Sagging collapsing roof, Amenia NY (C) Daniel FriedmanSagging Rafters, Leaning Walls, Collapsing Roofs

  • FORCES on ROOFS CAUSE MOVEMENT - CONTENTS: what are the forces that cause sagging rafters, collapsing roofs, leaning walls? How does a missing collar tie or rafter tie contribute to roof damage, sagging, or collapse? What are the sequences of movement in a collapsing or sagging roof?
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Roof Sag or Roof Collapse Forces:

How defects in roof framing or damage to roof connections or structural members lead to building damage & collapse. This article describes & explains the directions of forces in roof structures, illustrating downwards force, horizontal thrust, and the causes of rafter sags, rafters that disconnect from the ridge or top plate, and why a sagging roof pushes the building walls outwards at the wall top. We include photographs of collapsing wood-framed roofs and photo sequences showing the stages in a roof collapse as well as the direction of forces that bend and finally destroy a building.

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Forces & Direction of Movement in Sagging & Collapsing Roofs

Increased Horizontal Thrust on Low-Slope Roofs

Roof sags and walls are pushed outwards if the roof is not properly tied or supported (C) Daniel Friedman

This article series describes and illustrates the different types of support that prevents roof sagging and wall bulging at buildings, including definitions of collar ties, rafter ties, and structural ridge beams. Without the proper support of rafter ties or a structural ridge, a typical gable or sloped roof will sag downwards while pushing the building walls outwards towards a catastrophe. We include sketches of collar ties, rafter ties, and structural ridge beams as well as illustrations of collapsing and collapsed structures where these roof rafter ties were lost or omitted.

Watch out: also for increased horizontal thrust loads on low slope roofs. More examples of the direction of forces that push down on roofs and out on walls for low slope roofs are found in this article series at COMPRESSION BRACING for RAFTERS (Canada).

This horizontal load - or thrust - can be considerable, especially on a low-pitched roof. To resist thrust, the IRC calls for a structural ridge (required for any roof with a roof pitch less than 3/12) or for each pair of rafters to be securely connected to each other [at the lower end of the rafter] by a continuous joist. (R802.3, 2006 IRC). Code does allow joists [serving as rafter ties] to be installed above the top plate, but only under certain conditions.

Previous building codes permitted rafter ties to be placed as high above the plate as two-thirds the distance between the top plate and the ridge, but the 2006 IRC now limits this height to one-third the distance between the plate and the ridge (see footnote A, Table R802.5.1, 2006 IRC). (Truesdell 2008)

Collapsed low slope roof (C) Daniel Friedman

At ROOF SLOPE DEFINITIONS we comment that this photograph illustrates a roof whose slope has become irrelevant after the building collapsed. I suspect the very low-slope roof in the photo was a bit steeper before the building fell in, and that the connections of its rafter ties to the rafter ends were inadequate or failed from rot or insect damage to the structure.

Let's look a bit more at the collapsing roof and walls of the Amenia New York Barn. The downwards load on the roof structure is combined of dead loads: the weight of framing, roof covering, and other things that might be mounted onto a roof surface, plus the live loads of snow, wind, water, and an occasional worker or animal clambering around on the structure. These forces press downwards or in the case of strong winds, the force may combine both horizontal and downwards forces on the roof surface. How these forces are carried down to the foundation and ultimately to earth determines what happens to the roof and to the rest of the building structure. Even without doing the engineering or the math, we can see in photos below what happens to a building wall and roof when its support is incomplete, lost, damaged or missing.

Collapsing barn roof, Amenia New York (C) DanieL Friedman

Let's press our imaginary thumb down on the roof of an old barn whose rafter ties have been cut out and carried off by someone who's decorating their Los Angeles apartment. As the ridge sags down and the centers of the front and rear walls push outwards at the wall top, the photo above provides a graphic illustration of direction of the forces at work.

  1. The gable ends of the structure want to hold the two ends of the ridge up. That's why most of the ridge sagging occurs in the center of the roof (page top photo).
  2. As the wall tops are pushed outwards at their center, either the entire wall leans over or, as the wall is held up at the building corners (gable ends), the wall center may break and bend over as in my photo above.
  3. Ultimately as the roof collapses downwards starting at the center of the ridge line, the tops of the gable end walls are pushed inwards (photo just above).

Just below we trace another building through sagging roof to ultimate and total collapse.

Stages in the Collapse of a Roof Without Rafter Ties

Roof sag and wall bulge outwards at a collapsing building in Dutchess County, NY (C) Daniel Friedman

[Click to enlarge any image]

What happens if rafter ties are omitted and the builder simply frames rafters butting against a ridge board with or without collar ties but with no rafter ties nor ceiling joists? Unless the building included a structural ridge beam the downwards loads on the roof rafters will cause the building walls to bulge outwards, most noticeably at the center of the walls, and the roof rafters will sag.

Our photo above shows a collapsing building is part of a study that I [DF] watched for years as the structure slowly settled to earth. This modest farm house served as a home to farm workers probably from the 1930's until around 1960. I watched its condition from 1969 until its disappearance a few years ago.

As the roof ridge sagged downwards (red line and arrow), the centers of the front and rear walls of this little house bulged outwards, telegraphing that movement into the outwards lean of the front porch as the front walls of the home also pushed the porch roof outwards (blue line and arrow). Above in snow is the sagging roof and bulging wall story of this home in 2003. By 2007 the front porch had collapsed, the roof had sagged further and the front wall leaned outwards precariously. In fact it's remarkable how plastic building materials can be when bent over a long timer.

Collapsing building from no rafter ties (C) Daniel Friedman

By 2011 the roof and most of the structure had collapsed (below).

Collapsed wood framed roof and walls (C) Daniel Friedman

And the next year, in 2012 this little house finally bit the dust, or I could say snow-dust (below).

Collapsed house after sagged roof (C) Daniel Friedman

Today all traces of the building have been removed and the cows seem to have wandered off as well.

Article Series Contents


Continue reading at CATHEDRAL CEILING / ROOF SUPPORT REQUIREMENTS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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