Having read up on it a bit more I believe the bigger heads are for fascias n smaller heads for soffits? It also depends how sound the timber behind is, if there's an inch of wet rot then it's better to use the 65 mm length nails to penetrate into sound timber, if you didn't want to fix cleats to the sides of the joists to provide good fixing points. Ideal for outdoor use as the soffit pins are UV stable meaning they won't fade over time from the outdoor elements, and are far more reliable.

The manufacturer, SEAC would like to remind the trade that POLYTOPS ® is a registered trademark and any use of the text “POLYTOP” or “POLYTOPS” is violation of the mark if products sold are not POLYTOPS ®. One 40mm pin every two feet is not enough long term to hold up the lengths of guttering, especially in snow or freezing conditions. The self-masking, decorative plastic top of Polytops® polymer nails provide a neat and unobtrusive fixing in one quick and simple step. The difference is, one are polytop nails, the other polytop pins, and the overall factor is strength.

POLYTOPS ® fixings are plastic top pins, nails and screws manufactured to a high UK standard using top grade polymer and the highest grade steel, that’s A4 marine grade stainless steel.

Catherine Thomas "Thank you very much for your information package never mind your impressive client list and various services offered i was sold on the chocolate bar. Plastic soffit fixing pins with stainless steel shanks designed for use with fascia, soffit and window boards.This is why Leading UPVC board manufacturers such as Eurocell, Celuform and Deceuninck recommend POLYTOPS ® on their boards.