HERBET Abel Aug 20, 2015:

Then you must say :
"la toiture comporte DES fermes pour en augmenter la portée/qui en augmentent la portée"

Tony M Aug 20, 2015:

@ Asker Abel in his response to my peer comment makes an important point that I had already been pondering over before posting.

Hopefully you may be able to tell from your wider context, but it is certainly not clear from this snippet of context here quite what the importance of the 'span-increasing' element it.
As Abel rightly says, it is inherently in the nature of a 'truss' to increase a span — as here, to allow shorter lengths of corrugated sheet material to span a greater distance; hence the addition of 'span-increasing' in this particular instance may have only a weak meaning, simply qualifying 'truss' with one of its natural qualities.
On the other hand, this qualification might be specific and intentional: a truss is being used to increase the span in this particular instance — in which case, it may be necessary to find some way to render that in FR.