Selecting materials

The materials library at Elmhurst is a long basement room lined with steel shelving, and on the first day of term Professor Teodora Vasić lets the four students loose in it before saying a word. There are bricks from a dozen plants, offcuts of cross-laminated timber, a slab of terrazzo, coils of copper, a sheet of perforated weathering steel, jars of cement and slag, cork, felt, a dented zinc panel and a cardboard box of failed sealant joints that she collected from a façade repair. After twenty minutes she asks each of them to bring back one object and say why they picked it. Wren holds up a rough Tyndall stone sample because it has fossils in it. Callum brings a brick that rings when he taps it. Ingrid has a piece of glulam whose end grain she likes. Joon-ho brings a cracked plastic window spacer and asks why anyone would make a window out of something that breaks like that. 'Good,' says Vasić. 'You chose by eye, by sound, by touch and by suspicion. This term you will learn to choose by evidence as well.'

Vasić trained as an architect, spent eleven years at a building envelope consultancy investigating leaks and failed cladding, and now runs the materials library and teaches this course. Her rule is that an architect does not have to be a materials scientist but must know enough to ask the right questions, read the answers and notice when something is wrong. This lesson sets out how to describe a material, where to find reliable data and how to choose between candidates in a way you can explain to a client, an engineer or a building official.

Why material choice matters

Materials are the physical substance of architecture. They decide how a building stands up, keeps out weather and fire, sounds, smells and ages. They make up most of a building's cost and, increasingly, a large share of its lifetime carbon emissions. A choice made casually in an early sketch, such as a dark metal panel on a south wall or a stone floor in a daycare, can commit a project to decades of consequences: heat, glare, cold feet, cracked joints, high maintenance or an early replacement.

Material choice is also cultural. Brick tells a story of craft and permanence; glass of openness; timber of warmth and, on the prairies, of a forest economy. In ARCH-130 the students learned how buildings are assembled from systems. In this course they look inside the systems at the materials themselves, ask what each one can and cannot do, and learn to combine them in assemblies that last.

Describing a material: families of properties

Engineers and scientists group the properties of materials into families. An architect should be able to name the important properties in each family and know roughly how the common building materials compare.

• Mechanical properties describe how a material carries load. Strength is the stress at which it yields or breaks; stiffness, measured by the modulus of elasticity, is how much it resists deforming; toughness is its ability to absorb energy before breaking; hardness is its resistance to scratching and denting.
• Physical properties include density, thermal conductivity (how readily heat flows through it), thermal expansion (how much it grows when warmed), specific heat and thermal mass, and its relationship with water: how much it absorbs, how vapour passes through it and how it swells or shrinks.
• Fire properties include whether it burns, how fast flame spreads across its surface, how much smoke it produces and how long it keeps its strength when heated.
• Acoustic properties include mass, which blocks sound, and absorption, which softens a room.
• Durability describes how a material resists weather, ultraviolet light, freeze-thaw cycles, corrosion, rot, insects and wear.
• Sensory properties include hue, texture, gloss, warmth to the touch, smell and how the material changes as it ages.
• Environmental and economic properties include embodied carbon, toxicity, recycled content and recyclability, cost, availability, lead time and the skills needed to install it.

Two terms cause particular confusion. Strength and stiffness are different. Glass is very stiff but breaks suddenly; rubber is flexible but tough. A floor can be strong enough not to collapse yet so flexible that it bounces uncomfortably. In building design, stiffness often governs: beams and floors are frequently sized to limit deflection rather than to avoid breaking. Similarly, thermal conductivity and thermal mass are different: concrete conducts heat fairly well and also stores a lot of it, while foam insulation does neither.

Comparing materials on a property chart

The materials scientist Michael Ashby popularized charts that plot one property against another on logarithmic scales, with each material family occupying a bubble. The most useful for architects plots stiffness against density. It shows at a glance why steel, concrete and wood behave so differently, and why wood is so competitive in light structures: along the grain, it is about a twentieth as stiff as steel but also about a sixteenth as dense, so its stiffness per unit weight is similar.

The chart also explains a basic fact of wood: its properties depend on direction. Along the grain wood is stiff and strong; across the grain it is ten or more times softer. Wood is said to be anisotropic. Steel, glass and, approximately, concrete are isotropic: the same in every direction. Engineered wood products such as cross-laminated timber arrange layers in different directions to even out these differences.

A few patterns in the table are worth remembering. Steel conducts heat about four hundred times better than wood, which is why a steel beam passing through a wall becomes a cold bridge. Aluminium and PVC expand about twice and five times as much as steel, which is why their joints must allow movement. Concrete and steel expand at nearly the same rate, one of the happy accidents that makes reinforced concrete possible. Mineral wool insulates because it traps still air, not because its fibres are special.

Where reliable data comes from

Architects meet material information in many forms, and they are not equally trustworthy. Vasić gives the students a ranking.

• Standards set minimum requirements and test methods. In Canada, the CSA Group publishes standards for wood design, concrete, masonry, steel and windows, among others, and the National Building Code of Canada refers to them. In the United States, ASTM International and bodies such as the American Concrete Institute and the American Institute of Steel Construction play a similar role, referenced by the International Building Code. A product that meets a named standard has been tested in a known way.
• Evaluation reports, such as those from the Canadian Construction Materials Centre or the ICC Evaluation Service, show that an innovative product meets the intent of the code where no standard yet covers it.
• Technical data sheets from manufacturers list tested properties, installation requirements and limits. They are reliable for the conditions stated, but read the footnotes.
• Environmental product declarations, or EPDs, are independently verified reports of a product's life cycle environmental impacts, prepared under ISO 14025 and product category rules. Health product declarations list ingredients and hazards.
• Trade literature, sample boards, showroom displays and online images are useful for inspiration but are marketing, not evidence.

