I use grey roll roofing material for my ballast and if you turn it over it's black. Cut your ties any size you need (cut from the back, score the roofing then snap it off)
One thickness is good for O27 and put two together for O Gauge. Depending on the thickness of the roofing you can just put the ties in place and screw down your track to hold the ties in place, If the roofing is a little thin put a dab of caulking adhesive on the ties.
If you can find a roll of damaged roofing , you can usually get it for a discount.