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Rafter ties

One very important structural problem that I look for on every building, but especially on detached garages, is the top of the wall pushing out.  Here’s a short but (I hope) thorough description of the problem, the cause, and the solution.

Most roofs are built so that the roof rafters lean against the ridge board, or even just lean against another rafter without a ridge board.  Then the rafters are nailed in place.  The other end of the rafter sits on the top plate of the outside wall.  Older roofs were almost always built this way.

So let’s look at this in the direction straight along the ridge beam.

Gravity is pushing down on the ridge board and the rafters, and maybe that’s even helped along by a big snow load.  There’s nothing holding the ridge board up – it’s just there for the rafter to lean against.  So the whole thing tends to settle a little bit.  And when it does the bottoms of the rafters naturally want to push out, and this takes the top of the wall with it.  This happens fairly easily, and the leverage from the bottoms of the rafters will easily pull out any nails that are attaching the top of the rafter to the ridge board.

This problem is especially bad with hipped roofs, because there’s really nothing holding the ridge board up.  On gable roofs the problem isn’t usually quite so bad, because the gable wall provides some support to hold the ridge board up and prevent it from settling.

To fix the problem, or to stop it from happening in the first place, a rafter tie should be installed.  This is a horizontal piece that attaches to the bottoms of the rafters.  It creates a very stable triangle shape and holds the bottoms of the rafters and the tops of the walls from pushing out.  A rafter tie is typically a piece of 2x lumber, but it could even be a metal cable or chain as long as it’s securely fastened.  And you don’t need a rafter tie at each pair of rafters.  In a detached garage you only need a total of two or three rafter ties to make the structure nice and stable.

In houses with cathedral ceilings, where you want the open vaulted ceiling and you can’t tolerate a rafter tie, the ridge board needs to be very solidly supported at both ends all the way down to the foundation.  Then it becomes a ridge beam instead of a ridge board.  Since the ridge beam can’t settle, the rafters are held in place and won’t push out at the top of the wall.

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