If the signal is weak, then there is a good chance that the yagi may help.
(Especially if you can mount it on the roof and run coax down to the
radio inside, since you know you have coverage from the roof.)
It's when you can't hear the signal at all that you can't predict whether
increased antenna gain is going to help or not.
My approach in such a case is to try it and see. That will give you an
idea of how much gain you need. The WA5VJB "Cheap Yagi" designs
are easy to build with copper wire and PVC pipe:http://www.wa5vjb.com/yagi-pdf/cheapyagi.pdf
Scale one of his 70cm designs to your operating frequency, hook it up
and see how it works. You may have to rotate it around a bit to maximize
signal reflections into and out of the building.
Otherwise, if the buildings are metal and the flat side of one faces
the other, you can install a dipole or other element on the outside
using the metal as a reflector and run the coax in through the wall.