The table below compares several useful current stair-stringer spacing references.

The wood entry comes from the American Wood Council’s DCA6 prescriptive residential wood-deck guide. Composite entries come from current manufacturer guidance for the listed product or assembly.

The correct question is not simply:

“How far apart should composite stair stringers be?”

It is:

“What is the maximum stair-stringer spacing for this exact tread product and this exact stair assembly?”

Once you know the maximum permitted stringer spacing, use the calculator below to estimate how many stringers the stair width requires.

Enter a preliminary stringer-layout width and the maximum spacing permitted by the tread product or approved stair detail.

The chart below applies the same preliminary formula as the calculator: divide the width into enough equal spaces that no space exceeds the selected maximum, then add one stringer to create supports at both sides.

Start with the maximum permitted stringer spacing for the tread system.

Then use:

Example: 36-Inch Stair at a 12-Inch Maximum

36 ÷ 12 = 3 spaces

Three spaces require:

4 stringers

Example: 48-Inch Stair at a 9-Inch Maximum

48 ÷ 9 = 5.33 spaces

Round up:

6 spaces

Then add one:

7 stringers

If the final outside-stringer centerlines are 48 inches apart:

48 ÷ 6 = 8 inches OC

That stays below the 9-inch maximum.

On-center spacing is measured from the centerline of one stringer to the centerline of the next.

It is not the clear opening between the faces of the stringers.

A nominal 2x stringer is approximately 1 1/2 inches thick. If two stringers are placed 12 inches on center, the clear distance between their facing surfaces is approximately 10 1/2 inches.

The same centerline convention is used throughout deck framing. See our Deck Joist Spacing Guide for the main deck framing application.

Stringer spacing and stringer span describe two completely different structural checks.

This article primarily answers the first question: how closely the tread needs to be supported across the width of the staircase.

Use our Deck Stair Calculator for rise, run, stair footprint, and preliminary stringer geometry.

Conventional wood stairs can often use wider stringer spacing than stairs surfaced with standard composite deck boards.

The American Wood Council's DCA6 prescriptive residential wood-deck guide shows cut stringers at a maximum of:

in the applicable wood-tread detail.

DCA6 also requires at least three cut stringers where only cut stringers are used and places additional limits on tread size, stringer span, geometry, attachment, and support.

That does not mean every wood stair should automatically be built at exactly 18 inches on center.

Closer spacing can make sense when:

• the selected tread material requires it
• greater tread stiffness is desired
• railing framing creates additional support locations
• tread joints require support
• the approved stair detail calls for closer framing

Cut Stringers vs. Solid Stringers

A cut stringer has material removed to create the rise-and-run profile, leaving a narrower structural section commonly called the throat.

A solid stringer remains unnotched and supports the treads using another approved structural detail.

Because the two configurations behave differently, spacing stringers closer together should not be treated as permission to exceed the allowable span or geometry of the individual stringer.

For the full stair system, see our Deck Stairs Guide .

Composite and PVC decking manufacturers commonly publish a separate support limit for stair applications.

That stair limit may be substantially tighter than the joist spacing permitted beneath the main deck surface.

A product approved across deck joists at 16 inches on center can still require stair stringers at 12, 10, 9 inches, or another product-specific spacing.

Trex Stair Stringer Spacing

Trex's current stair-building guidance provides a clear example.

In its Trex Enhance stair example, the decking is supported at a maximum of:

9 inches on center

Trex also instructs the installer to divide the remaining stair width so that no space between stringers exceeds the permitted decking span.

The important lesson is not that every Trex product equals 9 inches.

It is that the exact Trex collection and board profile must be checked against the current installation instructions.

Fiberon Stair Stringer Spacing

Current Fiberon technical specifications for Concordia list:

• 16 inches perpendicular to ordinary deck joists
• 12 inches when installed diagonally up to 45 degrees
• 12 inches for stair-stringer spacing

Current ArmorGuard Designers Choice specifications also list 12-inch stair-stringer spacing.

This illustrates why the stair requirement should be checked independently from the main deck's framing.

TimberTech Stair Stringer Spacing

TimberTech's current composite installation guide shows how much spacing can vary even inside one manufacturer's product family.

The Terrain row is especially important.

A larger spacing is possible there because the decking is being used as part of a specified supported stair assembly.

Deckorators Supported Stair Assemblies

Deckorators likewise publishes a supported tread detail that permits stringer spacing up to 16 inches on center when the specified support assembly is constructed.

Again, the support system is part of the requirement.

Main-deck boards cross a repeating field of joists and form a broad walking surface.

Stair treads experience a different support condition. Each tread bridges across relatively narrow stringer supports and receives repeated foot traffic near its leading edge.

Manufacturers therefore publish separate stair-support limits.

That is why a board that performs properly over deck joists at 16 inches on center may need stair stringers at 12, 10, or 9 inches on center.

Related: Deck Joist Spacing Guide .

Yes — if the permitted maximum spacing stays the same.

A wider stair does not get permission to spread the existing stringers farther apart.

Instead, more stringers are added.

