If the aim is to kill your opponents, then it comes down to the same as standard combat, though there are tricks you can do in order to change the nature of the combat/slant the results in your direction. Below, you will see a general breakdown, with the specific example of Jimmy Johns the demolition expert, and his accomplice in B&E Grant Humphries as they plan their hit against the hideout of the Deadboys, a local gang. (As I'm more familiar with DFRPG, I'll use Craftsmanship: Breaking (
YS125) for the skill)
Stage 1: Casing the joint. (ref
First, you'd make your job a lot easier by actually doing research on the facility, and using your demolitions skill (and any others that might be applicable) to put aspects on the location.
Grant does his face man work and finds out that the Deadboys hold up in an old warehouse on the waterfront. A few pictures from a few angles, and Johns goes to work, making assessment rolls against the building and spending fate points to put the aspects of
Cracks in the Foundation,
Shoddy Construction, and
Not up to Code on the building. These set up assessments that are not subject to the usual time limit for tagging- thereby getting free tags at the time the explosives are set off.
Stage 2: Setting the explosives.
After acquiring the explosives in one fashion or another, the demolitionist provides these to the infiltration expert (or does it himself). How this goes down is a matter of the sneaky type vs the security present on site (either mechanical, mystical, physical, or a combination of the three). The security specialist can also place additional assessments on the location, representing his on site adjustments to the location of the explosives based on what he sees.
After creating the 'packages', Johns passes them off to Grant, with specific instructions on where they should be placed. Grant waits until the Deadboys are mostly away from the warehouse by observation/assessment/placing aspects and makes his way stealthily to the warehouse. As he arrives, he assesses (using his alertness) a large crack in the base of the wall near where Johns told him to place the first package, and he wedges it into the
Crack in the Wall. Going to the second location, he sees that he missed one of the Deadboys, but after harrowing use of his Skulking to get past the sentry, he places the second one. The others are placed with similar care, though Grant doesn't try for an assessment knowing that someone is there, and not being willing to spend the time and risk getting caught.
Stage 3: Bringing down the House. (ref
The explosives then need to be triggered, and the Craftsmanship: Breaking used to attack the structure. There are two ways to do this. First, the explosives can be given a damage rating and the structure a defense rating and stress. The Craftsmanship: Breaking would be an attack vs the structure's defense rating, and the damage done as normal vs. the structure's stress; in general, a structure doesn't have consequences, so if the attack does more after defense than the structure's stress, the structure comes down. Multiple placed explosives help with this as each would be an attack. Second, the whole thing can be considered a challenge, with the structure of the building representing the total number of successes that must be garnered in one explosion or multiple explosions to bring the building down.
Grant hides with his cell phone, waiting for their targets to come back to the warehouse so that Johns can bring the whole thing down around them. For all of its faults, the warehouse is pretty sound with 4 defense and 12 stress. Each explosive has a rating of 3; Johns decides to stagger the explosions, using his Craftsmanship: Breaking of Good(+3). On the first explosion, the first roll comes up -3, so he tags
Cracks in the Foundation for free, and rolls again, getting +1+3-4=0 so the explosion goes off successfully and does damage, adding the rating of the explosive(3), so 3 stress- not an auspicious start. On the second, he rolls +3+3-4=+2. Another successful explosion for +2+3 = 5 stress. On the third explosion, he invokes Grant's
Crack in the Wall for effect to bypass the defense of the warehouse, as it's already in the wall, and rolls +3+3=+6. He utilizes his prior assessments for
Shoddy Construction, and
Not up to Code, and spends a Fate point to tag
Cracks in the Foundation again for a total of 14 stress, successfully bringing down the warehouse. The fourth explosion is rolled just for effect- after all, there were people in there! +2+3-0(cancel out the defense since the building is wrecked)=5+3=8 stress
(Alternatively, a challenge level could be set. As you can see from the example above, without the assessments, it becomes more difficult to get enough stress in one roll to bring the structure down; challenges, as they let you accumulate successes, would allow for an easier experience)
Stage 4: The people within (ref
Depending on the relative importance of those within, casualties can be claimed by the number of stress inflicted on the building and/or environmental hazards/explosions and whole-zone effects. The rating for the hazard/effect should be based on the skill roll of the craftsmanship:breaking.
The Deadboys was a pretty big gang- was being the operative term. With 22 nameless grunts in the gang, the GM rules that the massive size of the 4 explosion was enough so that they'd get consumed in the conflagration, and only worry about the leader and his 2 right hand men. For expediency sake, the GM had them roll 4 rolls vs the hazards of the explosions, using the base rolls for the breaking, i.e. 0, 2, 6, and 5 vs the thugs athletics. Not looking too good for the thugs.
A lot of this is supported by the rules, and a lot of it is fill-in, but this approach has worked pretty well in my games.