I am in the local DIY shed when Dad calls to ask me to pick up some roofing felt for his shed.
Me: How big is the shed roof?
Dad: Oh, about 20 feet long 5 feet wide.
Me: OK, that's about 6 metres by 1.5 metres. The rolls are 10 metres by 1 metre. You will need at least two layers....
Dad: I want to have three layers of felt.
Me: Well, if you don't mind having a join in one of the layers you can do it with two rolls as you will get one and 2/3 layers per roll. You can put the 2/3 long bits on first overlapping and then two long lengths over to get you the three layers you want.
Dad: will that give me the three layers?
Me: Yes, if you don't mind the join in one of the layers. Or I could get three rolls and you you can do it with no joints.
Dad: How much is a roll?
Me: One roll is £22 but if I get three rolls they come down to £17 each.
Dad: Just get me one roll for now.
Me: But one roll will only cover one and a half layers, it won't be enough.
Dad: That's ok, I'm not going to do the job straight away.
Me:
It doesn't matter when you do it, it still won't be enough to do the roof.
Dad: We can sort that out later.
Me: So you want me to get you one roll of felt now, and then when you start doing the job and you see there isn't enough then you will want me to come out and get another roll for you?
Dad: Well there might be enough so I might not need another roll.
Me:
Are you sure about that, Dad?
Dad: Yeah, it will be OK, we can sort it when I an ready to do it.
Me:
I bring back one roll of felt for him.