Roof to Wall Structural Connections
Loads and Load Paths:
Wind resistant homes are not only built to hold up their roofs but to also hold the roofs down. The weight of the roof is typically about 7 to 10 pounds per square foot (psf); but, the uplift pressures averaged over the roof in a strong hurricane can be four to five times that amount. On some areas, the uplift loads can be 10 to 15 times higher than the weight (70 to 130 psf). The uplift at the wall connections are magnified by the size of the roof because the pressures act on every square foot of the roof. For example, if the average uplift pressure on the roof is 50 psf, the weight of the roof is 10 psf and the roof is 30 feet wide, the uplift at the wall would be 600 pounds per foot of roof length [(50 psf – 10 psf) times one foot wide, times 15 feet (half the roof width)]. If the trusses or rafters are spaced every two feet, each connection of the rafter or truss to the wall would have to be able to hold down with a force of 1200 pounds.
A way to visualize what is needed is to think in terms of how you would connect the roof if you intended to turn the house upside down and shake it up and down. Hurricane straps are used to anchor the roof trusses or rafters to the tops of the walls. Recent storms have shown that roofs anchored using modern wind resistant connections have not failed. However, it is not enough to just connect the roof to the tops of the walls. The uplift loads have to be carried far enough down into the house so that the weight of the house including the floors becomes greater than the uplift forces caused by the wind. This is called developing a continuous load path.
But, wind doesn’t just apply uplift forces to the roof that try to rip it off the top of the house, the wind also creates pressures that push and pull on all exposed surfaces of the house. It tries to lift it up, tip it over, slide it sideways and suck the side walls away from the connection to the windward and leeward walls. Consequently, the walls and roof have to be tied together like a well built box and anchored to the ground with enough weight to keep it in place. In addition to these “external” wind forces, if windows, doors or garage doors fail on the side facing the wind, and allow wind pressures to build up inside the house, these pressures will try to push up on the roof and push out the walls. This is another major reason it is important to protect openings. There already pressures being applied to the outside of the house, trying to tear it apart, the added wind pressures from the inside can exceed the capacity of the connectors to hold the building together.
Sorting Through the Options – Sources of Design Help:
Each home has its own features and vulnerabilities. This makes it difficult to prescribe a one-size-fits-all set of recommendations for structural retrofits. Consequently, this discussion of structural retrofits describes typical situations and outlines the types of retrofits that will raise the wind resistance of the home and bring it closer to the kinds of wind resistant construction features prescribed in modern building codes and standards. However, we have also tried to focus on what may be practical and beneficial, realizing that bringing the structure all the way up to the latest high wind requirements could entail almost rebuilding the house, which (unless you are rebuilding after a major event and the house is essentially gutted) is neither cost effective nor practical. With structural retrofits it is smart to work from the top of the walls downward as each retrofit will engage more and more of the weight of the house. The parts of a house do have a way of working together to help each other out when they are properly connected. By starting at the top, each retrofit you accomplish will increase the ability of the home to survive a hurricane.
Usually the easiest and least intrusive structural retrofit is strengthening a gable end wall. If the house has a gable end, look at some of the options available below for retrofitting this condition.
The rest of the structural retrofits are organized according to the type of walls you have in the house. Wood frame walls (regardless of the type of cladding from vinyl to brick) present several possible options for retrofitting. While masonry walls (concrete block) require different retrofit options.