I should have suspected something when I noticed softness in the hardwood floor in the back corner, but I wasn't using that area much. Then, when a couple of exterior shingles fell off and I noticed that the others were out of line. I investigated, and found extensive rot in that corner of the house (an addition to it built in the 1960s) which was concealed by the Transite siding.
The two paired 2x4s which formed the corner post are entirely gone; they came out in pieces. The wood sheathing behind the Transite is also heavily damaged. At least the base timbers on the cinderblock piers seem to have survived (picture below); the one on the left running front-to-back appears sound and the one on the right running crosswise has some rot damage, but may be salvageable.
It appears that the damage was caused by water entering through a gap in the wooden trim at the top of the wall; the rotten trim strip is visible at the top of the first picture. At the present time turning to professionals is not an option for budget reasons. First, what do I need to do to prevent further damage until I can tackle the repair work? Second, how should I go about repairing and restoring this damage, and then protecting the job for the future?
Note: I have several bundles of GAF fiber-cement shingles which are a cosmetic match for the Transite. My carpentry skills are basic, but I do have good tools including a Craftsman table saw and a ShopSmith. I also have a couple of hydraulic bottle jacks and automotive jacks if you would recommend raising the corner somewhat when installing the new corner studs. I'm grateful for any helpful advice.
Editing to add additional information:
I pulled back the siding to expose essentially all of the rotted wood. Note: The eaves are covered in aluminum sheet, so it's possible there could be more bad stuff in there. I didn't want to open that up just now. Here's the extent of the damage:
That's bad enough, but that appears to be it. There's a stud in that north wall (bottom picture) behind the wood sheathing which appears to be sound. The timbers on the piers look to be 4x6 timbers with a half-lap joint at the corner, and the floor joists look to be 2x8.
First priority looks to be running to grab some plastic sheeting to cover the opening, along with some really good duct tape. My idea for moving forward from there is to saw the sheathing back to the far side of that stud, then rebuild the corner studs and move on from there. Again, suggestions from those with experience are welcomed.
Yes, it's the roof. There's a little valley where the roof of that addition joins the main roof, and the roofers who worked on it a few years back didn't use flashing. Water pooled in the corner and rotted through the decking:
As it turns out, though, I had a 10' 2x6 in the rack above the Shopsmith. Cut it to length and wedge it in, and I've got a temporary support for that corner while I work on repairing the damage.
It's only money....