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It seems 'coffrage' has several meanings in engineering terms, and not an area in which I am specialised - but according to this link (dictionary of civil engineering), the term 'chimney breast' may be the most relevant, although it uses the term 'coffre'. Lots of other possibilities for 'coffrages' here including 'shuttering', 'planking', 'casing' etc, any of which might be appropriate.

http://books.google.fr/books?id=nhetF1UI7mYC&pg=PA126&lpg=PA...

Sorry for the huge link...but hope it helps anyway, even if I haven't chosen the right definition!

Coffrage is often translated as the decking of a roof. Could work here depending on the broader context. A diagram would help!
Coffrage can also be translated as "formwork" for concrete but that translation doesn't seem to fit here unless you have a concrete roof deck.

What precisely do you/they mean by "embouts de cheminée"? Are these what are habitually called "mitrons", or chimney pots, the ceramic (for ex.) bits atop the chimney stack?

When installing chimneys with steel liners, a "coffrage" can be the concrete elements actually forming the shaft through which the pipe is routed, i.e. what traditionally constitutes the chimney flue itself. The flue protects the tube from mechanical damage and supports it, the pipe protects the flue from excessive heat.

More context is needed because coffrages means more than formwork; but it gives me the idea that we are talking here about cladding the opening of a fire place and therefore formwork seems appropriated.