That.ay include stairs, patios, walkways, retaining walls, masonry fireplaces and hearths. They may also install sheet materials air and vapour retarders that prevent moisture problems in walls and attics. For the class of mathematical functions, see Sub contraction map . According to the remodelling Sentiment Report, 35% of home-owners bypass the general contractor and hire tradesmen themselves including plumbers, electricians and roofers.  Our relationships with subcontractors are crucial, and SOS Building and Development SOS relies on them from the day we bid a project until completion.