Hanging heavy window boxes to house

I need to hang some heavy window boxes. The brackets are wrought-iron, the boxes are wood. When loaded up, they are heavy. My house is shingled. The brackets have two vertically-spaced holes on each end. Would a lag screw work? Does a lag screw hold if the tip has gone through the sheathing? What about large hollow wall anchors? I can't be sure that I will hit studs.
Thanks.
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a handyman with a drill and longer lag bolts and a studfinder may be all that is needed. you might lighten the weight by reducing the ingredients of the window box dirt. check with your gardening store for their tips appropriate to your climate and rainfall for a filler. alternatively, consider hanging them from the deck or porch railing, or perhaps placing them along a walkway or on the porch.
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