Sistering

There is a term in carpentry called Sistering. I was reading about it on Glennon Doyle Melton's blog today, and this is how one Momastery reader defines it:

"Sometimes an existing joist, which was designed to handle a certain load, becomes too weak. Maybe it was damaged by water or fire. Maybe it still has structural integrity… but,  now it is not as sturdy as it needs to be…the new load [it receives] is going to be a lot heavier than before.  When a builder needs to strengthen that joist, she puts a new member right next to the original one and fastens the two together. Sometimes, two new joists are needed- one on either side. Do you know what they call that? A Sister Joist.  And builders use “Sister” as a verb, like, “We need to Sister the joists in the east bay about four feet.” Even better is the nonsensical: “Sistering” as in, “Are they finished Sistering the roof rafters?” "

It got me thinking. Is this not the perfect solution to when the load of life gets too heavy for any one person? For times when we might be mangled, bouncy, or sagging and need additional support? So, I did some additional research into how Sistering is used in construction and engineering to restore structural integrity and provide long term rigidity.

Here's the thing.  When houses (or people) are around long enough, something in their basic structure is inevitably going to get broken, twisted or split.  Right?  Typically, if additional support is added, it needs to happen right alongside the existing structure so the two are fastened tightly together. The attachment is the key.  If the original structure is good to start with then doubling it with the equal or larger sister support ups the safety margin.  But, ONLY if and when it is properly attached in the first place.  

For example, a damaged floor joist or roof rafter generally causes a low or sagging spot on the structure.  You need to raise this bouncy low spot to stiffen the floor or roof and raise it to it's original height. You brace the compromised joist along it's low point by jacking it up with a sister board of the same length and depth that is then carefully glued and nailed in place. You always want to be sure the sister board has enough extension to bridge the defect in the broken board. Glueing and nailing on a piece of wood that is the same depth and full length as the original board really increases the load carrying ability of that board.  Of course, you must take care not to jack the low point too quickly or the the walls above or below will crack.  When you put a new piece of wood against an old piece you want to strengthen it not take the old piece out or bring the house down entirely.

So, there you have it.  Sistering is the solution.  Makes perfect sense to me.  This old house has been relying on it for a long time now and I'm grateful for the support.

              This post is dedicated to Beth Willson Bassett with love.  She knows why.