How wood moves — the basic principle
Wood changes dimension as it absorbs and releases moisture. The movement is not uniform: it is greatest in the tangential direction (parallel to the growth rings), roughly half as much in the radial direction (perpendicular to the growth rings, toward and away from the pith), and negligible along the grain (longitudinal). In eastern white cedar, this anisotropic movement pattern is less extreme than in denser hardwoods, but still meaningful in outdoor construction where wide seasonal moisture swings are normal.
Understanding where the growth rings are in a given board — relative to its faces, edges, and to the structure it is installed in — predicts how it will move and in which direction.
Flat-sawn vs. quartersawn cedar
Most cedar lumber produced in eastern Canada is flat-sawn (also called plainsawn or tangential-sawn). Flat-sawn boards are cut so that the growth rings run roughly parallel to the wide face of the board. This is the most economical cut because it yields wider boards from a given log diameter and wastes less material.
Quartersawn boards are cut so that the growth rings run roughly perpendicular to the face. This orientation has practical advantages in outdoor applications:
- Quartersawn boards cup and warp less across their width, because the dominant movement direction (tangential) is now aligned with the board's thickness rather than its width.
- Surface checking (small splits along the face grain) tends to be shallower on quartersawn stock.
- Exposure of medullary ray cells on the face can create a distinctive appearance, though this is less pronounced in cedar than in oak or sycamore.
Quartersawn cedar is less commonly stocked at standard lumber yards in Ontario and Quebec. It is available from some specialty sawyers and from portable-mill operators processing locally harvested logs. For projects where minimizing movement is a priority — tight-clearance decking, for instance — it may be worth sourcing deliberately.
Identifying cut orientation at the lumber yard
The end-grain pattern reveals the cut. Flat-sawn boards show growth rings as arcs across the end. Quartersawn boards show growth rings as roughly vertical lines across the end. Mixed or riftsawn boards show something in between, with rings at approximately 45 degrees.
Heartwood and sapwood placement within boards
As discussed in the rot resistance article, eastern white cedar heartwood (darker, inner wood) has natural decay resistance that sapwood (lighter, outer wood) lacks. But the heartwood-sapwood boundary also marks a zone of different moisture behaviour.
Sapwood has a higher moisture content when green and reaches equilibrium moisture content faster than heartwood. In boards that contain a significant sapwood zone, differential movement between the heartwood and sapwood portions can create internal stress that leads to checking at the boundary. This is most noticeable in fence boards and deck boards that contain both zones.
For fence boards specifically, orienting the board so that any sapwood is on the less-exposed (sheltered) face can modestly reduce differential drying stress, though in practice the effect is minor once the board has reached equilibrium with its environment.
Bark-side up or down on deck boards
The orientation question that comes up most commonly in deck construction is whether to install flat-sawn deck boards with the bark side (crown of the growth rings) facing up or down.
The conventional advice in North American framing guides has been to install boards bark-side down, on the reasoning that flat-sawn boards tend to cup toward the bark side as they dry, and bark-side-down would cause the board edges to rise while the centre remains in contact with the joist — making it easier for water to drain off the crowned surface.
Some wood technologists have challenged this as oversimplification, noting that installed boards already constrained by fasteners respond differently from free-floating boards, and that the direction of cupping also depends on how quickly each face dries relative to the other. The installed condition (fastened, in contact with joists, covered vs. shaded) matters as much as the grain orientation alone.
In practice, the most consistently cited benefit of bark-side-down installation is that any surface checking that does develop — particularly on the bark side — tends to remain smaller and less likely to hold standing water when the checked face is the underside of the board.
Grain orientation in posts and vertical members
For fence posts and pergola posts, grain orientation matters less than heartwood content and moisture management at the base. However, in full-sawn posts (square section cut from the log), if the pith (centre of the log) falls near or within the post cross-section, star checks radiating from the pith are likely as the post dries. These surface checks are a normal drying characteristic of cedar and do not indicate structural failure, though they can affect appearance on visible post faces.
Posts sawn to avoid the pith — with the pith outside the section — dry more consistently and with less severe surface checking, but this requires larger-diameter source logs and generates more waste.
Fastener placement and grain
Splitting at fastener locations is more likely when fasteners are driven close to board ends or edges, and when the fastener is large relative to the board thickness. Cedar's relatively low density (compared to pressure-treated SPF or Douglas fir) means it is more prone to splitting at fastener locations than harder framing lumber. Pre-drilling is recommended for screws near board ends, particularly on 1× lumber.
Stainless steel or hot-dipped galvanized fasteners are required with cedar. Bright zinc-plated or electroplated steel fasteners react with cedar's acidic extractives and stain the wood surface within one to two seasons. The staining is cosmetic, not structural, but it is difficult to reverse.
| Member type | Grain consideration | Practical note |
|---|---|---|
| Deck boards | Flat-sawn typical; bark-side orientation debated | Crown-side down limits visible surface checking when cracking occurs |
| Fence boards | Flat-sawn; sapwood at back face preferred | Effect minor once acclimated; heartwood proportion more important |
| Fence rails (horizontal) | Wide face vertical to shed water | Avoid flat-face-up orientation that pools water |
| Posts (square) | Pith location affects checking pattern | Star checks from pith are cosmetic; heartwood content governs durability |
| Pergola rafters | Edge-grain (vertical grain) preferred for dimensional stability in deep members | Reduces twist and bow in long members between spans |