You can always (1) copy the Base mat, (2) paste to Preview, (3) copy your Layer 1 mat to Base mat, and (4) copy the Preview mat to Layer 1. I always use Preview as my working storage. Good luck with your roof. 

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W11,Intel i9-14900KF @ 3.20GHz, 64.0 GB RAM, 64-bit, GeForce GTX 4070 Ti SUPER, 16GB. 

Old lady hobbyist.

All visual art or fiction is "playing with dolls."