Strength, Grain & Moisture

Engineers describe wood with a family of properties, but two matter most to a designer's intuition: strength (how much load it can carry before it fails) and stiffness (how much it deflects under load, measured by a property called the modulus of elasticity). A strong-but-flexible beam might not break but could sag or bounce; a stiff one holds its shape. In timber, both are dominated by the grain. Along the grain, wood is strong in tension and compression and stiff in bending, which is why beams, joists and columns are cut and used with their length along the grain. Across the grain it is weak in every mode - it splits in tension and crushes in compression - so we design to avoid loading it that way.

The modes of loading behave differently and it is worth knowing them by feel. Along the grain, wood is generally excellent in compression (a post carrying weight) and very good in tension (a tie being pulled), though real tension members are sensitive to any defect. In bending, a beam is squeezed on one face and stretched on the other, so it uses both - and bending strength is a headline number for timber floors and roofs. The weak mode is shear and, above all, tension perpendicular to the grain, where wood splits easily; good detailing keeps stress out of that direction. Density is the other big lever: denser woods (more cell-wall material, less void) are generally stronger and stiffer, which is part of why a dense hardwood outperforms a light softwood pound for size - though softwoods win on strength-to-weight and are the workhorses of mass timber.

Crucially, these are not single fixed numbers. The strength of a real piece of timber depends on its species, its density, its defects and its moisture, and it varies from board to board because wood is a natural, grown material. That natural variability is precisely why timber is graded (Lesson 1.3) and why the engineer designs to conservative characteristic values from the code, not to the average. As a designer your job is to understand the pattern - strong along the grain, weak across it, cut by defects, changed by moisture - and to let the engineer assign the binding values for the species and grade you actually use.

Because wood is a grown material, no board is perfect, and its imperfections are not random blemishes - they are places where the orderly grain is disturbed, and disturbed grain means lost strength. The most familiar is the knot: the buried base of a branch, around which the trunk's grain had to sweep. A knot itself is dense, but the grain deflected around it now runs partly across the direction of load, creating a local weak point - especially in tension and bending, where a knot near the stretched face of a beam can be the place it fails. This is why the number, size and position of knots are central to how timber is graded.

Other defects follow the same logic of disturbed or discontinuous grain. Sloping grain - where the fibres run at an angle to the length of the board, often because of how the log was sawn or because the tree grew with spiral grain - weakens the piece and makes it prone to twisting as it dries. Checks, splits and shakes are separations along the grain, some from growth stresses in the tree and many from drying too fast; they reduce shear strength and can be entry points for water and decay. Wane (bark or missing wood on an edge), reaction wood (abnormal wood formed in leaning trees), and decay or insect damage all downgrade a piece. None of these mean the wood is useless - much of it is perfectly good structural timber - but each has to be seen and accounted for.

The designer's takeaway is twofold. First, timber is graded, not assumed: because defects vary, every structural piece is sorted - visually or by machine - into strength grades so the engineer can design to a reliable value, and specifying the right grade for the job is part of good design (Lesson 1.3). Second, and this is the hinge to the whole course, engineered wood exists largely to beat defects. If you chop timber into small pieces, the defects get chopped up and scattered too, and when you glue many such pieces back together into a glulam beam or a CLT panel, no single knot or slope of grain sits at a critical point - the flaws are dispersed and their effect averaged out. That is a major reason engineered products are stronger, more uniform and more reliable than the sawn timber they are made from, as Lesson 1.3 explains in full.

If you learn one thing about timber in service, learn this: wood and water are never done with each other. A living tree is full of water; freshly cut "green" timber can weigh far more than dry timber and be more than half water. As wood dries, water leaves in two stages that matter enormously. First the free water sitting in the hollow cell cavities evaporates - and while that happens the wood barely changes size or strength. Then, below a threshold called the fibre saturation point, water starts leaving the cell walls themselves, and this is when things change: as the walls give up water they shrink, and as they shrink the wood shrinks - and gets stronger and stiffer as it dries. Below fibre saturation, drying and strength and movement are all linked.

Because the shrinkage happens in the cell walls, and because wood is anisotropic, it moves far more across the grain than along it - and even differently in the two cross-grain directions. Along the grain, timber shrinks almost negligibly. Radially (across the rings) it shrinks moderately. Tangentially (around the rings) it shrinks the most - often roughly twice the radial amount. This uneven, direction-dependent movement is why boards cup, twist and check as they dry, why the way a log is sawn changes how a board behaves (quarter-sawn moves and looks differently from flat-sawn), and why solid-timber joinery must always be detailed to let the wood move.

And it never truly stops, because wood is hygroscopic: it keeps absorbing and releasing water vapour to stay in balance with the humidity of the air. That balance point is the equilibrium moisture content (EMC) - the moisture content wood drifts toward for a given temperature and humidity. In a dry, air-conditioned interior wood settles low and shrinks; in a humid monsoon it takes up moisture and swells; and it moves back and forth with the seasons for its whole life. This single fact explains the stuck door, the gap that opens in a timber floor each dry season, and the discipline of seasoning timber to near its in-service EMC before it is installed. Moisture governs strength, movement, decay risk and dimensional stability all at once - which is why the next lesson's drying step, and Module 6's moisture detailing, are treated so seriously.

Step back and notice how many separate timber issues are really the moisture issue wearing different clothes. Strength depends on moisture: the same piece of wood is weaker and more flexible when wet and stronger and stiffer when dry, so structural timber must be at a controlled, known moisture content for its design values to hold. Dimensional stability depends on moisture: because wood swells and shrinks across the grain as its moisture changes, floors gap, doors jam, panels cup and cladding moves unless the movement is anticipated and detailed for. Durability depends on moisture most of all: the fungi that decay wood need it to be persistently wet, so wood kept reliably dry can last centuries, while wood that stays damp will rot - which is why "keep it dry" is the first commandment of timber detailing (Module 6.2).

This is why the industry invests so much in getting moisture right before and during construction. Timber is dried (seasoned) - in a kiln or in the air - down to a moisture content close to what it will experience in service, so that most of its shrinking is done before it is built in and its strength is reliable (Lesson 1.3). Engineered mass-timber products are manufactured from dried timber at controlled moisture and glued in factory conditions precisely so the finished panel is dimensionally stable and predictable. And on site, mass timber must be protected from getting wet during construction, because soaking a CLT floor and trapping that moisture is a real and well-documented risk. The whole logistics of timber construction is shaped by respect for water.

For you as a designer, the discipline is clear and it is a matter of judgement rather than calculation. Specify timber dried to a moisture content suited to its use and location. Detail every junction to shed water, ventilate, and keep structural timber dry for its whole life. Allow for the movement that remains - never fix wide timber rigidly across its grain. And defer the binding specifics - the required moisture content for a given use, the strength values at that moisture, and the durability and treatment requirements - to the timber engineer, the manufacturer's data and the current code. Understand that moisture is the master variable, design to keep it under control, and timber will reward you with a material that lasts for generations.