Hiya.
My suggestion would be this: Stop trying to "play by the same rules the PC's play by".
NPC's don't follow the same rules (re: class, HD, abilities, etc) that PC's do. Monsters don't follow the same rules that the PC's do. Why should traps be any different? Just think about the situation, imagine what would "make sense" for this wizards trap, write down what you think would make the game interesting and fun, then assign a DC to detect and/or disarm based on the *guidelines* in the DMG (as someone laid out earlier in the thread). You don't have to "find a rule" or "find a spell" to do what you want, nor do you have to then figure out the specific AC and HP of every item in the room and if it is/isn't in the initial "blast radius". That's 3.x/PF/4e thinking. You should have left that kind of thinking back with those books.
So...just imagine what would be cool and make sense. Then write it down that way.
The wizard was a paranoid fellow, always seeing shadows out of the corner of his eyes. Convinced that someone...or someTHING...was waiting for the opportunity to steel his secrets, he placed a ward on his writing desk. It is crafty and hard to spot (DC 20), but if found, not difficult to remove (DC 10). If it is triggered, a magical blast of sparks, heat and a loud, thunderous BOOM erupts from the desk itself. All those in the room can make a DC 15 Dexterity save. Failure means they are thrown prone and take 2d10 damage. Success means they take half damage and are not thrown prone. The more fragile things in the room (loose paper/scrolls, glass bottles, feather pens, light clothing, etc.) will likely catch on fire as the heated sparks touch them. If not extinguished quickly the whole library will be engulfed in flame.
There. Simple, interesting and logical. How to handle the specifics like what exactly 'catches fire' or what can put out the fires and how many 'fires' are there? In the immortal words of Richard Carlson..."
Don't sweat the small stuff". In other words, make it up on the spot. Stop just "
running the game", and start "
DM'ing the game". If your players dice have been rather cold and unforgiving that session, make it relatively easy to put out the fire with any reasonable idea (e.g., "I smother the flames with my cloak"). If they are a group who like to use their equipment a lot, let them use up all their water/wine/beer dousing the place. If they are dice-monkeys and just want to roll a lot of dice, pick a DC and have them roll some particular check each round, subtracting failures from successes, with whichever number (positive or negative) getting to 5 first, "winning" (e.g., if the success:failure ends up at 5:2, they put it out...if it ends up 2:5, the place is too far gone and is now an inferno).
Why do I suggest this way? Well, to quote the Famous Bard: "
The play's the thing" (ok, so I'm using 'play' differently...sue me

). And with regards to running 5e the way it is intended to be run (heavily IMHO), fast and loose is ALWAYS the better road to travel. The DM needs to have fun too. I may be a bit different than other DM's, but I get immense enjoyment out of trying different ways of handling things "game mechanics wise". I've DM'ed Pathfinder multiple times, and after a certain number of games, without fail, I get bored as H-E-doublehockeysticks! I don't get to flex my DM'ing muscles. I don't get to really try new things. My players become used to the ho-hum drudgery of "Oh, ok. So let me guess...I roll a d20, add some stat/skill adjustment and have to beat some particular number? *yawn*..." If EVERYTHING in the game comes down to simply rolling a d20, with a binary "100% success or 100% failure", the game gets predictable and bland. Fast. So spice it up with a bit of unexpected "mechanics play"! Get creative with your chances! Turn a natural 1 into some sort of humorous (or disastrous!) result in stead of just the "OK, you don't do it". In short, surprise yourself with your creativity and give your players something a bit different. I bet you (and them) will find running and playing in a D&D session a LOT more entertaining!
^_^
Paul L. Ming