Shrinking Stringers: Understanding and Preventing Stair Framing Problems from Lumber Shrinkage

Few things are more frustrating for a builder than returning to a project after months of settling to find that carefully cut stair stringers no longer fit correctly. The rise and run cuts that once met at perfect right angles have shifted, and the stairs that were level are now noticeably out of true. This phenomenon — shrinking stringers — is cause

Wood is a hygroscopic material — it constantly exchanges moisture with the surrounding air. When lumber leaves the mill, it typically has a moisture content (MC) between 19% and 28% for “green” or S-Green lumber, or around 19% for S-Dry (surface-dried) lumber. In a conditioned

The critical characteristic of wood shrinkage is that it is highly anisotropic — that is, it shrinks differently in different directions. Wood shrinks very little along its length (longitudinally), modestly in the radial direction (across the growth rings), and significantly in the tangential direction (along the growth rings, perpendicular to the grain). For a typical flat-sawn 2×12 used as a stair stringer, the tangential shrinkage is the primary concern.

ttle along its length (longitudinally), modestly in the radial direction (across the growth rings), and significantly in the tangential direction (along the growth rings, perpendicular to the grain). For a typical flat-sawn 2×12 used as a stair stringer, the tangential shrinkage is the primary concern.

Why Stairs Are Particularly Vulnerable

Stair stringers are typically cut from 2×12 lumber, which is almost always flat-sawn (tangentially cut) from the log. Flat-sawn boards expose the widest face to tangential shrinkage, which means a 2×12 can shrink by 1/4 inch or more in its 11-1/4 inch width as it dries from 19% MC down to 8% MC. This change in stringer depth directly alters the geometry of the stair.

Consider a typical stair with a 7-inch rise and 11-inch run. The stringer is cut so that the tread (horizontal) and riser (vertical) cuts meet at precisely 90 degrees. If the stringer shrinks by 1/4 inch in depth, the geometry distorts: the rise dimension effectively decreases at the top of the stringer, and the tread nosing angle shifts. The result is treads that slope slightly outward and risers that are no longer perfectly vertical.

Solutions for Minimizing Stringer Shrinkage

Select Dried Lumber

The most effective solution is to start with lumber that is already at or near its equilibrium moisture content. Look for lumber stamped S-Dry (surface dried to 19% MC) or KD15 (kiln dried to 15% MC). Even better, specify KD (kiln-dried) lumber with a moisture content of 12% or less. This lumber costs more but dramatically reduces post-installation shrinkage.

Pre-Dry Your Stringers

If you can’t source kiln-dried lumber, buy your 2×12s several weeks or months ahead of time and allow them to acclimate in the space where they’ll be installed. Stack the boards flat with 1/2-inch stickers (narrow spacers) between each layer to allow air circulation around all surfaces. Weight the stack to reduce the cupping that flat-sawn boards are prone to as they dry. An indoor environment with stable humidity (such as the basement or garage of the house being built) is ideal.

Use Smaller Lumber When Possible

A 2×10 stringer will experience less absolute shrinkage than a 2×12 because the dimension across the grain is smaller. However, building codes require that the uncut portion of a stair stringer (the solid wood remaining above the notch cuts) must be at least 3-1/2 inches deep. If your stair design allows and the structural calculations support it, 2×10 stringers can be a viable alternative that reduces shrinkage issues.

Switch to Engineered Lumber

Laminated veneer lumber (LVL) and laminated strand lumber (LSL) are manufactured at low moisture content (typically around 12%) from thin veneers or strands oriented primarily along the beam’s length. Because of their construction, these engineered products exhibit minimal shrinkage after installation — practically nil in most cases. LVL stringers are dimensionally stable, consistent, and free of the knots and defects common in solid lumber. The cost premium is significant but may be justified for high-end finish stairs.

Consider Prefabricated Stairs

Another option is to use a prefabricated stair system whose treads and risers are leveled and plumbed with wedges. These systems allow for field adjustment after the stringers have stabilized, compensating for any minor shrinkage that does occur. Prefabricated stairs are common in commercial construction and are increasingly available for residential applications.

Construction Techniques to Minimize Damage

Regardless of which stringer material you choose, construction techniques can help mitigate the effects of shrinkage:

• Use construction adhesive: Apply a generous bead of construction adhesive to the stringer face before fastening treads and risers. The adhesive bonds the finish materials to the stringer across the entire contact surface, reducing the likelihood of nail pops and squeaks even if minor separation occurs.
• Use ring-shank or spiral nails: These nails have significantly greater holding power than smooth shank nails. While smooth nails can gradually back out as wood shrinks, ring-shank nails resist withdrawal forces effectively.
• Install treads after stringers stabilize: If possible, install the rough stringers and allow them to acclimate for several weeks before cutting and installing finish treads and risers. This is easiest in new construction where the stairwell can be temporarily decked for access.
• Avoid fastening before drying: Fastening finish treads and risers to a green stringer is counterproductive. As the stringer shrinks away from the back of treads and risers, it will pop nails and create squeaks — the exact opposite of what you want in a finish stair.

