How to Build Composite Deck Stairs and Railings?

Stairs are the most safety-critical part of any deck. Every riser must be the same height. Every tread must be the same depth. Inconsistent steps cause trips and falls.

This guide walks you through measuring, calculating, building, and installing composite deck stairs from start to finish. It includes IRC building code requirements, composite-specific tips, and railing installation.

Project Overview

• Difficulty: Intermediate to advanced
• Time: 6–10 hours (one weekend for most projects)
• People needed: 2
• Material cost: $300–$800, depending on stair height and width
• Tools required: See full list below

Building Code Requirements (IRC)

Deck stairs must meet the International Residential Code. Inspectors check these measurements carefully.

The most important rule: All risers must be the same height, and all treads must be the same depth. A variation greater than ⅜ inch between any two steps is a code violation and a serious trip hazard.

Stair Anatomy: Key Terms

Understanding the correct terminology prevents confusion during planning and building.

• Stringer: The diagonal support board (typically cut from a 2×12) that runs from the deck frame to the landing. Notches cut into the stringer create the step shape. Most stairs need at least 3 stringers.
• Tread: The horizontal board you step on. Made from composite decking for this project. Must be at least 10 inches deep and thick enough to support foot traffic (minimum 1 inch).
• Riser: The vertical board between each tread. Risers close the gap between steps. They are not structurally required outdoors, but improve appearance and prevent feet from slipping through.
• Rise: The total vertical distance from the ground to the deck surface.
• Run: The total horizontal distance the stairs cover from top to bottom.
• Nosing: The front edge of the tread that overhangs the riser below. Provides a visual edge and a secure foothold.

Tools and Materials

Tools

• Tape measure
• 4-foot level
• Framing square with stair gauges (clamp-on guides)
• Circular saw
• Jigsaw or handsaw (for finishing notch cuts)
• Cordless drill and impact driver
• Speed square
• Pencil
• Clamps
• Safety glasses, hearing protection, N95 dust mask

Materials

• 2×12 pressure-treated boards (for stringers — one per 16 inches of stair width, minimum 3)
• Composite deck boards (for treads — use solid boards, not hollow)
• Composite or pressure-treated boards (for risers)
• Stair gauges (attach to framing square)
• Stringer hangers or angle brackets
• Exterior-grade structural screws (3-inch for framing, 2½-inch for treads)
• Concrete landing pad or pre-cast pier blocks
• Joist tape (waterproof flashing for stringer tops)

Step-by-Step Installation

Step 1: Measure the Total Rise

Place a long, straight board or level horizontally from the deck surface out over the landing point. Measure the vertical distance from the bottom of this board to the ground. This is your total rise.

Example: If the total rise is 55 inches, calculate the number of risers: 55 ÷ 7.5 = 7.3. Round up to 8 risers. Then calculate each riser height: 55 ÷ 8 = 6⅞ inches per riser. This is below the 7¾-inch maximum. If you use the deck surface as the top step, subtract one riser from the total (7 risers in this example).

Step 2: Calculate the Total Run

Each tread should be 10–11 inches deep. Multiply the tread depth by the number of treads (one fewer than the number of risers if the deck is the top step).

Example: 7 treads × 10.5 inches = 73.5 inches total run (about 6 feet 1.5 inches from the deck edge to the front of the bottom step).

Step 3: Prepare the Landing

The bottom of the stairs needs a solid, level landing surface. Options include a poured concrete pad (4 inches thick on 4 inches of compacted gravel) or pre-cast pier blocks. The landing should extend at least 36 inches from the bottom step.

Step 4: Cut the Stringers

This is the most critical step. Mistakes here affect every step above.

1. Set stair gauges on a framing square. Clamp one gauge at the riser height (6⅞ inches in our example) on one arm and the tread depth (10.5 inches) on the other arm.
2. Place the framing square on a 2×12 board. Align both gauges with the top edge of the board.
3. Trace the first step. Slide the square along the board and trace each subsequent step.
4. At the top, draw a vertical plumb cut line where the stringer meets the deck frame.
5. At the bottom, subtract the thickness of one tread from the first riser height. Draw a level cut line. This ensures the bottom step is the same height as all other steps once the tread is installed.
6. Cut along all marked lines with a circular saw. Stop each cut just short of the inside corner. Finish the corners with a jigsaw or handsaw. Do not overcut — overcutting weakens the stringer.
7. Use this first stringer as a template to mark and cut the remaining stringers.

Stringer quantity: Use one stringer per 16 inches of stair width. A 36-inch-wide staircase needs 3 stringers. A 48-inch-wide staircase needs 4.

