Certain hardwoods currently used for exterior shutters, specifically Spanish cedar, Honduran mahogany and teak, are resistant to rot and decay, and far more durable than a softer wood like untreated pine. Because of the tannin in these woods they are also not prone to problems from insects. Pressure treatment with wood preservative makes pine, cedar and other types of wood suitable for exterior shutters. Non-functional exterior shutters can be constructed from wood, vinyl, fiberglass or other synthetic materials. Non-functional shutters can be simply screwed or bolted directly to the building, with spacers or other hardware to prevent water accumulating against the wall. Fiberglass, a material widely used for boat hulls, is not prone to rotting or twisting, is dimensionally stable and has low expansion and contraction. It is a reinforced plastic, unlike vinyl or PVC, so fiberglass shutters do not have a tendency to warp or sag. Its strength is comparable to aluminum; however, it does not suffer from corrosion and electrolysis.