Feature
When a Spec Says 'Or Equal'
Errol Nakamura · · 2 min
Specifications name products, then add “or equal approved” so a substitute can be used. The phrase does real work: it admits the named item is one valid answer rather than the only one. The question is what “equal” is being measured against.
Three reasons a product gets named
Performance. The spec needs a measurable property — a fire rating, a compressive strength, a vapour permeability. Here “equal” is testable: match the standard and the number, and the substitution is sound.
Compatibility. The product is part of a system — a membrane and its tape, a render and its primer, an adhesive and its backing board. Here “equal” is much narrower, because the manufacturer tested the combination, not the component.
Habit. The specifier has used it for years. Entirely legitimate, and the easiest to substitute against, provided you can establish which of the three you are looking at.
The warranty question
System warranties are the usual casualty. Manufacturers warrant an assembly on the condition that its components are theirs; substituting one layer can void cover on all of it.
That is worth pricing. A cheaper membrane inside a warranted roof build-up may cost more than it saves the first time something leaks.
How to compare properly
Compare against the datasheet, not the marketing page, and match the tested standard rather than the adjective. “Breathable” is a claim; a stated vapour resistance to a named test method is a number.
Where the spec cites a standard, that is the actual requirement — the product name is shorthand for something that meets it.
When to just use the named product
When it is part of a warranted system, when the substitution saves little, or when nobody can tell you which of the three reasons applies. “Or equal” permits substitution; it does not oblige it.
Mortar Desk publishes general reference information about building materials and specifications. It is not a contractor and does not provide engineering advice. Work involving gas, structural elements or main electrical supply belongs to a licensed trade, working to local code.