Mine screw onto the roof purlins. https://www.midmichiganmetalsales.com/product/snow-defender-4500-color-powder-coated-snow-guard/?gclid=Cj0KCQiA_P6dBhD1ARIsAAGI7HDv_pUL8_W4YSVWS29T3oNJ8bLpUfGrWFRLWTc7UNB7bZjrGK8F8LoaAjUfEALw_wcB

They don’t necessarily hold all of the snow on the roof all winter. They mainly just prevent huge overhangs from dumping off all at once. To enable still slowly comes off, just a little at a time.