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Lumber Stacking and Sticker Placement: Why Sticker Spacing Controls Drying

A stacker turns a stream of green boards into stickered packages, and the spacing, thickness, and alignment of the stickers it places help decide how flat the lumber comes out of the dryer. UTEC Industrial designs, engineers, machines, fabricates, and installs custom material handling systems for aerospace and heavy industry from its Spokane Valley, WA facility, integrating Allen-Bradley PLC and motion control with in-house CNC machining, heat treating, and stress relief. This article sets out what two USDA Forest Products Laboratory general technical reports, the USDA Dry Kiln Operator's Manual, and a laboratory guidebook-checklist say stickers control, with each figure's scope, air drying or kiln drying and hardwood or softwood, and what those figures ask of the stacking machine. Stack quality is also decided along one build chain, design → engineering → parts machining → fabrication → assembly → weld fatigue → stress relief → drives → controls → tuning → monitoring, from the straightness of the sticker guides to the sensing that confirms every sticker landed.

Why is lumber stacked with stickers before drying?​

Federally, 29 CFR 1910.265(b)(40) defines a sticker as a strip of wood or other material used to separate layers of lumber. The USDA Forest Products Laboratory's general technical report on drying hardwood lumber, FPL-GTR-118, states that for almost all drying processes, the most notable exceptions being some vacuum drying processes, lumber must be stacked in horizontal layers, each separated from the next by stickers, and that the major purposes of stacking lumber in a specified manner are to promote uniform air circulation, which in turn results in good drying, and to reduce or eliminate warp.

The laboratory's general technical report on air drying, FPL-GTR-117, lists what stickers do in air drying:

  • They separate the courses of lumber, permitting air to circulate over the broad faces of the boards.
  • Each sticker carries its proportionate share of the weight of the pile above it.
  • In combination with the loads imposed on them, stickers tend to keep the boards flat, restraining warp.
  • Stickers at the two ends of a pile may reduce end checking, splitting, and warping near the board ends.

The same page adds the cost: stickers contact wet boards, and the contact area, somewhat retarded from drying, may develop stain (OSHA 29 CFR 1910.265-2016: Sawmills, §1910.265 paragraph b.40; Denig, Wengert, and Simpson 2000, FPL-GTR-118, p. 30; Forest Products Laboratory 1999, FPL-GTR-117, p. 39).

How do automatic and semiautomatic stackers place stickers?​

FPL-GTR-117 describes both machine types for air-drying yards:

  • Automatic stacker. Identified by the magazines that hold the stickers. The stickers are mechanically fed out the bottom of the magazine onto the lumber layers, with a magazine at each sticker tier, and the magazines are manually kept filled.
  • Semiautomatic stacker. It lays up the courses of lumber onto the package, but the stickers are laid manually into sticker guides on both sides of the stacker frame.
  • Feed. Bulk units from the sorter move on a roller or chain conveyor to a breakdown hoist, and the boards go from the hoist to an unscrambler, where they are separated into a single layer and moved to the stacker lay-up table. A grading and tally station might be located between the breakdown hoist and the lay-up table.

When a course has been laid and the stickers placed, the package is lowered mechanically to receive the next course. FPL-GTR-118 says that today most lumber is stacked with automatic or semiautomatic stackers without internal flues, and the Dry Kiln Operator's Manual says most plants use semiautomatic or automatic stackers. The kiln manual's hoist-and-magazine description is summarized in the sawmill material-flow article. For customer Gillingham Best, UTEC Industrial fabricated sawmill lumber stacking and material handling equipment (Forest Products Laboratory 1999, FPL-GTR-117, p. 35; Denig, Wengert, and Simpson 2000, FPL-GTR-118, p. 31; Simpson 1991, Agriculture Handbook No. 188, p. 109).

What sticker spacing do the USDA manuals give, and for what lumber?​

The manuals give spacing as guidance tied to species, thickness, grade, and drying method, not as one figure. The Dry Kiln Operator's Manual states that optimum sticker spacing is governed by the lumber's tendency to warp, its thickness, and its resistance to crushing, and that in general hardwoods require closer spacing than softwoods. It adds that warp-prone species like sweetgum and the elms benefit by spacing of less than 2 ft, that hardwood stickers thinner than 1 in should also be spaced less than 2 ft apart, and that spacing may need to be reduced to avoid crushing stickers between the bottom courses of heavy loads.

