This project wasn’t a full roof replacement. Instead we installed a roof onto a porch that was added on to the house. The original shingles were fairly new so it would have been a waste to replace the entire side. The porch roof was installed right over the top of the shingles, then we tore out what we needed to and blended the two roof surfaces together.
We matched the color and brand of shingle to help it blend as well as we could. Then underlayments were installed and the shingles over top. We always put 3 layers in the valleys: ice shield, valley metal, then the shingles over top. This ensures there will never be a problem in the valley.