I finally gave up on screws for cabinet boxes after a job in Toledo changed my mind
Always figured glue and brads were fine for face frames and screws were overkill for boxes, did it that way for 6 years. Then a kitchen job in Toledo last month, homeowner loaded a lower cabinet with cast iron and the thing racked about a quarter inch after 3 weeks. Went back and pulled it apart, the joints had opened up where I only nailed. Rebuilt those boxes with glue and 2 inch screws every 8 inches and they feel dead solid now. Anyone got a rule of thumb for screw spacing on 3/4 birch ply boxes, or am I overdoing it?
Screws every 8 inches sounds like a lot to me, no offense. In my experience glue does most of the work on a ply box and the screws are just there to hold it tight while the glue sets, so a few well placed ones per joint go a long way. The part that actually bit you in Toledo was probably the nails, since brads have almost no pull out strength and glue alone on end grain ply is weak. Your mileage may vary but I'd say 2 screws per joint plus glue, and make sure the box is square before you drive them. The cast iron thing would've wrecked most lowers anyway, so I wouldn't rebuild your whole method around one heavy load.