My redwood deck was on top of a flat, tar and gravel roof. Open to all the elements and wind, lots of weeds and grass grew up between the flat boards spaced 1/2 inch apart. First, I considered using RoundUp then pull up the dead growth. Problem: the roof belonged to the homeowners association and I was hesitant to use a harsh chemical on the roof. I called 3M for advice and spoke with their chief chemist. He agreed that RoundUp might damage the roof. He suggested I use white vinegar. I poured it on without diluting it. I used 6 gal. for a 225 sq.ft. deck. Worked beautifully and my deck was weed free for four months. Since then, I use vinegar if there is a rubber or vinyl membrane under a walkway or brick patio. Inexpensive, ecologically sound and easy to do.
Thanks for sharing your experience with white vinegar. I had read that it could kill weeds and had limited success, but from your description I probably didn't use enough of it. Next time I go after the weeds, I'll treat them to a stronger mixture.
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