Vasić's test for a claim is simple: who measured it, how, under what conditions, and who checked? A brochure that says a panel is 'fireproof' or 'maintenance-free' fails all four questions. A data sheet that says the panel achieved a certain flame-spread rating when tested to a named standard passes most of them.

A method for selection

Choosing a material is a design problem with many answers, so it helps to work in a structured way. Ashby's method begins by stating four things.

• Function: what must the component do? Carry load, keep out rain, insulate, finish a floor, shade a window.
• Constraints: what must it achieve without exception? A fire rating, a minimum strength, a maximum thickness, a code requirement, a budget ceiling.
• Objectives: what should be minimized or maximized? Cost, weight, embodied carbon, maintenance, thickness.
• Free variables: what can the designer change? The thickness, the profile, the finish, the spacing of supports.

Constraints screen out candidates that cannot work. Objectives rank the survivors. Where there are several objectives, which is nearly always the case in architecture, a weighted score makes the trade-offs explicit. Weights are a judgement, not a fact, and the point of writing them down is that the team and the client can see and question them.

The class tests the method on the warming hut at Kestrel Point. In ARCH-130 they converted the little boathouse into a heated shelter where skaters and skiers warm up beside the lake. Its cladding faces wind-driven snow, children's skates and boots, a wood stove inside and almost no maintenance budget. The constraints are that the cladding must be non-combustible within 1.2 m of the stove flue where it passes through the wall, resist impact up to shoulder height and last at least thirty years. The objectives are low embodied carbon, low maintenance, low cost and a good fit with the lakeside setting. Four candidates survive the screen: vertical wood boards with a stain, cement board panels reinforced with fibres, corrugated galvanized steel and thermally modified wood.

# Weighted comparison of cladding for the warming hut (scores 1 to 5, 5 is best)
weights = {"carbon": 0.3, "maintenance": 0.25, "cost": 0.15, "setting": 0.2, "impact": 0.1}
options = {
    "stained wood boards":       {"carbon": 5, "maintenance": 2, "cost": 4, "setting": 5, "impact": 3},
    "cement board panels":         {"carbon": 3, "maintenance": 4, "cost": 3, "setting": 3, "impact": 3},
    "corrugated galvanized steel": {"carbon": 2, "maintenance": 5, "cost": 5, "setting": 3, "impact": 5},
    "thermally modified wood":   {"carbon": 4, "maintenance": 4, "cost": 2, "setting": 5, "impact": 3},
}
assert abs(sum(weights.values()) - 1) < 1e-9
def score(o, w):
    return sum(w[k] * options[o][k] for k in w)
ranked = sorted(options, key=lambda o: -score(o, weights))
for o in ranked:
    print(f"{o:<30} {score(o, weights):.2f}")
# Sensitivity: swap the two largest weights
w2 = dict(weights)
big = sorted(w2, key=lambda k: -w2[k])[:2]
w2[big[0]], w2[big[1]] = w2[big[1]], w2[big[0]]
print("Swapping", big[0], "and", big[1], "->", max(options, key=lambda o: score(o, w2)), "ranks first")

stained wood boards            3.90
thermally modified wood        3.80
corrugated galvanized steel    3.70
cement board panels            3.25
Swapping carbon and maintenance -> corrugated galvanized steel ranks first

The score is close, which is common. Vasić points out that the result depends on the weights: if the city valued low maintenance above low carbon, the galvanized steel would come first. She also points out what the table hides. Thermally modified wood is new to local installers; galvanized steel would be noisy in hail and cold to lean against; cement board needs careful cutting with dust control, because its dust contains silica. The score is a starting point for discussion, not the decision itself.

Stiffness per unit weight

One property ratio deserves special attention: stiffness divided by density, called specific stiffness. When a component must be stiff but light, as with a long roof beam or a cantilevered canopy, the material with the highest specific stiffness does best. The surprising result is that steel, aluminium, glass and wood along the grain have similar specific stiffness. That is why the decision between them rests on other properties: strength, cost, fire, durability, connections and carbon.

Thinking in the whole life of a material

A material's story does not start at the building site or end at the ribbon-cutting. It begins with extraction (a quarry, a forest, a mine), continues through processing and manufacture, transport, installation, decades of use and maintenance, and ends with demolition, reuse, recycling or disposal. Good selection considers the whole sequence. A cheap finish that must be replaced every eight years may cost more over a building's life than an expensive one that lasts forty. A low-carbon product shipped across an ocean may lose much of its advantage. A material that cannot be separated from its neighbours at the end of life will probably go to landfill.

Lesson 11 returns to these questions with the tools of life cycle assessment. For now, the students add two questions to their checklist for every candidate: how long will it last here, and what happens to it afterwards?

The course project

Throughout the term each student keeps a material notebook: samples, sketches of joints, photographs of weathering on real buildings, and notes from data sheets. In Lesson 12 each will research one material in depth for the community hub they designed in ARCH-200, test it against the hub's needs and recommend whether and how to use it. Vasić asks them to start looking now. Wren is drawn to the cross-laminated timber of the hub's wing, Callum to clay brick for the hall's base, Ingrid to low-carbon concrete for the slab on grade and Joon-ho, still suspicious of plastics, to glazing and its frames.

Exercise 1: selecting materials

• Visit the materials library (or a building supply yard) and handle at least ten materials. For each, note what you can learn by sight, touch, sound and weight.
• For one component of the warming hut, state its function, constraints, objectives and free variables.
• Shortlist three candidates and find a data sheet, a relevant standard and, if possible, an EPD for each.
• Build a weighted comparison like the one above, and test how the result changes if you swap the two largest weights.
• Photograph a building at least thirty years old and record how its materials have aged.