In the following sentence:

"Wall and roof thickness are always considered when repairing a chimney."

The sentence refers to the thickness of both the wall and the roof. Is the sentence above correct (i.e., even though "thickness" is singular and "are" is plural") or should the non-count noun "thickness" be repeated to match the plural verb "are"?

i.e., "Wall thickness and roof thickness are always considered when repairing a chimney."

Is there a rule regarding constructions like these? Hopefully, someone can help. Thanks!

  • 1
    See all of these posts: english.stackexchange.com/search?q=plural+verbs For your example, though, I would say 'Wall and roof thicknesses are always...'
    – JLG
    Sep 7, 2014 at 14:46
  • 3
    Wall and roof thickness is the output of conjunction reduction, from wall thickness and roof thickness. That's a plural NP, so it gets a plural verb. It's not necessary to repeat the plural in the noun, though. The plural makes it clear enough they're considered separately. Sep 7, 2014 at 14:48

2 Answers 2


John Lawler's answer in the comments above is dispositive: The phrase "wall and roof thickness" attached to the plural verb "are" unmistakably indicates that this expression is a short form of "wall thickness and roof thickness."

Because it permits this formulation, English doesn't require us to say "wall and roof thicknesses," which a reader or hearer might very reasonably interpret to mean "wall thicknesses and roof thicknesses." If the idea we have in mind is "the thickness of the wall or walls, and the thickness of the roof," then the prospect of being obliged to use plural "thicknesses" in the shortened form "wall and roof thicknesses" might impel us to repeat "thickness" (as in "wall thickness and roof thickness") to ensure that we are properly understood to be talking about each thickness as a unitary idea, and not as a series of variable measurements.


That would be a correct usage of the singular thickness if the thickness were cumulative or if thickness of the roof only were considered, leaving all or other attributes of the wall to be implied. If the wall's thickness and the roof's thickness are considered together but separately, thicknesses would be the correct choice.

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