior wood glue such as Titebond III can work. For wider cracks, use a thickened epoxy such as Abatron WoodEpox or a general epoxy mixed with a wood filler additive.
Work the adhesive into the crack. Use a putty knife, a plastic spreader, or a syringe to push the epoxy deep into the split. If the slat is loose, flex it very gently a few times to pump the adhesive deeper into the grain. Do not flex hard enough to extend the crack.
Clamp across the crack using bar clamps or pipe clamps with small wood blocks to protect the bench surface. Pull the crack closed until the board is flush, but do not overtighten. Overtightening can squeeze out too much adhesive and starve the joint. Wipe off excess epoxy with a rag and mineral spirits before it cures.
If the crack left a void or small missing piece, fill it after the first epoxy has cured. Use exterior wood filler or another layer of thickened epoxy, shape it slightly proud, and sand it flush once hard.
Sand the repair with 80-grit paper, then 120- or 150-grit. Feather the edges so the patch blends into the surrounding wood. If the edge feels sharp, soften it to match the other slats.
For cracks near a bolt hole or the end of a board, add extra reinforcement underneath. A stainless steel mending plate or a small angle bracket can reduce flex and prevent the crack from reopening. If you reinforce from the top, use corrosion-resistant screws and countersink them.
Seal the repaired slat with an exterior penetrating wood sealer, spar varnish, or paint that matches the bench. Coat the top, edges, end grain, and underside if you can reach it. Two thin coats provide better weather protection than one thick coat.
Reinstall with stainless or galvanized hardware. If an old screw hole is stripped, fill it with epoxy, let it cure, and drill a new pilot hole. That gives the screw solid material to bite into.
Let the bench dry for the full cure time before heavy use. Check the repair after the first wet season, because freeze-thaw cycles can reveal hidden gaps. If the crack reappears as a hairline, clean it and work in a little more epoxy before it spreads.
This approach costs much less than replacing the whole bench or even the slat, and with proper sealing it can last for years.