You can do this with collision mask and collision group.



These options are found in the physics panel. To fix your particular problem, select the invisible collision box for the player and put it in collision group 2. 

Next, select the bullet and while holding Shift click on the 2nd box in collision mask. This will stop the bullet from colliding with anything in collision group/layer 2.

Invisible Physics to Certain Objects in the BGE
We're hiring a contractor to take off the existing roof, repair/replace any rotted sheathing and rafters, and install a new roof. We've also asked that they add a new plumbing vent stack for a new bathroom we'll add later, a bathroom exhaust vent, a whirlybird to vent the attic, a dryer vent, and supports in case we want to install a roof deck later. 

This is a 4 floor row house with shallow pitch roof in mid-Atlantic US. There's a 6" to 3' pitched attic area with r30 batts on the ceiling joists. Also have HVAC compressor on roof and unit in attic. 

What else should we think about along these lines? Green roof supports? Solar panel mounts? Additional insulation? What should we prepare for now, or at least consider?
