Carpentry and Woodworking - Door jambs not standard width?

I can't figure it out, but I first noticed it when I had purchased a jamb set to finish an opening, and realized it wasn't wide enough. Ended up ripping some 1x6 to do the job. However, I'm now looking to replace all the interior doors in the house, and when measuring the jambs, am consistently getting 4 & 3/4". Did the use fire code throughout the house?

Anyway, I have to decide if I'm going to special order a bunch of pre hung doors or if I should just get slab doors. I know hanging a slab door can be more difficult, but I'm wondering if special ordering pre hung doors with the correct jamb width is really worth it.

If there is 5/8 sheetrock used, it would account for the 4-3/4" thickness. I would just order pre-hung doors with the wider construction. It may not cost that much more to order the doors with the wider jamb. Worth asking the door shop in town anyway. If nothing else, it may be worthwhile to have the jambs made. That way all you have to do is fit the doorway and hang the door.

I would buy standard 4 9/16 prehungs, since they are in stock, and install the doors flush with one side of the wall. Then come back and shoot on a 1/4" extension. (3/16" x 11/16" oak or pine screen moulding might work just fine, since it's already sanded, with a rounded edge.)

Personally if the jambs are in good shape I would buy slabs and hang them on the existing jambs.It would save you alot of money and probably alot of time in the long run since if you use prehungs you'll probably need to get new casing and re trim all the doors.

From a business stand point I make it a point to tell homies it'll cost a whole lot more to have us install slab doors than if we just order teh correct door in the first place.

I'm all for the stock doors and building out the jamb myself, once it's stained you'll never no the difference, if people have the coin we just order the thing the way it's supposed to be. Slab doors are time consuming and PITA to say the least. But I just do this for a living so the faster I complete a job the more oney I make:) Time is money in this profession :D But if your a home owner with nothing better to do with the colder weather outside, the slab owuld make for a great learning expereince and something to do.

I'm really up in the air. I'm going to check out exactly what the price difference would be for the slabs vs. the custom pre-hung. If it's not an extraordinary difference, I'll probably go with the pre-hung, based off the fact that I'm trying to get about 12 doors done. Some of the trim around these doors has already been replaced/upgraded, but I'll just have to carefully remove it.