Step 5: Install the Stringers

Attach the top of each stringer to the deck frame using stringer hangers or heavy-duty angle brackets. The top of the stringer should sit flush with the top of the rim joist.

Secure the bottom of each stringer to the concrete landing using angle brackets and concrete anchors.

Check that all stringers are plumb, level across the treads, and equally spaced. Use a level across the tread notches to verify.

Tip: Apply waterproof joist tape to the top of every stringer notch before installing treads. This prevents water from soaking into the pressure-treated lumber at the most vulnerable contact points.

Step 6: Install the Risers

Starting from the bottom, attach a riser board to the vertical face of each step. Use 2½-inch exterior screws. Pre-drill pilot holes to prevent splitting.

Risers can be composite boards or pressure-treated lumber. Composite risers provide a matched appearance with the treads. Pressure-treated risers are cheaper and hidden from direct view.

Step 7: Install the Composite Treads

Start from the bottom step and work upward.

1. Place the tread board on the stringer notch. Position it so the front edge overhangs the riser by ¾–1¼ inches (the nosing).
2. Leave a 3–5 mm expansion gap between the back of the tread and the riser above.
3. Pre-drill and drive two screws per stringer into the tread.
4. Repeat for each step.

Composite-specific tips:

• Use solid composite boards for treads. Solid boards are stronger and provide a cleaner front edge. Hollow boards expose the internal chambers at the nosing, which looks unfinished even with end caps.
• Use textured or grooved boards. Stairs are the highest-risk area for slips. Grooved composite provides better wet traction than smooth profiles.
• Orient the board correctly. Place the finished (textured) side up.
• Seal cut ends. Cutting exposes the uncapped core of capped composite boards. Apply manufacturer-recommended end sealant to any cut edge that remains visible.

COOWIN solid composite decking is ideal for stair treads. The dense core provides full structural support. The co-extruded HDPE shell resists moisture and staining on the high-traffic tread surface. Request free samples to evaluate the material before ordering.

Step 8: Install Stair Railings

Stairs with four or more risers require a handrail by code. Most deck stair railings include posts, a top rail, a bottom rail, and balusters.

1. Install posts. Mount a post at the top and bottom of the stairs. For long stairways, add intermediate posts every 6 feet. Posts should be bolted through the stringer or attached with heavy-duty post brackets. Never rely on screws alone for stair railing posts.
2. Set the rail angle. Stair railings follow the slope of the staircase. Measure the angle of the stringer and cut the top and bottom rail ends to match.
3. Attach rails. Mount the top rail at 34–38 inches above the stair nosing, measured vertically. Mount the bottom rail parallel to the top rail.
4. Install balusters. Space balusters so that no gap exceeds 4 inches. Pre-drill and screw or clip each baluster into place.

COOWIN composite railing systems include stair-compatible components with angle-adjustable brackets. They coordinate with COOWIN decking colours for a unified appearance.

Final Thoughts

Building composite deck stairs requires careful measurement and consistent execution. Every riser the same height. Every tread has the same depth. No exceptions.

Take your time with the stringer layout. Double-check every measurement before cutting. Use solid composite boards for treads and apply joist tape to every stringer notch. These details determine whether your stairs are safe, durable, and code-compliant for years to come.

Frequently Asked Questions

How many stringers do I need for deck stairs?

Use one stringer per 16 inches of stair width. A standard 36-inch-wide staircase needs 3 stringers. A 48-inch-wide staircase needs 4. Never use fewer than 3 stringers for composite treads.

Should I use solid or hollow composite for stair treads?

Always use solid composite boards for stair treads. Solid boards are stronger and provide a clean front edge at the nosing. Hollow boards expose internal chambers at the front edge and offer less structural support under concentrated foot traffic.

How do I calculate the number of steps I need?

Measure the total vertical distance (rise) from the ground to the deck surface. Divide by 7.5 to get an approximate number of risers. Round up to a whole number. Then divide the total rise by that number to get the exact height of each riser. Each riser must be under 7¾ inches.

Do deck stairs need a concrete landing?

Yes. The bottom of the stairs needs a stable, level surface. A poured concrete pad (4 inches thick on compacted gravel) is the most common solution. Pre-cast pier blocks work for lightweight stairs. The landing should extend at least 36 inches from the bottom step.

Can I install composite stair treads over existing wood stairs?

Yes, if the existing stringers and framing are structurally sound. Remove the old treads and risers. Inspect the stringers for rot, cracks, or warping. Replace any damaged stringers. Then install composite treads and risers on the existing stringer framework.