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There are certain conditions where metal fasteners are appropriate.  In trusses or other joinery that is in tension, metal may be required to supplement wooden joints.  We avoid the use of metal fasteners for repairs whenever possible.  We have seen many poor repairs using metal plates to shore timbers that are rotten or otherwise compromised.  Repairs that use metal are often short term solutions at best that may cause greater damage to the structure in the long run.  Compromised timbers that metal is attached to is often not able to maintain a strong connection under load and will likely fail in the vicinity of the added metal.   Metal hardware often attracts condensation that can accelerate the decay of timbers.  In addition to attracting moisture to the timbers, condensation causes rust.  Rust can expand, with great force, and further weaken connections with the timbers.  This kind of damage in wood that is suitable for a viable wooden repair often leads to the loss of the timber in the vicinity of the metal hardware for future repair efforts.  The shrapnel left behind from metal fasteners causes great damage to a joiner’s edge tools and hinders future preservation efforts.  Metal hardware should only be used as a last resort when it proves to be  the most conservative repair possible.