When in doubt, ask an engineer. :) Question 1: What’s the meaning of ‘the final roof undercoating of three-by-fives?’ ( final roof undercoating? ) The wood making up the final structural layer of the roof, was made of (probably wood) cut to three inches by five inches, by something long. Example A four by two is a structural piece of wood 4 inches by 2 inches by however long you need it. Question 2: What’s the meaning of ‘it might be nearly two feet through the center?’ ( its diameter might be nearly two feet? ) Yes, diameter, if you can define that for something irregular. A measure form side to side, "thorugh the centre". Question 3: Can I also say '‘the final roof undercoating of three-by-FIVE?’ If not, what's the difference between '‘the final roof undercoating of three-by-fives' AND '‘the final roof undercoating of three-by-FIVE?’ No, sorry. inches, they are relatively small things, so it is made of many of them, plural. The total of many of them makes up the roof structure, as well as any structure below them. . That much clearer?

Flashing is a covering often lead or zinc or some metal alloy, that is shaped around where a roof joins another wall or structure, to stop rain seeping in through the gap/join. The undercoating is a section made of wood that is 3x5 inches (only wood is described this way in old imperial measurements) 2) yes it's diameter is nearly two feet. 3) it is traditional to say Three by Five. or write 3"x5" (three inches by five inches). if you keep to these old traditions people will understand. We are talking of construction and engineering, so GRAMMAR like in a text book does not apply. I am not totally sure as to how, American English, New Zealand English or Australian English or Canadian English would write 3"x5" for wood measurements, but it won't be too much difference. Flashing picture, everywhere a roof meets another wall etc, or something comes through it it is covered with a made to fit seal. https://www.mrcentralheating.co.uk/glow-worm-pitched-roof-flashing?utm_source=google_shopping&gclid=CjwKCAjwxILdBRBqEiwAHL2R8y_Di3lJXl-4QonKFFYs6hvYX3eGkM8SsVlTx5Dt-lF5uAQeDUAvEhoC4BMQAvD_B Roof undercoating IS where a roof has been fixed with water proof adhesive , so there was an area made from 3"X5" timber pieces and the roof tiles/slates/felt or whatever was stuck to it.

Thank you :-)

Simon is correct. 'Three by fives' is short for '_pieces_ of (probably) wood that are 3x5"'. You could have one three by five, but here, it is made of several three by fives.

3. Using "five" as a singular there means that the entire roof is only three by five (feet) as opposed to the parts of it. "Three by fives" here is a shortening, e.g. of "three-by-five tiles".