There are a four basic physical forces in the universe which govern all things great and small. Two of these forces I would believe are familiar to all humans, electromagnetic and gravity. We are all intimately acquainted with gravity. It is the force that adheres us to the earth and causes us to hit the floor when we roll past the edge of the bed. Surprisingly it is the weakest of the four basic universal forces.
Gravity acts on everything with mass and it theorized it can even act on light. What this very simply means is the more the mass of an object, the more it weighs and the greater the force it exerts. Again something we are all familiar with. A baby weighs much less than an elephant. Scientifically speaking the elephant has a greater mass than the tiny baby. The basics of gravity and mass (weight) were glaring apparent to me during a recent home inspection. And just as apparent was someone’s seeming lack of understanding.
Entering the attic of a ranch style house, the first thing that grabbed my attention upon popping myself through the scuttle was that a section of the ceiling insulation had been pushed aside and never replaced. The reason for moving the insulation was to facilitate the installation of several boards fastened to the ceiling joists and the above rafters. The boards were installed with the intent of providing structural support for the ceiling structure. Normally the support of the joists is accomplished by the supporting wall below. This wall is installed in line with the main support beam found in the basement.
Going back into the house to look at this section of ceiling, I found the support wall had been removed to enlarge the bedroom. The new boards I saw above are intended to replace the missing wall. The gravity of the situation was now fully apparent, at least to me.
The ceiling above where the wall had once been was cracking, with past repairs in evidence. There are two reasons the cracking is occurring due to gravity and the force it exerts on the house, specifically the roof.
Here in Connecticut we get snow. Sometimes a lot of it. Snow has mass. When snow accumulates on a roof, it exerts force (good old gravity at work). The more snow, the greater the weight. The weight acts on the roof in two ways as shown in the diagram. The bottom board in the diagram is typically the ceiling joist, but can be a rafter tie in some instances. The rafter tie prevents the rafters from spreading apart and transferring the pushing force to the exterior walls.
So what happens when we tie the rafters directly to the ceiling joists, remove the structural support from below and add weight? The ceiling joists (rafter ties) flex and move with the force of gravity, cracking the ceiling and acting on the framing in the ceiling and to some extent the exterior walls.
The repair is to install a beam at the ceiling supported by strategically placed posts. In addition the ceiling joists should be examined for stress and repaired as necessary.
Gravity is ubiquitous, knowledge is founded in knowing one’s ignorance.