What are collar ties and what do they do? Collar ties are wood frame members that are installed across the rafters. They are connected from one side to the other side of the opposing rafter. Collar ties add stiffness to the rafter and roof structure. They help in keeping the roof from sagging and also prevent rafter spread. They are simple members but rather important for the integrity of a roof structure. Ideally there should be one collar tie for each opposing rafter.
During a New Jersey home inspection I viewed a deep sag in a house roof. There was only one layer of roof covering installed and there was plywood decking. There were however no collar ties installed. You could see the bow from the interior of the attic which is very rare. Usually roof sagging is more easily viewed from the exterior of the home. I felt the reason for the defects was the lack of collar ties and roof rafters that were too light. I am not an engineer so I directed my home inspection client to have the condition evaluated by a structural specialist to determine what the proper corrective actions would be to correct the issue.
Some defects that I find with the collar ties themselves are there are two few installed. Usually I find them installed on every third rafter as opposed to each rafter set. This is typical of construction in older homes. This is easy to fix because additional collar ties can be easily added. Another issue is that the wrong framing is sometimes used for the collar ties. Any wood member that is lighter then a 2×4 is inadequate for a good collar tie. If collar ties are longer then eight feet long they are susceptible to buckling. These long collar ties need to be reinforced. Collar ties should be installed in the middle of the rafter but often they are not because the proper installation requires the reduction of head room in the attic space. Collar ties that are installed too high will not be effective in adding stiffness to the rafter.
LookSmart (John Martino) is a member of the American Society of Home Inspectors New Jersey.