Source and scopeGuidance, in the source's words where quoted
FPL-GTR-118, hardwoodMost hardwood drying operations use 3/4 in thick stickers "spaced 24 in. (610 mm) on center"
FPL-GTR-118, upper grades of 4/4 through 6/4 sizes of most hardwoodsStickers "should be placed every 12 to 18 in." along the length
FPL-GTR-118, hardwood thicker than 6/424 in spacing "is usually acceptable"
FPL-GTR-118, hardwood lower grades, 4/4 through 6/4"should probably be 12 in. (30 cm) on center"
FPL-GTR-118, warp-prone species such as red gumStickers should be spaced 12 in apart
FPL-GTR-117, air drying, 1 in by 16 ftFive stickers "generally" for softwoods; five to nine for hardwood
FPL-GTR-117, air drying, hardwood16 in to 2 ft apart "is common", particularly where sticker guides hold them

As this article's arithmetic, assuming end stickers flush with the board ends, five stickers on a 16 ft board leave four 4 ft spaces, and nine leave eight 2 ft spaces, so the air-drying counts span 2 ft to 4 ft spacing. FPL-GTR-118 also says that for mixed lengths at a normal 24 in spacing, an extra set of stickers should be placed 12 in from each end of the packs; for 12 ft lumber that puts stickers at 0, 12, 24, 48, … 120, 132, and 144 in (Simpson 1991, Agriculture Handbook No. 188, p. 108; Denig, Wengert, and Simpson 2000, FPL-GTR-118, p. 31; Forest Products Laboratory 1999, FPL-GTR-117, p. 41).

Does sticker spacing control drying, or warp?​

As this article reads the sources, spacing is mainly a warp control, although stickers also slow drying where they touch the boards, while sticker thickness sets the air path that dries the wood:

  • Spacing and warp. FPL-GTR-117, writing on air drying, says stickers retard drying to some extent where they contact the boards, so the fewest number of stickers that will produce the desired warp control is best, and that fewer stickers are used with softwoods than with hardwoods, so softwood stickers are wider to provide sufficient bearing surface and avoid crushing the sticker or the lumber.
  • Thickness and drying. FPL-GTR-118 says sticker thickness is an important variable in drying, affecting drying speed and uniformity; typical thickness is 3/4 in for air drying, with 7/8 and 1 in occasionally used, and usually 3/4 in for predrying and kiln drying, with 9/16 to 7/8 in also used. It calls uniform thickness from sticker to sticker extremely important and says nonuniform thicknesses will increase warp.
  • Capacity. As a rule of thumb, FPL-GTR-118 says each 1/8 in increase in sticker thickness reduces kiln capacity by 7%, and that a properly designed kiln will work very well with 3/4 in stickers.
  • Other controls. The same manual notes that in many operations with good stacking procedures, low relative humidity is often a more effective control measure than the use of stickers and bolsters.

As this article's arithmetic, applying the rule of thumb twice, moving from 3/4 in to 1 in stickers, two 1/8 in steps, costs about 14% of kiln capacity, assuming the 7% steps add linearly. As engineering reasoning, the spacing a stacker is set to and the stickers it is loaded with are two separate decisions (Forest Products Laboratory 1999, FPL-GTR-117, p. 41; Denig, Wengert, and Simpson 2000, FPL-GTR-118, p. 31).

Why must sticker tiers be aligned, and how much misalignment matters?​

The manuals give the mechanism and a tolerance. FPL-GTR-117 says that if stickers are not aligned, the superimposed weight will not be transmitted from one sticker through the lumber directly to the next by compression, but will act on the span of the boards to cause warp. The Dry Kiln Operator's Manual says misaligned stickers, particularly in stacks of green lumber, invariably cause nonuniform distribution of weight and result in sharp kinks where the stickers contact the lumber, that the thinner the lumber the greater the possibility of kinking, and that considerable waste results from incorrectly aligned stickers in 4/4 lumber. FPL-GTR-118 states that even a small amount, 1 or 2 in, of misalignment can greatly accentuate warp.

