Joining faces

I am quite new to sketchup, I am hoping someone can help me out. I have imported an IFC file and want to Push/pull a surface to adjust the heights. However, the surface consists of many faces and I cannot join them. Is there a way I can do this so that I get just one surface for each building. (Image attached)

It depends on the situation and what exactly you need

This worked. Thanks!

Is the surface flat? If so, you could erase those hidden edges before running Push/Pull. If the surface is flat you really should get rid of those edges anyway.