For example, using a 12-inch maximum preliminary spacing:

• 36-inch layout → 4 stringers
• 48-inch layout → 5 stringers
• 60-inch layout → 6 stringers
• 72-inch layout → 7 stringers
• 96-inch layout → 9 stringers

At tighter composite spacing, the count grows quickly.

That is worth considering before choosing a tread product for broad patio transitions, wraparound stairs, or oversized architectural staircases.

Stringer spacing is usually expressed as a maximum distance.

But some stair details can also establish:

• a minimum number of stringers
• required supports at stair edges
• dedicated support around newel or guard posts
• support beneath tread joints
• additional framing beneath a supported tread assembly

Therefore, the calculator result is not always the final count.

Stair guard and handrail framing can interrupt an otherwise simple, evenly-spaced stringer layout.

Newel or guard-post locations can require:

• dedicated blocking
• adjacent framing
• additional stringers
• reinforced connection zones
• manufacturer-specific post hardware

The stringer layout therefore should be coordinated with the railing layout before the entire flight is framed.

For broader guard and handrail requirements, see our Deck Railing Code Guide .

Tread boards should not be allowed to meet at an unsupported joint unless the product's approved installation detail specifically allows that condition.

A joint may require:

• a dedicated stringer
• paired support
• blocking
• another manufacturer-approved support detail

This becomes particularly important on very wide stairs where a single tread board does not run continuously across the entire stair.

The stringers support the stair tread, so the tread's ability to span between those supports directly affects the permitted stringer spacing.

Two products that look nearly identical from above can have different:

• board thicknesses
• core structures
• solid or scalloped profiles
• material properties
• approved stair-support details

A thicker or structurally different tread can sometimes be permitted to span farther than a standard deck board.

Composite decking systems commonly use square-edge boards for stair treads because the tread edge and fastening method differ from the hidden-fastener field of the main deck.

Grooved boards are primarily designed to accept concealed fasteners along their side grooves. Stair treads frequently need face-fastened, plugged, or otherwise product-specific attachment.

The exact allowed profile depends on the manufacturer.

See our Decking Materials Guide when comparing tread-material options.

1. Confirm the stair geometry. Calculate the rise, run, tread depth, and total stair layout first.
2. Select the exact tread product. Identify the manufacturer, collection, thickness, and profile.
3. Find the stair-specific support limit. Do not rely on the ordinary deck-joist spacing.
4. Check for a minimum stringer count. Some product details impose both a maximum spacing and a minimum count.
5. Determine the preliminary number of stringers. Use the calculator or formula above.
6. Establish the outside stringer locations. Account for tread edges, skirts, fascia, railing posts, and fastening.
7. Measure between actual outside-stringer centerlines.
8. Divide that distance evenly. Keep every resulting space at or below the permitted maximum.
9. Add any required support. Account for newel posts, tread joints, and manufacturer-specific framing.
10. Verify one complete stringer before duplicating it.

For the cutting and duplication process, see How to Cut Stair Stringers .

Building the full project? See our Deck Building Tools Guide .

Every correctly spaced stringer still needs an intentional structural load path at both ends of the stair flight.

At the Top of the Stairs

The stringer reaction must transfer into suitable rim, header, or other supporting deck framing through an approved connection.

The connection can involve:

• a properly designed stair header
• rim/header reinforcement
• blocking
• approved stringer connectors
• specified structural fasteners

See our Deck Rim Joist & Header Guide for the load path at the deck perimeter.

At the Bottom of the Stairs

The lower end of the stringer needs stable bearing on the designed landing or support.

Settlement, frost movement, drainage, and local foundation requirements can affect this part of the stair assembly.

Also see Deck Screws vs Structural Screws before choosing fasteners for structural stair connections.

Stair stringers create repeated horizontal bearing surfaces beneath the treads where water can collect around fasteners and between materials.

Good drainage, appropriate treated lumber, required cut-end treatment, and compatible flashing or protection details can reduce repeated moisture exposure.

This can be especially relevant beneath long-lived composite or PVC decking, where the surface material may remain serviceable for decades while the wood framing below remains structural.

See our Deck Joist Tape Guide for framing-protection considerations.

Sometimes.

When existing wood stairs are resurfaced with composite or PVC, the original stringers may be spaced farther apart than the new tread product permits.

Adding intermediate stringers can help satisfy the tread-support requirement, but it does not automatically make the existing staircase suitable for resurfacing.

Check:

• condition of every existing stringer
• remaining wood at the notches
• upper attachment
• bottom bearing
• landing stability
• riser consistency
• new tread thickness
• railing connections
• required stringer spacing for the new product

Use our Deck Inspection Checklist when evaluating an existing deck and stair structure.

1. Assuming Every Stair Uses 12-Inch Stringers

Twelve inches works for some tread systems, but current wood and composite details also use 18, 16, 10, 9 inches, and other product-specific limits.

2. Copying the Main Deck's Joist Spacing

A composite board allowed over deck joists at 16 inches on center may require substantially closer support when used as a stair tread.

3. Looking Up the Brand Instead of the Product

Different collections and profiles from the same manufacturer can have different stair-support requirements.