FPL-GTR-117 adds two related points. Sticker guides not only ensure good sticker alignment within a package but also regulate the spacing so it is the same from package to package, which is necessary if stickers, bolsters, and foundation cross beams are to line up in a pile of several packages. And even dry stickers may vary in thickness, and when that is associated with poor sticker tier alignment, the introduced warp may be significantly increased. The federal piling rule that stickers be uniformly spaced and aligned one above the other is set out in the sawmill material-flow article (Forest Products Laboratory 1999, FPL-GTR-117, pp. 40 and 49; Simpson 1991, Agriculture Handbook No. 188, p. 109; Denig, Wengert, and Simpson 2000, FPL-GTR-118, p. 32).

How straight and uniform must stickers be for a stacking machine?​

FPL-GTR-117 is direct about machines. Automatic stackers eject stickers from the bottom of the magazine, and if the stickers are not straight and uniform in cross-sectional dimension and length, the stacker will not function properly; warped and end-battered stickers create problems in automatic stacker operation, so poor stickers must be replaced. In semiautomatic stacking, some sticker warpage can be tolerated, but very crooked stickers should be discarded, twisted stickers rejected because they cause the same problems as excessively thick stickers mixed in with others of uniform thickness, and thin stickers rejected because they restrict air flow. It also says automatic stackers seem to be less troublesome when stickers are made from surfaced lumber.

The FPL's 1993 predryer guidebook-checklist, written for hardwood lumber predryers, turns this into measurements. Its ratings are the guide's own self-rating scale, not standards:

  • Thickness range. Stickers are usually 5/8 to 1 in thick, and all stickers within a predryer charge should be surfaced to a uniform thickness. Measure 20 stickers in use, selected at the stacker or the unstacker, and take the range: 1/32 in or less rates 4, 1/16 in or less rates 3, 1/8 in or less rates 2, and more than 1/8 in rates 1.
  • Straightness. In hardwood plants using semiautomatic stackers and stickers up to 7 or 8 ft long, a significant number of stickers are likely to develop crook or kink after repeated use; stickers with more than 3 in of crook or side kink should not be used.
  • Length. Stickers with broken ends that are shorter than the package width by 3 in or more should not be used (Forest Products Laboratory 1999, FPL-GTR-117, pp. 40–41; Wengert and Boone 1993, FPL-IMP-GTR-3, pp. 9–10).

What do bolsters and package dimensions add?​

Bolsters separate packages in a pile so lift-truck forks can enter, and the manuals treat their placement as part of sticker alignment:

  • Air drying. FPL-GTR-117 says bolsters are usually 4 by 4 in wood members as long as the stickers, that bolster placement is critical because out-of-line bolsters between packages can cause warp, and that while bolsters at every sticker tier would give maximum restraint, it is not considered practical, so five bolsters are usually used on 16 ft lumber.
  • Hardwood drying. FPL-GTR-118 says bolsters should be placed at every sticker, especially if warp control is important.
  • Bolster thickness. The predryer guidebook says different bolster thicknesses can cause warp and kink as boards bend over the bolster, and unequal bolsters under side-by-side packages can misalign the sticker spaces and block horizontal airflow. Bolsters tend to become compressed the first few times in use and damaged from dropping and being hit by forklift tines; a bolster thickness range of 1/8 in or less rates 4 on its scale.
  • Package size. FPL-GTR-117 says package width is determined by the subsequent drying process and the load capacity of the forklift truck, and gives 4 ft as a standard height. FPL-GTR-118 gives a typical hardwood package of 4 to 8 ft wide and 2 to 6 ft high, and says the maximum forklift load capacity and kiln dimensions often determine width and height.

FPL-GTR-118 also says extra stickers may have to be added in the bottom four or five layers at the lifting points if the forks are not aligned directly under a sticker. The federal bolster and pile-foundation rule is set out in the lumber-mill industry article (Forest Products Laboratory 1999, FPL-GTR-117, pp. 37–38; Denig, Wengert, and Simpson 2000, FPL-GTR-118, pp. 31–32; Wengert and Boone 1993, FPL-IMP-GTR-3, pp. 9–10).

How do board lengths and package ends affect stacking?​

FPL-GTR-118 states that stacking uniform lengths of lumber within one package is the best method for consistently minimizing warp, maximizing kiln productivity, and maximizing the ease and quality of stacking. When random lengths must be stacked, the lumber is either box piled, its preferred method, with the longest pieces on the outside edges and shorter pieces staggered from end to end, or even-end piled. Even-end piling, it says, usually leads to excessive warp and end checking losses and therefore cannot be recommended for lumber of any value, and it often lowers kiln production efficiency.

