Cap fasteners are a recommended best practice installation by housewrap and roofing underlayment manufacturers like DuPont, Typar, Grace, and Titanium. Why? Because caps help seal out moisture that can penetrate sidewall and roofing fabrics. Caps provide better holding power than a nail or staple alone and that means reduced blow-offs and lateral slippage! It also means fewer fasteners are needed to get the job done.