Solid timber roof decking does two jobs at once: it is the structure and it is the finished ceiling. That is exactly why it gets sized wrong. The span depends not just on thickness and load but on the layup pattern, and controlled random layup does not behave like simple spans. This calculator sizes tongue-and-groove roof decking from the published design values, pattern by pattern.
Open the CalculatorThickness. Nominal 2, 3, and 4 inch T&G decking cover very different span ranges, and the jump from 2 to 3 inch is larger than most people expect because stiffness grows with the cube of depth.
Layup pattern. Simple span, two-span continuous, and controlled random each have their own design equations. Controlled random, the most common pattern on exposed timber roofs, gains capacity from continuity but only if the joint layout rules are actually followed on the roof.
Load and deflection. Roof decking is usually deflection-governed, and on an exposed ceiling, deflection is not abstract. A sag you can measure is a sag you can see from the living room.
Species and grade. Decking grades carry their own design values, separate from dimension lumber grades of the same species.
Exposed 3x6 Douglas Fir T&G decking, controlled random layup, over glulam purlins, 30 psf snow and 10 psf dead.
Enter thickness, pattern, loads, and deflection limit. The calculator returns the allowable purlin spacing and identifies whether bending or deflection governs. If the purlin spacing is fixed by the design, run it in reverse: enter the spacing and the calculator returns the required thickness and grade.
In the range of feet, not the 16 or 24 inches of sheathing, but the exact answer swings with pattern, load, and deflection limit. Controlled random layup over three or more supports spans meaningfully farther than simple spans of the same material. Run the pattern you will actually install.
A pattern where board end joints fall at controlled locations between supports rather than on them, letting each course act as a continuous member. It is efficient and beautiful, and its span values are only valid when the layout rules are followed during installation.
T&G decking has recognized but limited diaphragm capacity compared to panel sheathing. On projects where the roof diaphragm is doing real lateral work, that question goes to the engineer of record early, not to a calculator.
Yes. Enter your ground or roof snow load per your jurisdiction. Load duration adjustments for snow are applied automatically.
Results from this calculator are preliminary sizing guidance for comparison and planning. They are not engineering, not sealed drawings, and not a substitute for design by a licensed professional under your local code. Final member sizing, connections, and load paths must be confirmed for your specific project.
Bardowl Design produces structural layouts and material specifications that builders and lumberyards can actually work from. Clear scope, defined assumptions, documented output.
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