Roll under D&D [Subterranean Adventures tinkering]

I'm seriously considering going roll-under for my edition-less houseruled D&D. I generally like roll under: it's simple, clean and gives you a good idea about your chances of doing something.

There is precedence for this: older editions of D&D, before there was an unified resolution system offered the same option: roll under your stat to succeed. So this is actually nothing new or unusual. This makes it kinda like a less granular percentile system where the smallest possible modifier to your chances of success is 5% (+/-1). Looking at the Warhammer RPG line that's not unprecedented either as the smallest possible modifier for anything there is 5%, too.

One problem is that characters can start out unusually powerful. Rolling an 18 on a stat with 3d6 is rare (0.46% chance, to be precise), but when it happens it means you have a starting character with a 85% chance of success, which is pretty darn high. It leaves little room for improvement (without the application of penalties).

Another problem is the disparity of systems: while ability checks are roll-under, combat is still roll-over in older editions. I don't like that kind of diversity, I want my game to use the same rolling mechanic, for simplicity's sake if nothing else. The solution would be to make combat roll-under as well.

Without taking into consideration class bonuses, the universal modifier of meelee rolls has always been Strength. So in a roll-under combat system, rolling under your Strength constitutes a basic attack roll. The enemy's armour would thus need to act as a modifier to the attacker's roll.

Again, there is precendence for this. I'm not really inventing anything new, just rationalizing what exists. Under the old THAC0 system that many find confusing, armour did not modify the "difficulty" treshold needed for a hit as in the newer roll-over editions, it modified the attacker's ability. That's why lower, negative armour class was better: it represented a penalty.

Clearing away with the confusion that is THAC0, an enemy's armour class can simply be applied as a penalty to the attacker's base Strength check.

A quick bit of math shows that a base armour of 0 isn't justified in the roll-under system, so let's make it 2 (thus the percentages correspond to the old roll-over probabilities). Or to put it otherwise, all attacker's base attack value is [their strength-2].

I like this. It's clean and concise and simple. I have just three problems with it:
-adding is, in general, easier for people than subtracting
-I have already invested some time into figuring out the specifics of roll-over mechanics
-you lose the excitement of rolling a natural 20

This last one bothers me the most. I really like that special feeling and I've made "critical successes" a big part of my system. Making it roll-under robs me of that. You can make a natural 1 a critical hit but that just doesn't have the same vibe.

So that's the last big decision I have to make: simple clean roll-under or the exhilarating feeling of a natural 20...


  1. The advantage to roll-over is that your ability depends more on your level than Str or Dex. So if you are 20th level and have Str 9, you still fight better than a 1st level guy with Str 18. You can also use an opponent's stats as roll-over target numbers.

    Of course, roll-under is super-simple. The way I use it is that if you have skills and equipment, you roll under your stat. If you need special skills or equipment that you don't have, you roll under half your stat. If you need both skills and equipment you don't have, you can't roll.

    What would you do for monsters and NPCs if you used the roll-under to-hit system?

  2. If I were to go down that way the simplest thing (for me) would be to have the monsters Just Do It. When the dragon breathes fire on you, he just does it, it's the player who rolls Dex (or Ref) to avoid being burnt to a crisp etc. When something tries to hit you, you make an AC save.

    (At worst the monsters would give you some sort of penalty on your roll based on their level, size & perceived abilities.)

    It's super-simple and I like it, but then it also starts looking more and more like a heartbreaker and less and less like a frankenstein houseruled D&D that's recognizable from B/X to 4E (which is my mission statement).

  3. Uh, dude, that IS how dragon's breath works in B/X. You save vs. breath weapon to take half damage.

    Anyway, what I mean is, that while it may seem like a simple solution, it's really not, because stats and level are two different gauges of effectiveness, and they work differently -- hence the roll-under alongside the roll-over.

    Have you looked at The Whispering Vault? I think that was the first game to get rid of GM dice-rolling and have NPC stats be target numbers for players to hit.

  4. I know! That's how it works in 3.x, too. I just picked a really bad example of "monsters don't need to roll". Dumb.

    I realize the stat-level disparity. For this game I was thinking about seriously flattening the numbers and cutting out a lot of the assumed progression, but as I said, then it really diverges from my goals.

    I've since decided I'll be going back to roll-over, even though the prospect of having a really simple, roll-under-only D&D is very attractive. Maybe for another game.


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