138 Section through doorframe at sill space filled with insulation, compressible gasket or sprayed urethane foam sheathing membrane siding shim door jamb caulking sloped doorsill exterior door trim water-impermeable sill membrane check for storm door 19 mm (3⁄4 in.) board caulking concrete step SUSTAINABLE HOUSING INSIGHT Affordability Exterior trim and millwork that needs to be periodically coated with paint, stain or clear finishes is becoming increasingly less popular. Chapter 24 provides more detailed information about the expected service life of coatings. Exterior, site-coated surfaces can be expected to require maintenance at least every three years. Exterior trim and millwork is often difficult to access without ladders or other special equipment, further increasing the cost of maintenance. Factory-finished metal and other trim materials are recommended because their appearance can be maintained for a long period of time by cleaning rather than recoating. Resource Conservation Exterior trim and millwork that uses finger-jointed wood, or medium/high density fibre, board conserves forest products.