What Is Rough-Cut Shiplap? (Sizing & Grading)
Rough-cut shiplap is lumber with unsurfaced faces and overlapping, rabbeted edges. “Rough-cut” describes the board’s surface; “shiplap” describes its edge profile. There is no universal thickness, width, or rabbet size, so measure the stock, specify the profile, and confirm the intended grade and moisture condition before ordering or installing it.
A board marked “rough-cut shiplap” can sound like a complete specification, but it may not tell you enough to buy or use the right material. The lumber’s actual size, edge profile, grade, and moisture condition all matter. Here is how to understand those terms and check whether a batch fits your project.
Shiplap Profile, Sizing & Safety Before Milling
Rough-cut shiplap combines two separate features: rough-sawn faces and edges cut with opposing steps, called rabbets. One board’s rabbet overlaps the next board’s rabbet. The name alone does not set the board’s dimensions or guarantee that it will suit a particular wall or exterior cladding system.
Rough-cut means the wide faces have not been surfaced smooth. The saw marks may remain, and thickness and width can vary. Shiplap means the board edges have complementary rebates, or rabbets, so adjoining boards overlap. A board can be rough-cut without being shiplap, and shiplap can be made from surfaced lumber.
How do you size rough-cut shiplap?
Start by measuring the boards, not by relying on a label such as “one-by-six.” Rough-sawn stock is often sold close to its stated rough dimensions, but the actual size depends on the product and sawmill. A common reference for surfaced-four-sides (S4S) nominal 1×6 lumber is 3/4 inch thick by 5-1/2 inches wide. Do not assume that size applies to rough stock.
Measure several boards from the lot with a tape and caliper, including the thickness and overall width. Record the smallest and largest readings. Then measure the rabbet depth and shoulder width, and fit two sample boards together to check their overlap and visible reveal. There is no universal rabbet depth or reveal to use as a default.
| Item to specify or check | What to record | Why it matters |
|---|---|---|
| Thickness and overall width | Actual measurements across several boards | Rough stock may vary from its stated size |
| Rabbet depth and shoulder | Dimensions from a profile drawing or sample | Controls how boards overlap and sit together |
| Reveal | Visible face width after fitting | Affects the finished pattern and coverage |
| Species and grade | Exact names, plus grading rules if applicable | Helps establish intended use and acceptable defects |
| Moisture content (MC) | Readings from several boards | Helps assess fit and expected movement |
A close-looking joint does not prove that a profile is weather-tight. For exterior work, the project detail must also address drainage, flashing, fastening, and coatings. A shiplap joint by itself is not a complete water-management system.
Before milling, write the required thickness, width, species, grade, moisture target, rabbet dimensions, and reveal into the order or drawing. Ask the sawmill or shaper operator to produce a sample first. Key takeaway: treat the fitted sample and project drawing, not a generic “standard shiplap” claim, as the reference.
Grading and Installation for Interior or Exterior Use
A lumber grade describes how a board’s characteristics compare with a set of rules; it does not simply mean “good” or “bad.” The right grade depends on the intended use. Appearance-grade boards are not automatically suitable for structural or load-bearing work, which needs an applicable recognized grade and design approval.
When a seller represents lumber as graded, ask for the grade name, species, and grading agency or rule. In Canada, consult the applicable National Lumber Grades Authority (NLGA) rules. In the United States, check the grading agency and rules that apply to the species and product. A broad label like “rough-cut” does not tell you how knots, splits, or other features were judged.
What is the difference between appearance and structural grade?
Appearance grading focuses on the visual features allowed by a stated grade. Structural grading evaluates lumber under rules intended for structural use. The categories are not interchangeable. Do not use ungraded or visibly defective boards as load-bearing members just because they look sound or have a tight shiplap joint.
For an interior finish, choose a grade and appearance that meet the project’s needs, then confirm that the profile and dimensions suit the layout. For exterior siding, also confirm the species, moisture condition, finish system, and installation detail. For structural use, follow the project design and use lumber with the required recognized grade.
Practical purchase check: Ask the supplier to identify the grade on the quote or delivery paperwork. If the product is sold without a grade, treat it as ungraded rather than assigning it a grade from appearance alone.
How should you check moisture and fit?
Moisture content, or MC, is the amount of water in the wood, stated as a percentage. Set an acceptance target based on the installation environment, species, climate, and product instructions. A target of 12–15% MC is practical for many siding projects, but it is not a universal code limit or a substitute for project requirements.
Use a calibrated pin moisture meter, and check several boards rather than taking one reading from one end of one board. Follow the meter instructions for the wood species and reading location. If readings vary widely, pause and ask the supplier or project lead how the lot should be handled.
A simple installation sequence is:
- Confirm whether the boards are for interior finish, exterior siding, or a structural purpose.
- Match the species, grade, dimensions, profile, finish, and moisture target to the project documents.
- Fit sample boards together and verify the overlap and reveal before milling or installing the full run.
- Follow the project’s fastening and joint details. For exterior work, include the specified drainage and flashing details.
Key takeaway: grade and moisture are separate checks. A board can meet the appearance you want while still being unsuitable for a structural use or too wet for the planned installation.
Inspection, Defect Sorting & Profile Maintenance
Inspection means comparing boards with the project’s stated dimensions, profile, grade, and condition. It is not a search for perfectly clear wood unless the specified grade calls for that. Sort boards by whether they meet the requirements and whether their defects interfere with the overlap, fastener zone, or intended appearance.
Check each board for decay, active insect damage, severe warp, splits through the intended fastener zone, and defects that prevent the specified overlap. Judge knots and other features against the specified grade, not against a generic idea of what rough-cut lumber should look like. A rough surface can be expected; a broken or unusable rabbet is a separate concern.
