How to Install Aluminum Railing on Deck | Set Posts First

Installing aluminum railing on a deck starts with setting posts, then cutting rails to fit, and ends with panels, covers, and caps tightened into place.

Aluminum railing is the rare deck upgrade that looks like a weekend project but punishes a skipped measurement for years. The biggest mistake isn’t cutting a rail an inch short—it’s bolting posts in the wrong spot before deck boards go down. Whether you’re using Deckorators, Trex Signature, or another system, the sequence stays the same: posts first, rails second, infill and trim last. Most manufacturers also expect you to check local code before you buy, because guard height standards are set by jurisdiction, not by the product box.

What Code Requires Before You Start

Under the International Residential Code (IRC), a guard is required on any open-sided walking surface more than 30 inches above grade. The minimum residential guard height is 36 inches, measured from the deck surface to the top of the guard—or to the top of the cap rail if you install one. Your local jurisdiction can raise that baseline, so confirm the exact height and opening limits before ordering materials.

This is the one step that isn’t optional. A beautiful railing built to the wrong height is a failed inspection and a safety hazard, no matter how clean the miters look.

Set the Posts First, Then Cut Rails to Fit

Posts go in before deck boards for most aluminum systems, and they must be plumb before anything is fastened. Skip the level and you’ll fight every subsequent step. The exact mounting method depends on your system, and the published specs differ enough that you should follow your own installation sheet.

Two popular systems show how much the details vary:

Deckorators Aluminum Railing specifies 4×4 posts spaced 6 feet on-center, bolted to the inside of joists with two 1/2″ x 6″ galvanized carriage bolts. Post sleeves should be at least 1-1/2″ longer than the overall railing height, plus another 1-1/2″ if you’re adding the optional cap rail. The distance between each post and the first baluster must be less than 4 inches and equal on both ends.

Trex Signature Aluminum Deck Railing takes a different approach: posts attach with four 3/8″ x 6″ HG bolts, washers, and nuts, backed by a Trex aluminum plate under the block. The lower bracket sits 1-7/8″ up from the deck surface to the bottom of the bracket, and the upper bracket fastens with three 2″ wood screws.

For most systems, an alternative guide suggests keeping a 5″ x 5″ base plate at least 2 inches from the deck edge to leave room for trim, then using shims and a level to plumb the post before tightening.

Assemble the Rails, Balusters, and Trim

Once posts are plumb and locked in, measure from inside to inside of the brackets—not from post face to post face. Transfer that measurement to the rails, then cut. A fine-tooth miter saw or portable band saw works, and you should subtract 1/2″ from the opening for brackets on systems that need it. Trex’s own guide has you measure the inside-to-inside bracket distance, transfer it to the rails, and cut both the bottom rail and its cover to that exact length.

Insert the balusters, then fasten the top rail to the brackets with self-tapping screws. A rubber mallet is recommended for tapping caps and skirts into place, and a foot block under the center of the bottom rail gets pre-drilled with a 1/8″ bit before its base is attached. For any opening greater than 4 feet, one product sheet requires support legs under the center of the bottom rail—don’t let a long span go unsupported.

When you’re choosing a system, the details are worth comparing side by side. Our roundup of the best aluminum railing for decks breaks down the top options by mounting style, finish, and price.

Mistakes That Cost Time and Money

• Skipping the level. Manufacturer instructions explicitly call for plumb and square setup before fastening. A post that’s off by half a degree shows up as a crooked rail you can’t fix later.
• Cutting rails before measuring. Almost every guide says measure the inside-to-inside opening first, then cut. One bad measurement means a rail you can’t return.
• Ignoring edge clearance. Base plates need room for trim—at least 2 inches from the deck edge on systems that use 2-piece trim.
• Using mismatched specs. Post spacing, bracket heights, and fastener sizes differ by manufacturer. What works for Deckorators won’t necessarily work for Trex.

If you’ve already bought a system, its installation sheet overrides any general guide, including this one. The sequence is universal—posts, then rails, then trim—but the numbers aren’t.