4. Treating the Maximum Spacing as a Required Target

Maximum spacing means the stringers cannot be farther apart than that distance under the stated conditions. They can be closer.

5. Rounding the Stringer Count Down

Rounding down can create a spacing wider than the selected tread permits.

6. Ignoring a Manufacturer Minimum Stringer Count

A product detail can require more stringers than the basic spacing formula alone suggests.

7. Ignoring Supported-Tread Assembly Requirements

A larger permitted spacing can depend on additional framing beneath the tread. Do not use the larger number without building the required assembly.

8. Letting a Tread Joint Float

Board joints can require dedicated support or another approved framing detail.

9. Moving Stringers Around Railing Posts Without Rechecking Support

Newel and guard framing must be coordinated with the tread-support layout.

10. Using Fascia as Structural Tread Material

Fascia and riser boards are finish components unless the manufacturer specifically approves another structural use.

11. Confusing Stringer Spacing With Stringer Span

Side-to-side tread support and the structural capacity of the stringer down the stair flight are separate checks.

12. Assuming More Stringers Fix Every Stair Problem

Additional stringers do not repair an inadequate throat, excessive unsupported span, weak top connection, unstable landing, or incorrect stair geometry.

How far apart should deck stair stringers be?

It depends on the tread material and stair system. AWC DCA6 shows cut stringers at up to 18 inches on center in its applicable prescriptive wood detail, while many composite products require closer product-specific spacing.

Is 16 inches on center acceptable for stair stringers?

Sometimes. Certain wood or specifically supported composite stair assemblies can permit spacing in that range, while many standard composite tread installations require closer supports.

Is 12 inches on center standard for composite stairs?

No. Some Fiberon products currently specify 12 inches, while current Trex and TimberTech examples include tighter spacing. Use the exact product instructions.

Can stair stringers be 18 inches apart?

AWC DCA6 permits up to 18 inches on center in its applicable prescriptive cut-stringer wood-stair detail. That number should not be transferred to a composite or PVC stair unless the exact tread system permits it.

Why does composite decking need more stringers?

Many composite stair treads have a shorter permitted unsupported stair span than conventional wood tread assemblies. Closer stringers reduce the distance each tread must span.

How many stringers do I need for a 36-inch stair?

Using the preliminary spacing formula, a 36-inch layout requires 3 stringers at an 18-inch maximum, 4 at a 12- or 16-inch maximum, 5 at a 9- or 10-inch maximum, and 6 at an 8-inch maximum. Manufacturer minimums or other framing details can increase the count.

How many stringers do I need for a 48-inch stair?

Using the preliminary formula, a 48-inch layout requires 4 stringers at an 18- or 16-inch maximum, 5 at 12 inches, 6 at 10 inches, and 7 at 9 or 8 inches. Verify final outside-stringer centerlines and product-specific requirements.

How many stringers do I need for a 60-inch stair?

Using the preliminary formula, a 60-inch layout requires 5 stringers at an 18- or 16-inch maximum, 6 at 12 inches, 7 at 10 inches, 8 at 9 inches, and 9 at 8 inches.

Should stair stringers be evenly spaced?

Generally, yes. Once the outside stringer locations and required count are known, divide the actual centerline distance into equal spaces that do not exceed the permitted maximum. Railing posts or other support requirements can create intentional exceptions.

Do I need a stringer under every composite board joint?

The joint needs the support required by the manufacturer's stair detail. That may involve a stringer, paired framing, blocking, or another approved support arrangement.

Can I use my deck's 16-inch joist spacing for the stairs?

Only when the exact stair-tread product and assembly permit it. Many composite products approved at 16 inches over deck joists require closer support on stairs.

Are stair stringers measured on center?

When the requirement is stated as OC, yes. Measure from the centerline of one stringer to the centerline of the next.

Does adding more stringers make stairs stronger?

More stringers provide closer tread support and can reduce tread flex. They do not automatically increase the allowable structural span or capacity of each individual stringer.

Deck stair stringers are not governed by one universal 12-, 16-, or 18-inch spacing rule.

Start with the exact stair-tread system.

For conventional wood stairs, an applicable prescriptive detail can permit relatively wide spacing. For composite and PVC stairs, use the current manufacturer requirement for the exact product, profile, and stair assembly.

Then determine the basic number of support spaces:

After establishing the outside stringer locations, recalculate the actual equalized spacing from centerline to centerline and add any supports required by railing posts, tread joints, manufacturer minimums, or other stair details.

Continue with our Deck Stair Calculator , Deck Stair Dimensions Guide , and How to Cut Stair Stringers when you are ready to move from support planning into stair geometry and cutting.

Last reviewed: September 2026

• American Wood Council — DCA6 Prescriptive Residential Wood Deck Construction Guide
• Trex Academy — How to Build Deck Stairs
• Trex — Current Installation Guides
• TimberTech — Composite Decking Installation & Maintenance Guide
• Fiberon — Concordia Decking Technical Specifications
• Fiberon — ArmorGuard Designers Choice Technical Specifications
• Deckorators — Vista Decking Installation Instructions