Historic Structural Detail: Clever Carpentry

by Don Friedman on August 1, 2016

Notice anything interesting about the header in front of the chimney masonry?


There’s no visible connection between the header and the trimmer. (There are also no visible connections between the joists and the header, but this photo doesn’t have the right angle to see, or not see, that.) This is a mid-1800s house, and the standard connection in the New York area at that time was to use a mortise and tenon, usually with the header’s tenon projecting several inches through the trimmer, sometimes pegged. Late in the 1800s, bridle irons became popular, followed by light-gage joist hangers in the 1900s.

So where’s the missing tenon? The carpenter went to the trouble to create a blind mortise, that is one that is a dead-end pocket within the trimmer. Since the floor framing was intended to be hidden by a plaster ceiling – and was hidden for most of the building’s existence – it will always be a mystery why the framer chose to do a lot of extra work for no benefit. It could be worse: in some furniture and some obscure semi-medieval heavy-timber framing, there are shouldered blind mortises, where the mortise and tenon change size inside the wood. They’re beautiful joints but very difficult to make.

In other words, there’s a reason that house framing is usually called “rough carpentry” in specifications. It’s not where you expect to see fancy joints and clever details.

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