The Tolerable Threshold

It’s important to recognize that some degree of stringer shrinkage is normal and often imperceptible in daily use. In a typical home with 2×12 stringers cut from S-Dry lumber, the tread slope might change by 1/8 to 3/16 inch over the tread depth — roughly 1 to 1.5 degrees from level. While a builder’s level will detect this, most occupants never notice it. Stairs are remarkably forgiving structures within reasonable tolerances.

For more on wood properties and construction techniques, see our articles on wood design and flooring types.

Conclusion

Shrinking stringers are a predictable consequence of using solid-sawn lumber in stair construction. While complete prevention is impossible without using engineered materials, the combination of dried lumber, proper acclimation, adhesive fastening, and appropriate nail selection can reduce the issue to negligible levels. Understanding the science behind wood shrinkage transforms this frustrating problem from a mystery into a manageable engineering consideration.

The Structural Implications of Stringer Shrinkage

Beyond the cosmetic issue of slightly unlevel treads, stringer shrinkage can have structural implications that affect safety. The International Residential Code (IRC) requires that stair stringers be designed to support a minimum live load of 40 pounds per square foot with a minimum concentrated load of 300 pounds applied at any point. When a stringer shrinks and distorts, the stress distribution within the member changes. Shrinkage-induced cracks can propagate, reducing the effective cross-section and potentially compromising the stringer’s load-bearing capacity.

The critical area is the “heel” of the stringer cut — the point where the notch for the bottom tread meets the uncut portion of the stringer. This is already a stress concentration point. Shrinkage can create additional stresses at this location, particularly if the stringer cups (curves across its width) as it dries. A cupped stringer bears unevenly on the supporting structure, creating rotational forces that the original design did not account for.

Species-Specific Shrinkage Characteristics

Not all wood species shrink equally. Builders who understand species-specific behavior can make informed choices that minimize shrinkage problems:

Note that SPF — one of the most common framing lumber species — has relatively high tangential shrinkage. Combined with the fact that 2x12s are almost always flat-sawn, this makes SPF stringers particularly susceptible to shrinkage problems. If you’re building with SPF, the case for using kiln-dried or engineered lumber is especially strong.

How Moisture Content Affects Stair Construction Timing

The ideal moisture content for stair stringers is within 2 percentage points of the equilibrium moisture content (EMC) of the finished building. The EMC of wood indoors in most of North America ranges from 6% to 10%, depending on climate and season. In a heated home in winter, indoor humidity drops to 20-30%, which corresponds to a wood EMC of about 6%. In summer, with humidity at 50-60%, the EMC rises to about 10-11%.

This means that stringers installed in summer at 12% MC will continue to dry and shrink through the first winter, reaching 6-8% MC. The total shrinkage from 12% to 6% MC for a flat-sawn 2×12 (11.25 inches wide) is about 1/8 to 3/16 inch — enough to cause noticeable tread slope changes. Stringers installed in winter at 8% MC and then exposed to summer humidity may actually gain moisture and swell slightly, though the change is typically less problematic.

The practical implication: install stringers in the same season and under the same environmental conditions as when the finish stairs will be used. If you’re building a summer cottage that won’t be heated in winter, seasonal moisture swings will be larger and the potential for shrinkage problems increases.

Special Considerations for Open Risers and Winding Stairs

The problems described above are amplified in stairs with open risers (where the vertical space between treads is open) and in winding or curved stairs. Open-riser stairs have fewer structural connections between stringers and are more sensitive to dimensional changes. Winding stairs — where treads taper from wide to narrow — create complex stress patterns in the stringer that can be significantly affected by shrinkage.

For these more challenging stair designs, engineered lumber stringers are strongly recommended. The dimensional stability of LVL or LSL eliminates the uncertainty of solid-sawn lumber shrinkage and ensures that the precise geometry of complex stair layouts remains stable over time.

For more on wood properties and construction techniques, see our articles on wood design and flooring types and estimating the life of a building.

Conclusion

Shrinking stringers are a predictable consequence of using solid-sawn lumber in stair construction. While complete prevention is impossible without using engineered materials, the combination of dried lumber, proper acclimation, adhesive fastening, and appropriate nail selection can reduce the issue to negligible levels. Understanding the science behind wood shrinkage transforms this frustrating problem from a mystery into a manageable engineering consideration. For the most demanding applications — open riser stairs, winding stairs, or high-end finish work — the small additional cost of LVL or LSL stringers is a worthwhile investment in long-term performance.