FPL-GTR-117 describes the machine side. In stacking untrimmed sorted lumber, one end of the package is made square at the bumper board end of the stacking stall or by the even-ender on the automatic stacker, with the sticker tier at or very near that end. Box-piled packages with a ragged end are difficult to transport without stickers being jolted out of alignment at the ragged end, so the package should be built with both ends square if possible. The Dry Kiln Operator's Manual adds that stickers should be placed flush with or very near the ends of boards whenever possible. The even-ender as a machine is covered in the unscramblers and even-enders article (Denig, Wengert, and Simpson 2000, FPL-GTR-118, pp. 30–31; Forest Products Laboratory 1999, FPL-GTR-117, pp. 35–36; Simpson 1991, Agriculture Handbook No. 188, p. 108).

Which upstream problems show up at the stacker?​

The stacker is where sawing and sorting errors become stacking errors. FPL-GTR-117 says lumber from the sawmill is generally sorted into classifications based on similar drying characteristics before it is sent to the stacker, and that 2 in lumber may require about three times longer to air dry than 1 in lumber of the same species, so it is usual practice to segregate rough lumber by thickness class for air drying. It adds that miscut lumber is difficult to stack, that one thick board in a course may allow adjacent well-manufactured boards to warp because of lack of restraint, and that presurfacing miscut lumber to a uniform thickness reduces warp, sticker breakage, and sticker deformation.

The predryer guidebook puts a number on it: some variation in lumber thickness is to be expected from normal sawing, but thickness variation greater than about 1/32 in is usually a result of sawblade or feed problems in the sawmill. When thick and thin pieces are mixed across a course, the thinner pieces are not restrained by the stickers and are free to cup, twist, and bow. As engineering reasoning, a stacker cannot restrain a board that its stickers do not touch, so saw-line accuracy is part of the stacking specification (Forest Products Laboratory 1999, FPL-GTR-117, p. 33; Wengert and Boone 1993, FPL-IMP-GTR-3, p. 10).

What moisture content do "dry" and "kiln dried" mean for softwood lumber?​

The stacked package is built to reach a moisture content, and for softwood lumber the national definitions are in NIST's voluntary product standard PS 20-25, the American Softwood Lumber Standard:

  • Dry lumber is lumber of less than nominal 5 in thickness which has been seasoned or dried to a maximum moisture content of 19 percent. For lumber of nominal 5 in or greater thickness, the standard notes that timbers are often manufactured and sold without drying, and that a specified maximum moisture content follows the applicable certified grading rules, which for some species define dry lumber as having a maximum moisture content higher than 19 percent.
  • Green lumber is lumber of less than nominal 5 in thickness which has a moisture content in excess of 19 percent.
  • Kiln dried (KD) lumber has been seasoned in a chamber to a predetermined moisture content by applying heat.

The 19 percent figure is a maximum for the thinner sizes, not a drying target. The standard states that the use of DOC voluntary product standards is voluntary and that NIST has no regulatory power in the enforcement of their provisions, but that when such a standard is made a part of a legal document, such as a sales contract or code, compliance with it is enforceable. The standard covers softwood lumber only and gives no stacking or sticker guidance (NIST PS 20-25, Secs. 2.7, 2.12, and 2.14 and front matter).

What sensing and PLC control keep stickers placed and aligned on a stacker?​

The manuals define what a good stack is; the controls confirm the machine built one. The magazine, placement, course-height and guide-setting items below are engineering practice, built on the failure points the manuals name; the PLC-logic item restates the Rockwell manuals:

  • Magazine level. FPL-GTR-117's magazines at each sticker tier are kept filled manually, so a low-level sensor on each magazine warns before a tier runs empty.
  • Sticker placement. A photo-eye or proximity sensor at each tier confirms a sticker dropped on every course before the hoist lowers, since a missing sticker is a spacing and alignment defect in the finished package.
  • Course height. A hoist position encoder lowers the package by the measured course thickness, and a mismatch between expected and measured height flags a thick board or a displaced sticker.
  • Guide settings. Where sticker guides are adjustable, their positions are read back to the PLC and recorded with each package, so the spacing used matches the species and grade being stacked.
  • PLC logic. Logix 5000 controller tasks can be configured as continuous, periodic, or event, and a periodic task performs a function at a specific time interval. Where a hoist or placement axis is servo-driven, Kinetix 5700 ERS3 inverters support DSL and Hiperface encoder feedback.
  • Guarding. The interlocked pit-gate and lockable-switch rules for stackers are set out in the sawmill material-flow article.

