Stop Sloping Deck Stairs: Smarter Stringer Layout That Accounts for Wood Shrinkage

Building deck stairs that remain level and safe for years requires more than accurate initial layout. One of the most persistent problems builders face is returning to a deck months or years after construction to find stair treads sloping forward, even though they were carefully set level during installation. The root cause is almost always wood shrinkage in the stringers. This article explains why this happens and presents practical strategies to prevent it, from selecting better materials to adjusting your cutting technique.

For a broader overview of outdoor stair construction, see our guide on building deck stairs that are safe and code compliant. Understanding the full stair-building process helps you see where shrinkage compensation fits into the bigger picture.

What Causes Deck Stair Treads to Slope Over Time

When a builder carefully lays out stringers with a framing square, cuts the notches precisely, and installs treads dead level, the assumption is that the stairs will stay that way. But dimensional lumber, especially pressure-treated wood commonly used for decks, is rarely at its final moisture content at the time of installation.

The Science of Lumber Shrinkage in Pressure-Treated Wood

Almost all lumber shrinks as it dries. The amount of shrinkage depends on several factors:

• Initial moisture content – Pressure-treated lumber is often delivered wet or even soaking, with moisture content well above 30 percent.
• Board width – Wider boards shrink more in absolute terms. A 2×12 stringer loses more across its 11-1/4 inch width than a narrower board would.
• Species and grain orientation – Flat-sawn boards shrink more tangentially than quarter-sawn stock, though this is harder to control when buying dimensional lumber.
• Climate conditions – Deck stairs in arid climates experience more total shrinkage than those in humid regions because the equilibrium moisture content is lower.

When a 2×12 stringer dries from green condition (about 30 percent moisture content) to equilibrium (about 12 to 15 percent in most U.S. climates), it can shrink roughly 1/8 to 1/4 inch across its width. This shrinkage does not happen evenly across the board. The outer edges dry and shrink first while the core remains wet, causing the board to cup slightly and the critical tread-to-riser geometry to shift.

How Shrinkage Disrupts Stringer Geometry

The problem is easiest to visualize by looking at the stringer notch. A stair stringer has a series of horizontal cuts (treads) and vertical cuts (risers). The tread cut must remain level for the stair to be safe and comfortable. As the 2×12 shrinks across its width:

• The distance from the top of the stringer to the tread cut decreases
• Each tread notch rotates slightly as the wood contracts
• The outer edge of each tread cut drops relative to the inner edge
• All installed treads tilt forward by a small but noticeable amount

The result is a staircase where every tread slopes toward the front. Even a 1/8 inch drop at the nosing creates a visible and measurable slope that can feel unstable and collect water rather than shedding it properly. Over a full staircase with multiple treads, the cumulative effect is both unsightly and potentially hazardous.

Understanding these moisture dynamics is the first step toward eliminating the problem. The second step is choosing materials and techniques that work with shrinkage rather than against it.

Material Choices That Eliminate Stringer Shrinkage Problems

The most reliable way to avoid sloping treads is to use stringer stock that does not shrink measurably after installation. Several alternatives to standard dimensional lumber are available, each with its own trade-offs.

LVL Stringers for Outdoor Deck Construction

Laminated Veneer Lumber (LVL) is an engineered wood product made from thin veneers bonded together with waterproof adhesives under heat and pressure. Unlike dimensional lumber, LVL is manufactured at a low moisture content (typically 10 to 12 percent) and remains dimensionally stable over time. Key advantages include:

• Minimal shrinkage – LVL does not shrink or swell appreciably with changes in humidity
• Elimination of defects – No knots, checks, or warps that weaken or distort stringers
• Consistent strength – Uniform engineering properties across every piece
• Straightness – LVL stays straight, making layout and installation more accurate

For deck use, select an LVL product rated for exterior exposure. Look for a stamp indicating the product meets PS 2-10 or similar standard for structural engineered wood and confirm the adhesive is waterproof (exterior or exposure 1 rating). Some manufacturers produce dedicated outdoor-rated LVL that includes additional treatment for moisture resistance.

Comparing Stringer Material Options

Each material choice comes with different performance characteristics and costs. The comparison below highlights the most important differences for deck stair construction.

For most residential deck stair applications, exterior-rated LVL offers the best balance of cost, performance, and ease of fabrication. When the budget requires using dimensional lumber, there are layout techniques that can dramatically reduce the sloping problem.

Choosing the right materials and framing approach is part of a broader strategy for durable outdoor construction. Our article on choosing deck materials and framing methods covers how material selection affects long-term performance across the entire deck structure.

Cutting Stringers to Compensate for Lumber Shrinkage

When dimensional lumber is the only practical option, experienced builders adjust the stringer layout to anticipate the shrinkage. This technique requires understanding how the wood will move and making precise adjustments to the cut angles.

