What size fixings for a 48kg driveway gate hinge plate?

A 40mm screw is basially a recipe for a sagging gate when you're dealing with nearly 50kg of timber per side. The leverage at the top hinge is immense, and as soon as a bit of wind catches those gates, those tiny screws will start to pull or wallow out the timber. You've got 150mm posts, so there is absolutely no reason to stay shallow when you have that much meat to bite into. I woud swap those out for **M8 or M10 coach screws** at least 80mm to 100mm long. If the holes in your hinge plates are large enough to accommodate them, go as beefy as possable. You want that thread biting deep into the heartwood of the post. Another solid option is using structural timber fixings like ForgeFast or Spax. They have a much higher shear strength than standard wood screws and the coating usually holds up better against the tannins in the wood which can rot out cheap zinc screws fast.

Since you're using two hinges per leaf, that top hinge is doing the vast majority of the work holding the gate up, while the bottom one mostly just acts as a pivot. Make sure you pre-drill your pilot holes carefully, especially with coach screws, or you risk splitting that expensive 150mm post. A 6mm pilot for an 8mm coach screw usually does the trick in standard treated softwood.

If you realy want to make sure it never moves, you could even go with **thru-bolts** on the top plate. Drill all the way through the 150mm post and use a large square plate washer and nut on the back side. It’s more effort and you’ll see the hardware on the back of the post, but it's the only way to be 100% sure it won't pull out over time. Given the weight and the inevitable seasonal movement of the wood, that’s what I’d do for at least the top two holes of the upper hinge plate.

That thru-bolt idea sounds solid but terrifying for a newbie like me. How on earth do you keep a long drill bit level through a 150mm post? I'd be so stressed about it poking out the back at a wonky angle and ruining the look. Also, does the type of wood make a difference for these beefier screws? My posts are pressure-treated softwood, and I've heard some fixings can react with the chemicals and just snap off or rust away after a few years. Is that a real risk with those M8 coach screws, or am I overthinking it?

If you're nervous about drilling thru-bolts level, just grab some 100mm Timberlok or similar hex-head structural screws. They’re basically modern coach screws but thinner and way tougher. They’ve got a specialized coating for pressure-treated wood, so you won't get those nasty black streaks or the screw snapping from corrosion in a couple of years.

Also, make sure you grease those hinge pins before you hang the gates. Metal-on-metal with a 48kg load will grind down the galvanizing fast. A bit of waterproof grease or even heavy-duty spray lube on the hooks will keep the swing smooth and stop the wood from taking the brunt of the friction vibration every time the wind catches them.

Definitely check the hole sizes on those plates before you buy your fixings. I’ve bought M10 coach screws in the past only to realize the hinge plate holes were only punched for 5mm screws. If they’re too small, you’ll have to drill the metal out with a HSS bit wich is a massive chore if you have twelve of them to do. Also, make sure you leave a bigger gap between the two gate leaves than you think you need—at least 10mm or 12mm. With 48kg of wood, those hinges will settle a few millimeters, and the posts always move when the ground gets soaked. You don't want them binding up the first time it rains.