UTEC Industrial, a Rockwell Automation Recognized System Integrator, builds UL 508A panels and integrates Allen-Bradley ControlLogix and CompactLogix control with PanelView and FactoryTalk operator interfaces (Forest Products Laboratory 1999, FPL-GTR-117, pp. 35 and 41; Rockwell Automation 1756-RM094N-EN-P-2025, Ch. 5 pp. 39 and 41; Rockwell Automation 2198-UM002E-EN-P, Kinetix 5700, p. 16).

Where does the build chain decide how straight a stack is?​

Each figure in the manuals depends on a machine that holds its geometry for years of cycles:

  • Design and engineering set the guide spacing range and its adjustment steps, the magazine design for the sticker sizes the mill uses, and the hoist's load, which, as engineering reasoning, is heaviest with green lumber.
  • Parts machining sets guide slots and magazine throats. FPL-GTR-117 notes that sticker guide slots sometimes are considerably wider than necessary, resulting in some offset stickers in the tiers, so as engineering reasoning a guide slot is a tolerance, not a clearance.
  • Fabrication, weld fatigue, and stress relief decide whether tall guide columns stay straight and parallel. As engineering reasoning, a guide column that bows by 1 in can by itself produce the 1 or 2 in of misalignment that FPL-GTR-118 says can greatly accentuate warp.
  • Drives, controls, and tuning set hoist positioning and sticker timing, and monitoring records misfeeds per tier.

UTEC Industrial stress-relieves welded frames, including by automated vibratory stress relief, before final machining, which is intended to keep guide and hoist geometry stable after machining (Forest Products Laboratory 1999, FPL-GTR-117, p. 41; Denig, Wengert, and Simpson 2000, FPL-GTR-118, p. 32).

What should a stacker specification state about stickers and packages?​

A specification is easier to test when it names the drying guidance it is built to:

  • the species, thicknesses, grades, and length ranges to be stacked, and whether lumber is sorted by length or box piled;
  • the drying method, air, predryer, or kiln, since the manuals' spacing figures differ by method and by hardwood or softwood;
  • sticker thickness, width, length, and the thickness range the magazines and guides must accept;
  • the sticker spacing settings required, including end stickers and extra stickers at lifting points;
  • package width and height, bolster positions, and the end-squaring method;
  • the sensing, alarms, and package records the PLC must produce.

As engineering practice, a factory test that stacks the mill's own stickers, including worn ones, shows whether the magazines feed the crook and thickness range the plant actually has. UTEC Industrial performs factory acceptance testing and on-site commissioning, where sticker placement and spacing can be demonstrated against the purchase order (Denig, Wengert, and Simpson 2000, FPL-GTR-118, p. 31; Forest Products Laboratory 1999, FPL-GTR-117, p. 41; Wengert and Boone 1993, FPL-IMP-GTR-3, pp. 9–10).

Related Articles

References​

  • OSHA 29 CFR 1910.265-2016: Sawmills. U.S. Department of Labor, 2016.
  • Denig J, Wengert EM, Simpson WT. Drying Hardwood Lumber, Gen. Tech. Rep. FPL-GTR-118. USDA Forest Service, Forest Products Laboratory, 2000.
  • Forest Products Laboratory. Air Drying of Lumber, Gen. Tech. Rep. FPL-GTR-117. USDA Forest Service, Forest Products Laboratory, 1999.
  • Simpson, W.T. (ed.). Dry Kiln Operator's Manual, USDA Agriculture Handbook No. 188. USDA Forest Service, 1991.
  • Wengert EM, Boone RS. Quality Drying in a Hardwood Lumber Predryer: Guidebook–Checklist, Gen. Tech. Rep. FPL-IMP-GTR-3. USDA Forest Service, Forest Products Laboratory, 1993.
  • NIST PS 20-25: American Softwood Lumber Standard. National Institute of Standards and Technology, U.S. Department of Commerce, 2025.
  • Rockwell Automation 1756-RM094N-EN-P-2025: Logix 5000 Controllers Design Considerations. Rockwell Automation, 2025.
  • Rockwell Automation 2198-UM002E-EN-P (2018): Kinetix 5700 Servo Drives User Manual. Rockwell Automation, 2018.

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