Your scheme will not work.
Yes there will be some load on the sensor as the roof is covered in snow, but you will not get the load per square inch, the roof material has stiffness and will deflect when a load is applied anywhere in the region of your sensor.
To be quite honest, this project sounds like managerial madness, a hare brained scheme dreamed up by a clueless suit, hell bent on having people carry on working in a potentially unsafe building.
I think that you will do just as well to measure the depth of snow at ground level and assume the same depth on the roof.
Knowing the depth of snow, you should be able to calculate the mass of snow on the roof once you know the density of snow.
Your next problem will be to find the density of snow which is far from constant.
JimB
Afterthought:
Why not measure the weight of snow on a 1sq metre board at ground level, and trigger your alarm from that.
(Still a daft idea).