Bolts, Nuts & Washers: How to Choose

A joint is only as strong as the fastener holding it — and only as long-lasting as the metal that fastener is made of. Pick the wrong grade, the wrong thread, or skip the washer, and you get a wobbly deck rail, a stripped bolt, or a rusted mess two winters from now.

Bolt, screw, or nut-and-bolt?

First, know what you’re actually buying. A bolt passes through a hole and is tightened with a nut on the far side — think a carriage bolt through a fence post. A screw threads directly into the material and needs no nut. A machine screw looks like a small bolt but usually threads into a tapped hole or a nut. If you’re fastening two pieces of wood or steel that you can reach from both sides, you want a bolt paired with a nut from the nuts bin. If you can only reach one side, you want a screw or an anchor.

The most common head styles: hex (the workhorse, driven with a wrench or socket), carriage (a smooth domed head with a square shoulder that bites into wood so it won’t spin), and lag (a heavy screw-tipped bolt for wood, no nut needed). Carriage bolts are the right call for decks, swing sets, and anywhere a finished look matters.

Grade is the number that matters most

The grade tells you how much stress a bolt can take before it yields. It’s stamped right on the head as radial lines (SAE) or a number (metric). For a mailbox post, grade is irrelevant. For a trailer hitch, an engine mount, or a structural connection, it’s everything — and a hardware-store “junk-drawer” bolt of unknown grade has no place there.

Tip: never mix a hardened Grade 8 bolt with a soft Grade 2 nut. The connection is only as strong as its weakest part — match the nut grade to the bolt.

Material and coating: where things rust or don’t

This is the decision people get wrong most often, and it shows up as orange streaks a year later. Coating buys you corrosion resistance; the right one depends entirely on whether the fastener lives indoors, outdoors, or in the weather and chemicals of a treated-lumber deck.

• Zinc-plated (plain steel): shiny silver, cheap, indoor use only. It will rust outdoors.
• Hot-dip galvanized: thick, dull-gray zinc coating — the standard for decks, fences, and anything touching pressure-treated wood, whose copper-based preservatives eat lesser coatings.
• Stainless steel (18-8 / 304): the go-to for wet, visible, or coastal jobs. 316 stainless steps it up for saltwater and pool areas.
• Silicon bronze: premium, for marine and fine woodwork where looks and salt resistance both matter.

One caution: don’t bolt bare aluminum with a plain steel fastener in a wet spot — dissimilar metals set up galvanic corrosion. Use stainless or an isolating washer between them. For fasteners that must survive treated-lumber decks and rails, your local store’s decking and roofing fasteners are rated for exactly that exposure.

Washers do more than fill space

Skipping the washer is the most common shortcut, and it costs you. A washer spreads clamping force so the nut doesn’t crush or pull through the material, and the right kind keeps the joint from loosening under vibration. There are three families you’ll actually reach for from the washers and O-rings section.

Rule of thumb: a flat washer goes under the part that turns. On a bolt with a wrench-turned nut, the flat washer sits under the nut; add a lock washer between the nut and the flat washer anywhere vibration lives.

Thread, diameter, and length — measuring right

Fasteners are sized by diameter, thread pitch, and length. In the U.S. you’ll see it written like “3/8″-16 x 2″” — that’s 3/8-inch diameter, 16 threads per inch, 2 inches long. Coarse thread (UNC) is the default: faster to drive, more forgiving, better in wood and softer metals. Fine thread (UNF) holds slightly more load and resists vibration, common in automotive. The two are not interchangeable — a coarse nut will not thread onto a fine bolt.

Measure length from under the head for hex bolts, but from the top of the head for flat-head (countersunk) bolts, since those sit flush. Bring the old part to the store, or use a thread gauge and caliper. If you’re unsure on a metric-vs-imperial call, the staff at your local hardware store can gauge it in seconds.

Pick by the job

Frequently asked questions

Fasteners are the cheapest part of any build and the one most likely to fail if you guess. When you’re unsure of a grade, a thread, or a coating, bring the part to your local independent hardware store and someone will size it on the spot — or ask us here. Getting it right the first time is what keeps the whole project standing.