Do I add modifiers to the Charisma check roll of 15 granted by the Glibness spell?

Question

While affected by the Glibness spell, a creature can replace any Charisma check roll with a 15:

Until the spell ends, when you make a Charisma check, you can replace the number you roll with a 15.

It's not clear to me from this wording whether the 15 replaces the roll before or after adding any modifiers. For example, if my modifier for a Charisma check is +5 and I choose to take the 15 from Glibness, is the resulting Charisma roll a 15 or a 20?

Answer 1

According to the SRD (p.77):

To make an ability check, roll a d20 and add the relevant ability modifier

Glibness can replace the Roll with a 15.

So, with a +5 CHA Bonus, instead of your Ability Check being 1d20 + 5, it can become 15 + 5, the d20 roll having been replaced by "15", if you so choose.

If you have Proficiency in the Skill you are using (e.g. you are making a CHA (Deception) check, and you have Prof. in Deception), you may also add your Proficiency Bonus.

Answer 2

"Roll" is not the same as "result"

The word "roll" literally refers to the roll of a die. Take, for example, the Great Weapon Fighting fighting style:

Great Weapon Fighting

When you roll a 1 or 2 on a damage die for an attack you make with a melee weapon that you are wielding with two hands, you can reroll the die and must use the new roll, even if the new roll is a 1 or a 2. The weapon must have the two-handed or versatile property for you to gain this benefit.

The "roll" refers to the actual number shown on the die.

The "result" is a different matter. It is the total value after applying all modifiers. Take, for example, the indomitable might class feature of the barbarian:

Indomitable Might

Beginning at 18th level, if your total for a Strength check is less than your Strength score, you can use that score in place of the total.

For glibness, you can adjust the d20 roll you make to be 15 regardless of what you actually rolled.