Roof Deformation

“One shall not use collar ties to try to hold exterior walls together.” P. DeBaggis, Easton, MA, Certified Building Code Official

This sums up the reason for a large majority of sagging roofs; collar ties serve to stabilize the connection between roof rafters and ridge beam, but do nothing to hold the exterior building walls together. It is the ceiling joists or rafter ties which primarily perform this function. This rigid triangle (two sloped rafters + bottom tension chord) press down on the wall plate, rather than pressing outwards. Without this or a structural ridge beam, the roof sags downward, and this then causes the rafters to kick the wall out. Any wall out of plumb by ¼ of an inch should be checked out. 

Visually it is easy to observe this phenomenon. This picture above is obviously an extreme case, but represents what eventually will happen if the problem is not addressed. Typically the downward loads on the roof are what drive this, and these loads are due to the dead weight of increased roofing layers, wind loads, and often snow loads. Poor construction could also play a role, such as an excessive birdsmouth cut on the rafter resting at the eave, or the classic move of the homeowner cutting the rafter ties so that they could “raise the ceiling” of the uppermost floor. Pest infestation, rot, and simply age are also contributory factors with roof sagging.  

BLSH&E will inspect the structural roof members to determine and validate the prospect that the ridge is sagging and/or the walls are bowing out. I will take measurements/photos to include in a written report with observations, conclusions, recommendations, and budgeted cost to cure. Call today for a free consultation. As with all of my engineering services, you will be presented with a detailed written proposal of the Scope of Work, including fee, prior to commencing.