DIY Wood Deck Built with 2×4 Lumber — Step-by-Step Guide

I built a backyard wood deck — entirely DIY!
It was a much bigger project than the small furniture I’d made before, but the result was completely worth it: it’s genuinely useful day to day, and I love how it looks.

I originally built the whole thing with cost-friendly SPF lumber and outdoor stain. After five years, the deck boards started showing their age, so I eventually replaced them with hardwood.

How a Wood Deck is Built — The Basics

Basic structure and terminology

A wood deck has a pretty consistent structure (see diagram below).
You start by deciding the floor footprint, then design the structural elements — concrete footings, support posts, and joists — to match.

For this build, I used 2x lumber for everything:
　Decking boards: 2×4 (38mm × 89mm / approx. 1.5″ × 3.5″)
　Joists: 2×6 (38mm × 140mm / approx. 1.5″ × 5.5″)
　Support posts: 4×4 (89mm × 89mm / approx. 3.5″ × 3.5″)
→ See “Materials” below for more on the lumber.

Planning

Deck size and height

I sized the deck at 1800mm wide × 1200mm deep (approx. 6 ft × 4 ft).
Honestly, I would have liked it bigger — but that’s about all the yard could spare.

For the deck height, I wanted it to flow nicely from the room, so I set the deck surface just below the sliding door track, as shown:

Post and joist spacing

The general rules of thumb for spacing are:

・Support post spacing: 600–1000mm (approx. 24″–40″) → I used 1000mm
・Joist spacing: 600–800mm (approx. 24″–32″) → I used 800mm

For my deck size, I just barely managed to fit within the upper-end spacing limits, as shown:

Footings — using existing concrete

There was an existing concrete step (a traditional feature in Japanese homes called kutsunugi-ishi, basically a stone “shoe-removal step” right outside the sliding door) under the window where the deck would go. I just used that existing concrete as a footing for one side.

Materials

Lumber

I prioritized cost, workability, and availability — so all the lumber for this build was SPF.

Wood used in decks is typically classified as either softwood or hardwood. SPF is a softwood and isn’t well suited to outdoor use on its own, so when you build a deck with SPF, staining/sealing is essential.

Stain and painting supplies

For most of my DIY work, I stain or paint after assembly — that way I can skip the parts no one will ever see.
But on a deck, that doesn’t fly: once the structure is together, you can no longer reach the bottom of the joists or the inside of the post connections. So this time, I painted everything before assembly.

Stain

I used water-based Xyladecor — a classic exterior stain that’s widely used for decks.

Brushes and a tarp

Just regular paint brushes (see below).
I also laid down a tarp to keep stain drips off the ground while I worked.

Screws and footings

Screws

I used stainless steel coarse-thread deck screws. Since I was working with 38mm-thick lumber, I followed the standard rule of thumb (screw length ≈ 2× the board thickness) and went with 75mm (3″) screws.

Footings (post bases)

I used pre-cast concrete footings with built-in metal mounting plates (“post base anchors”) — they let you screw the support posts directly to the metal plate, which is super convenient.
At my local home improvement store these were about ¥700 each (about $5 USD).
※ Heads up: the same footings sell online for ¥2000+ each. Buying in person at a hardware store saves a lot of money on this kind of bulky item.

Building the Deck

Step 1: Assemble the joist frame and dry-fit it

Screw the joists together with stainless steel coarse-thread screws to form the frame.

Place the assembled frame in position. Leave the spots open where the support posts will go — those get added in the next step.

Step 2: Install footings and support posts

Dig down about 10cm (4″) at each post location, drop in a few centimeters of crushed gravel, and tamp it down firmly with a piece of scrap lumber.

Step 3: Connect joists to support posts

Check for level, then screw the joists to the posts. Once that’s done, all that’s left is laying the deck boards.

A confession — I didn’t actually follow this order

OK, full disclosure: I wrote up a clean step-by-step above, but in reality I didn’t have anything to prop the joist frame up at the right height for the dry-fit, so I went ahead and screwed the front-side posts to the joists first (visible in the photo below). Do as I say, not as I did.

Step 4: Lay the deck boards (you’re done!)

Last step — screw the deck boards down across the joists.
Leave a small gap (a few mm / about 1/8″) between boards so rainwater can drain through.
I used a scrap of wood as a 5mm spacer between boards as I screwed them down.

All boards screwed down — and we’re done!