Rating is the general structure of the building, CF is how robust it is, and how much weight it can support.
Light: generally open structure, not damage resistant. Residential houses, small businesses, that sort of thing.
Medium: larger businesses, more solid construction, like museums.
Heavy: Factories, concrete or steel docks.
Hardened: military bunkers, bank vaults.
If your tonnage exceeds that of the building you are on (or in, and not on the bottom floor), it will collapse under you, and you will fall into the rubble. This will usually be a bad thing, but there might be circumstances when you want this to happen (like if your opponent has holed up lots of infantry in it). If you are on the bottom floor and your tonnage exceeds the CF, any basement in the building will collapse under you, so it is a very valid tactic to do enough damage to get a building's CF under it's occupant's, if he's using it to absorb damage. Get lucky with that basement, and that tactic will backfire on him:
- 2 Double basement. The unit falls 2 levels. Apply all damage to a ’Mech’s legs (use the Front column of the ’Mech Kick Location Table).
3 Basement. The unit falls 1 level. Apply all damage to a ’Mech’s legs (use the Front column of the ’Mech Kick Location Table).
4 Basement. The unit falls 1 level (for ’Mechs, use the Front/Rear column of the ’Mech Hit Location Table).
5-8 No basement.
9 Small basement. Infantry may move into the basement as though it were a new level of the building (Sublevel 1); ProtoMechs cannot enter this level. No effect on ’Mechs or vehicles. Not coded in megamek.
10 Basement. The unit falls 1 level (for ’Mechs use the Front/Rear column of the ’Mech Hit Location Table).
11 Basement. The unit falls 1 level. ’Mechs fall headfirst (use the Front/Rear column of the ’Mech Punch Location Table).
12 Double basement. The unit falls 2 levels. ’Mechs fall headfi rst (use the Front/Rear column of the ’Mech Punch Location Table)
(CF/10)*#levels (CF is the construction factor at the beginning of the phase, rounded up when you divide by ten), AND any unit in the building's basement will automatically be destroyed (it's buried in the rubble).
So, a 6 level building that started out as CF 90, been reduced to 25 at the beginning of the phase, will inflict 18 points of damage when it collapses. (2.5 rounded up to 3, times 6 is 18)
Entering the building: when you enter a building, you will pay additional MPs based upon the rating of the building, and you will roll a PSR to avoid damage. The PSR modifier will depend on the building rating; if you fail, you do not fall, but you do take damage equal to the CF/10 rounded up. Once in the building, though... watch for basements (see above). Also note that you will also inflict your tonnage/10 to the building's CF when you enter it.
If you are in a building and you are shot at, if you are not infantry, the building will absorb its CF/10 rounded up, where the CF is the CF at the beginning of the phase. So, with the above building, say you're in it, and there's a Locust on top of it. CF is 25, so no problem, but you get shot by a PPC, 4 medium lasers, and 12 points of LRMs. Every attack gets 3 points peeled from it and into the building instead, you you take 7 points from the PPC, 2 points from each of the mediums, the 5/5/2 groups from the LRMs get the same treatment so the first two groups do 2 points, and everything else goes into the building. 3 points from the PPC, each of the 4 mediums, and the LRM groups, so the building just took 22 points. Now the CF is only 3, and that Locust is easily enough to collapse it on top of you, then try to fall on you.
Last note: a building level is one level high, and while a mech is nominally 2 levels tall, when it's in a building, it's only one. So you can hide by entering a L1 building behind another L1, and you can punch ground vehicles, infantry, other mech's legs... options abound.
Infantry units do not pay a cost to enter the building; just the 1 MP to enter the hex (2 for mechanized infantry).
When infantry are in a building that is fired on, damage is passed through according to the rating: no damage for hardened, 25% for heavy, 50% for medium, 75% for light. This can result in infantry taking more damage than if they were not in a building.
Infantry take triple(!) damage from collapsing buildings.