Measuring Moisture Content Before Cutting

The first step is knowing what you are working with. A moisture meter is an essential tool for anyone building deck stairs with dimensional lumber. Here is the process:

1. Take moisture readings at several points across the width and along the length of each stringer blank
2. Average the readings to determine the current moisture content
3. Estimate the equilibrium moisture content for your climate (typically 12 to 15 percent)
4. Calculate the expected shrinkage: roughly 1 percent of the board width for every 4 percent change in moisture content
5. Use this estimate to determine how much compensation to build into your cuts

For a 2×12 stringer at 30 percent moisture content drying to 12 percent in a dry climate, the expected shrinkage across the 11-1/4 inch width is approximately 1/2 percent per percentage point of moisture loss, or about 1/4 inch total. Remember that shrinkage happens from the edges inward, so only half of that total affects the tread level on each side.

Cutting Stringer Notches Out of Square

John Spier, a builder on Block Island, Rhode Island, and the source of this technique, describes adjusting the framing square layout to cut both the riser and tread sides of each notch slightly out of square. The goal is to angle the cuts so that as the wood shrinks, the tread cut rotates back to level.

The procedure works as follows:

1. Lay out the stringer normally – Use a framing square with stair gauges to mark tread and riser positions as you would for any staircase.
2. Adjust the tread cut angle – Instead of cutting exactly on the layout line, angle the circular saw blade or handsaw so the tread cut tilts upward at the outer edge by the estimated shrinkage amount.
3. Adjust the riser cut angle – Cut the riser side of the notch with a complementary angle so the notch remains properly shaped.
4. Cut trial stringers first – Test your compensation on a short mock-up before cutting production stringers.

How Much to Compensate

The compensation amount depends on how damp the lumber feels:

• Damp but not soaking (moisture content around 20 to 25 percent) – Compensate by about 1/8 inch
• Soaking wet (moisture content above 30 percent) – Compensate by up to 1/4 inch
• Partially dried (moisture content around 15 to 20 percent) – Compensate by about 1/16 inch

These adjustments add approximately 1 to 2 degrees of angle to the tread cut. The exact amount requires judgment based on the moisture readings you take before cutting. When in doubt, err on the side of slightly more compensation, because a tread that slopes very slightly backward (toward the riser) is safer and less noticeable than one that slopes forward.

Practical Steps for Level Deck Stairs That Stay Level

Beyond material selection and cutting technique, several practices help ensure your deck stairs remain level and safe for their entire service life.

Proper Support and Fastening

Stair stringers need solid bearing at the top and bottom, regardless of the material used. Follow these guidelines:

• Top connection – Attach stringers to the deck framing with galvanized or stainless steel stair hangers or metal angles rated for the stringer size
• Bottom bearing – Rest stringers on a concrete landing or pressure-treated bearing plate that is below frost depth and sloped away from the house for drainage
• Lateral bracing – Install solid blocking between stringers at mid-span for runs exceeding 6 feet, and at the top and bottom of every run
• Fastener selection – Use hot-dipped galvanized or stainless steel fasteners for all connections to avoid corrosion and galvanic reactions with pressure-treated wood

Ensuring the entire stair structure is built to code-compliant standards is critical for both safety and durability. Local building codes specify minimum stringer dimensions, maximum span lengths, and load requirements that your design must meet.

Staging Installation for Best Results

Timing matters when working with pressure-treated stringers. Consider this sequence:

1. Cut and install stringers as early as possible in the deck-building process, ideally several weeks before installing treads
2. Allow the stringers to dry and stabilize in place
3. Check tread level after two to four weeks of drying
4. Shim or adjust the bearing points as needed before installing final treads
5. Install treads with a slight back-slope (1/8 inch over the tread depth) as insurance against future movement

This staged approach lets the stringers do most of their shrinking before the treads go on, so you can correct any slope before the final installation. While it adds a logistical step to the deck build, the results are far more professional and durable.

Inspection and Maintenance

Even with the best materials and techniques, deck stairs should be inspected annually. Look for:

• Treads that have developed a forward slope of more than 1/4 inch from front to back
• Gaps between treads and stringers that indicate shrinkage or movement
• Loose or corroded fasteners at stringer connections
• Rot or insect damage at bearing points, especially where wood meets concrete

Our free DCA 6 deck construction guide includes detailed inspection checklists and maintenance schedules that apply to all deck components, including stairs. Following these guidelines helps catch small problems before they become safety hazards.

By understanding how lumber shrinkage affects stair geometry, selecting appropriate materials, and using proven compensation techniques, you can build deck stairs that remain safe, level, and attractive for decades. Whether you choose exterior-rated LVL for a worry-free solution or master the out-of-square cutting technique for dimensional lumber, the investment in better stair construction pays off every time you walk up and down your deck stairs.