An Interesting Case of Roof Spread

Roof SpreadI was carrying out a dilapidation survey in the West Midlands quite recently and whilst checking the roof space came across an unusual case of roof spread. I’m sure you’re familiar with the basic cause of roof spread but if you aren’t then I’ve included a basic diagram that shows how unrestrained rafters can apply horizontal thrust to the walls and make them unstable. Generally this is due to inadequate collar ties or ceiling joists or indeed failure of those timbers.  In this particular case there were no external signs of cracking or structural problems and neither was I particularly looking for, or asked to investigate structural cracking in the building. However, on inspecting the first floor store room I noticed substantial cracking to the dividing party wall that warranted more detailed investigation.

There was a long crack to the internal corner of the ceiling to wall junction on the front elevation of the property and a further diagonal crack cutting down through the dividing party wall.

SRCrackThe damage was indicative of roof spread but was not quite serious enough to manifest any obvious visual problems to the front elevation of the property.

The roof space to this particular property was not fully accessible or safe to access so I had to inspect as best as I could from the loft hatch and after a couple of minutes I looked directly above my head and immediately noticed the problem. The ridge board was completely split which had resulted in a section of the rafters dropping just enough to apply horizontal thrust to the wall plate on the front elevation. If looking for the obvious and common cause of roof spread then this particular issue could so easily have been missed, especially  when limited to a head and shoulders inspection from the loft hatch.


Split Ridge
Split ridge board causing rafter instability and subsequent applied horizontal thrust to the wall plates.



5 responses to “An Interesting Case of Roof Spread”

  1. Neil Hewitt avatar
    Neil Hewitt

    The ridge board looks like it had been joined at this section, not a good idea, especially as there probably was a suitable length of timber available. This survey does show the need though for a proper loft inspection. If this were a building survey, I would have been up there, boiler suit on, and torches at the ready. I had a property yesterday, where I could only see a hole in the roof, when I was next to the chimney, and nothing visible from the outside. Though a damp patch in a bedroom below.

  2. Joe Malone avatar
    Joe Malone

    I too would normally have been up there with disposable coveralls but as I said in the blog, the roof space was not accessible.

    Joe Malone

  3. Dermot Russell avatar
    Dermot Russell

    I think, possibly, the situation described, i.e. split and dropped ridge board and rafter would have the effect of lifting the wall plate and hence the top of the wall, causing the cracking, rather than an outward trust, (I think the photo is consistent with my analysis) wouldn’t the tie still be effective in the circumstances?


  4. Mike Kirkham BSc CEng MIStructE avatar
    Mike Kirkham BSc CEng MIStructE

    This is a typical scarf splice joint used in purlins > it is capable of transferring vertical loads ( ie shear ) but not bending moment. Its a perfectly adequate detail for a VERTICAL purlin provided a vertical prop is nearby so that the bottom section cantilevers over slightly and supports the top section ( in other words the cantilever moment occurs at the purlin support and reduces to zero at the joint )
    Problems occur in purlins where there is a big gap between the support and the scarf joint . The purlin cant cantilever over without deflecting excessively and the rafters drop and roof spread occurs.
    There are usually no props to a ridge so a scarf joint is not a good idea and what we see in the photo is due to there being zero resistance to the banding forces that are acting . Unfortunately in most cases with cracking there is more than one thing going on and i suspect thats the case hear . If no tie is present then the rafters have to deal with the induced bending moments themeselves and any projection of the rafter below a collar or a tie . The rafters in the photo are way undersized to deal with these cantilever moments and this maty be the predominant problem as near the ridge the rafters tend to act together as an inverted V beam which would tend to reduce bending moments in the ridge beam.
    Im writing a report at the moment where factory trusses were used everwhere except near the corners where non designed hip rakers were added on site resulting in the corners being pushed out.
    In my opinion roofs are complex structures and should always be designed by an experienced engineer
    as ive lost count of the number of buildings that have been severely damaged by an undesigned roof built by a carpenter using traditional detials that didnt work in the first place in places that they were never supposed to be.

    1. Joe Malone avatar
      Joe Malone

      Great comments Mark. Thanks very much for posting.


      Joe Malone

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