Lew is right on every particular.

Yes... but. Lew's Knitted glass - when bought in 'roll' quantites is
about a dollar-a-pound more expensive than woven roving. Traditional
thinking is that woven fabric is used primarily with fiberglass 'mat'
in a polyester layup, one between the other. The basic reason for
this is that the woven, i.e. "over, under, repeat" nature of the w.r.
causes "hollows" where the 'high' strands cross-over the 'low' strands
of glass. The mat (in addition to providing significant bulk, and
sucking up a lot of resin) is thought to "tie" the woven layers
together, by bridging these 'hollows.'

Now, in a large epoxy resin layup one can significantly reduce the
cost, while maintaining adequate strength by continuing to use woven-
roving. The mat is forsaken in favor of a slurry of milled fibers and
a little cab-o-sil mixed to a 'light cream' consistency. That is,
layer of w.r. wet out with straight epoxy. Squeege. Trowel in a
slurry of epoxy / milled fibers / cab-o-sil, then next layer of w.r.
Finish wet-out with straight epoxy. Squeege. Repeat.

The milled fiber + cab-o-sil slurry is filling the hollows and
providing a tie for the next layer of woven fabric. A 10 lb bag of
cab-o-sil is like way cheap, and will last the entire project. A 50
lb bag of milled fibers is similarly inexpensive.

And, becuase we've eliminated the mat, resin-to-glass ratio's of 50 /
50 are easily achived.

So, in a large project where cost is a concern my belief is that one
ought not write-off woven roving as being a suitable material.

MW