What should you reject or set aside?
The right action depends on the project detail and grade. A board that is unsuitable for one location may be usable elsewhere only if the project allows it. Do not quietly use a rejected board in a structural role or where it cannot form the required overlap.
| Finding during inspection | Check next | Practical response |
|---|---|---|
| Decay or active insect damage | Confirm the defect and lot requirements | Reject or segregate; do not use structurally |
| Severe bow, twist, or cup | Fit the board with a sample and check the detail | Set aside if it prevents the required joint or finish |
| Split through the planned fastener zone | Compare with fastening details and grade | Reject or segregate if the split prevents secure installation |
| Uneven or damaged rabbet | Fit adjoining sample boards | Re-mill only if the specified profile can be restored |
| Knots or other visible features | Check the stated grade | Accept or sort according to that grade |
Before a full production run, check that the sawmill or shaper can make the profile consistently. Compare sample pieces at several points along their length. The rabbet should produce the overlap shown on the drawing, and the boards should fit as intended without forcing the joint.
Illustrative job-site scenario: A homeowner receives boards described only as “rough-cut 1×6 shiplap.” The first sample pair shows a smaller reveal than expected, and the board widths vary. Rather than start installation, the homeowner measures the lot, agrees on the required face reveal and overlap with the supplier, and checks a revised sample. This avoids treating an informal product label as a complete specification.
Storage, Acclimation & Seasonal Moisture Care
Storage and acclimation help keep boards in a condition suited to installation. Wood changes size as its moisture content changes, so a dry-looking stack or a tight sample joint cannot replace moisture readings and the project’s requirements. Keep boards protected while allowing airflow, then recheck their condition before use.
Store lumber flat, supported, stickered to allow airflow, and protected from ground contact and weather exposure. Keep stickers aligned so the stack remains supported. Protect the boards from getting wet, but avoid wrapping them in a way that traps moisture. Follow supplier instructions if they differ for the product or coating system.
Moisture movement can change the joint. Wet boards installed tightly may shrink as they dry and leave gaps. Very dry boards installed without room for expected moisture gain may swell, cup, or buckle. Do not force boards together to hide a size or moisture problem; follow the project’s spacing and fastening details.
What should you recheck before installation?
Measure MC again after storage or acclimation, especially if the boards were exposed to changing conditions. Check several pieces, inspect for new damage or warp, and fit a sample joint. If the measurements or fit no longer meet the project requirements, stop and resolve the mismatch before continuing.
Illustrative job-site scenario: An exterior siding order arrives during wet weather and is stored outdoors without adequate protection. Before installation, the crew checks the boards for moisture and fit instead of assuming the delivery condition is acceptable. If the readings do not meet the project target, they consult the supplier or project instructions rather than forcing the joints tight.
For exterior work, apply coatings to faces and exposed cut ends as required by the specified product system. Keep offcuts and unused boards protected, and follow the project’s maintenance instructions. Key takeaway: storage does not make an unsuitable lot suitable; it helps preserve material that meets the project requirements.
Conclusion
Rough-cut shiplap is defined by its unsurfaced faces and overlapping rabbeted edges, not by one fixed size or grade. Measure the lot, specify the profile, check the grade and moisture condition, and test-fit sample boards. For exterior or structural work, follow the project’s approved details rather than relying on the shiplap label alone.
Frequently Asked Questions
Is rough-cut shiplap the same as rough-sawn lumber?
No. Rough-sawn describes lumber with unsurfaced faces. Shiplap describes boards with complementary rabbeted edges that overlap. Rough-sawn lumber may have shiplap edges, but it may also have square edges or another profile.
What size is rough-cut shiplap?
There is no single standard size. Measure the actual thickness and width of the lot and confirm the profile dimensions on the order or drawing. A nominal lumber label does not guarantee the dimensions of rough stock.
Is rough-cut 1×6 shiplap actually one inch by six inches?
Not necessarily. Rough stock may be sold close to its stated rough dimensions, but sizes vary. The common S4S reference for nominal 1×6 lumber is 3/4 by 5-1/2 inches; do not apply that measurement to rough-cut stock without checking.
What is a rabbet on shiplap?
A rabbet is a step-shaped cut along a board edge. Matching rabbets let adjacent boards overlap. The depth and shoulder width should be specified for the project, because there is no universal shiplap rabbet size.
What moisture content should rough-cut siding have?
A 12–15% MC target is practical for many siding projects, but it is not universal. Use the target set by the project, product instructions, species, and local conditions. Check several boards with a calibrated pin meter.
Can appearance-grade shiplap be used structurally?
Not automatically. Appearance grade and structural grade serve different purposes. Structural use requires an applicable recognized grade and design approval. Confirm the required grade and use with the project documents or a qualified professional.
How do I know if shiplap boards fit correctly?
Fit sample boards together and compare the overlap and reveal with the project drawing. Check the rabbet along the board, not just at one point. A tight-looking joint does not by itself prove that an exterior wall is weather-tight.
Can shiplap alone keep water out?
No. Overlapping edges are not a complete exterior water-management system. Follow the specified cladding details for drainage, flashing, fastening, and coatings. Do not rely on the joint alone as weatherproofing.
What defects should I look for in rough-cut shiplap?
Check for decay, active insect damage, severe warp, splits through the planned fastener zone, and damage that prevents the required overlap. Evaluate knots and other features against the specified grade rather than a generic label.
How should rough-cut shiplap be stored before installation?
Store it flat, supported, stickered for airflow, protected from weather, and off the ground. Recheck its condition and moisture content before installation, especially if storage conditions have changed. Follow any product-specific storage instructions.
(This article was written by one of our staff writers, Evan Bradley. Visit our Meet the Team page.)
