Attaching coroplast

For self tappers I've used #10 or #12 screws with a fender washer. 1/2 inch or less is all you really need. They only need to be long enough to get through the frame. A second person is really useful with getting the sags out. One person holds up sections while another screws.

The only sealant I use is Great Stuff foam on the edges and penetrations. I squirt a line of foam about an inch from edge of the underbelly then put all of the screws in place. Makes a nice seal. Works much easier if you have all your screws pre drilled in their locations beforehand.

X2 on the overtighting. That is the cause for a lot of the screws braking off. They are self taping screws and the workers use air guns to run them into the frame. When they hit bottom there is a lot of force on them when the worker don't let off soon enough. Thus a broken screw.
Close to half of all the screws on my TT was broken.

# 8 x 3/4 sdst 5/16 head screws. Add a fender washer, be careful not to overtighten.

I found that using stainless steel fender washers worked well.

You might want to think about using ss screws all around. Match up the size thats already used.
Also, look into doing the underbelly in sections. It makes easier handling if you need to make a repair down the road.

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