Exterior trim and millwork include window and door trim, soffits, rake, gable and fascia trim that are cut and installed on site and other accessories such as louvers and shutters that are usually shop-manufactured.
Trim should be low maintenance, resistant to weathering, workable and dimensionally stable (low tendency to warp, shrink or contract and expand).
Prefinished sheet metal is now widely used because it is impermeable to water and can maintain its finish for many years without having to be repainted. Correct detailing and sealing are a must to prevent water from running behind the metal trim. When wood trim is used, seal the end joints or mitres of lumber exposed to moisture.
Nails or screws used for fastening trim should be corrosion-resistant, aluminum, galvanized or stainless steel. They must be compatible with the metal trim to avoid galvanic reaction between dissimilar metals, such as aluminum and steel. If finishing nails are used, they must be countersunk and then sealed after the prime coat is applied to prevent rust stains at nail heads. When the siding is prefinished, matching pre-painted nails may be used.
Source : Canada Mortgage and Housing Corporation (CMHC)