Which tool for slicing protruding screws?

Reciprocating saw with appropriate blade?

 

I think an angle grinder is the only way as most screws these days are hardened and very hard to saw, if on the other hand you intend to lay say Celotex over the roof timbers just leave them they will push into the insulation no problem.

 

I have used a reciprocating saw with the right blade many times to cut screws and nails

 

True with good access a power saw makes light work of it, but if he cannot get an short angle grinder in he will not have much luck with 2ft of reciprocating saw
From what he describes a hacksaw blade in a pad saw handle might be the only option to cut flush to the underside of the board, OK for one odd nail but darn hard work and lots of broken blades for the number he has

 

Short angle grinder only has a small amount of blade showing. Reciprocating saw has a much longer blade

 

Shame the roof was redone without removing the infestation.

I'd remove it.

But small angle grinder will do the job. It doesn't have to be horizontal to make the cuts.

Or as said above, jab the jab straight up!

Mr. HandyAndy - Really

 

Screws should come out through the orherside of plywood by approx 20mm if they are holding insulation or a membrane in place, is so leave them as this is to help the roof being blown off by wind action, if not cut away.

 

A hammer works well.

 

snapem orf with pair of pliers in a leisurly fashion

 

If long enough tap them back and forth with a hammer or use a multi tool with a decent metal cutting blade.