Designing Kitchens and Bathrooms - Drywall rotted after removing tub surround

Okay, I removed the tub surround and noticed that the drywall was soft and crumbling....do I just remove that portion that's rotted and patch it with new drywall or do I remove the entire section of drywall that was under the surround? the rest of it seems to be fine. How do I patch it?

You should be able to score a straight line above the rotted area with a razor knife and snap the affected area out, using the razor knife to cut the paper on the back of the drywall. Examine boards in walls to assess damage. If boards are black, poke with screw driver to see if they are rotted. If so, then you may need to call a professional to replace the joists. Make sure you keep the tub covered with a drop cloth to protect it from dirt and grit that will scratch the surface. Use a shop vac to remove all dirt and dust in the area where you are doing the repairs. Let the area remain open so that it will dry out. You want the wall cavity to be thoroughly dry before you seal it up. If you see mold/mildew, you may want to spray 50/50 bleach/water solution on affected areas to disinfect and kill mold/mildew spores. Don't seal up the wall if it not thoroughly dry. Perhaps running a fan and/or dehumidifier in the area will expedite the drying time.

It is recommended that you use cement board to replace the rotted drywall. It will hold up well to water. Green board is an option. It's a treated dry wall, but it will not hold up to water like the concrete board. Measure and cut the pieces to fill the areas where you removed rotted drywall. You can cut concrete board by scoring several times with razor knife and snapping, using knife to cut backing. Make your cuts and measurements as accurate as possible. You can trim with razor knife if the piece does not fit easily into the hole. If you need to raise the piece a little to match the level of the drywall, you can use shims. Construction adhesive can be used to attach the concrete board to the wall. Then, screw the concrete board to the wall. Use only galvanized screws. You can usually get these where they sell the backer board. Gaps between boards can be filled with thinset mortar.

I want to thank you, Twelvepole, for your response to my question. I followed your directions and Voila it worked so well. Thank you so much. I want to also thank everybody that actually responds to questions. I am a do-it-yourselfer novice and really need advice. Thank you.

Glad to be of service.