Aligning a pattern or texture from a line?

[rickk]

Just an unsolicited suggestion from the peanut gallery, but shaped gradients (spherical, dimpled) will follow the perimeter of whatever shape they are entered. Following is made with the "four bars" gradient that comes with gimp, but you could make your own  in colors and spacings that more  closely approximated shingles. What I did was to (after the fact) take a pattern of gravel, use "color to alpha" to remove the body color of the stones, leaving the grid lines only, and then bucket fill with a high threshold level that remaining grid over the top of the shaped gradient. Because the lines of the gradient dominate the image, the lines from the pattern drawn over them, appear subtle in comparison



Again, you'd want to draw your own source gradient to approximate roofing.  But if you make a lot of these, then I'm sure you would get good use out of it
FWIW