Gable End Walls
If the house has a gable end wall that is not properly braced and anchored, it can collapse causing a catastrophic amount of damage to the home. The good news is that of all the possible structural retrofits for a house, gable end walls are often the easiest to strengthen and deserve to be a high priority on the retrofit list.
There are three things to be concerned about with gable end walls. First, the most common type of failure is loss of roof sheathing from the gable end that results in the gable wall loosing its bracing along the top edge. The second most common failure is the connection of the gable wall to the wall below. The third potential weak link is the actual framing members that make up the gable end wall structure. In many houses, these members are the structural members of the last roof truss. Consequently, they are 2x4 lumber members that are being bent by the wind pressures applied to the wide side of the 2x4s – the direction in which they are the weakest. In homes with rafters and ceiling joists, the wall structure will often be made of 2x4s turned so that wind forces are applied to the narrow face of the 2x4s (the orientation with the strongest resistance to bending of the 2x4s) but they may only be just toe-nailed to the rafters and ceiling joist and not be sufficiently fastened to resist the wind loads experienced in a hurricane.
The most common gable end failure is one where the wall loses support along its top edge.
The second most common type of gable end failure is at the connection between the rectangular and triangular walls. – It is rare that you actually find one that looks like this because usually the wall below fails and the whole end collapses.
This is the more common look of a gable end failure when the triangular wall and rectangular wall separate.
The taller the gable end triangle, the greater the risk of failure and damage. For gable ends that are shorter than 4-feet, the forces applied by a 140 mph gust along the top and bottom edges of the gable end wall will be less than 100 pounds per foot of gable width and most nailed connections can handle these forces. In addition, if the gable end is less than about 4-feet tall, it will be difficult access for retrofit work.
Gable end walls on rooms with vaulted or cathedral ceilings, while common, do pose special problems for retrofitting. Unless special care was taken in the design and construction of these walls to provide the kind of bracing they need to stand up to strong winds, they are very likely to fail. The structural solutions usually involve beams that either span across the width of the wall or columns that span from floor to ceiling. In many cases, when the wall was originally constructed, the builder could have used continuous members that run from the floor to the ceiling and avoided the weakness.
Gable end wall with cathedral ceiling – note the horizontal joint running across the wall just below the semicircular window.
The red lines indicate where continuous wall framing members could have been used by the builder to avoid the weak connection between the rectangular wall at the bottom and the triangular wall at the top.
For gable end walls with a flat ceiling behind it (not a cathedral ceiling or vaulted ceiling), a new series of retrofits have been developed to strengthen the gable end wall and bring the bracing up close to modern code requirements.