align building prefab to a road prefab at runtime

I was wondering if anyone knew a way to align a building prefab to a road prefab at runtime when a building is placed. I already have scripts that let me place buildings and such so I only need help with aligning (or snapping in other words) a building prefab to a road prefab as the building prefab is moved closer to the road. The terrain is completely flat and roads will be pre-placed on the terrain at 90 degree angles. I myself cant think of any way to do that so maybe somebody can point me in the right direction on how to go about it. Thanks.