Total = Sum of its Parts?

At a recent inspection I noticed that some places on the roof felt “spongy.”  Actually bouncy may be a better word for it.  The whole roof felt a bit soft but these spots were different.  On the rest of the roof I could tell that the soft spots were between rafters or trusses.  The cause of that could easily be thin plywood as roof deck.  The bouncy places were much wider. 
I made a mental note to have a good look in the attic.  I figured I was going to find a few collar ties missing or maybe a few cracked rafters.  A collar tie is the wood member, usually a 2x4, that ties together the rafters that are opposite one another.
If there is a lot of weight, like snow, on one side of the roof, the collar ties transfer some of that weight to the other side.  The one rafter cannot bend down without the opposite rafter bending up.  If a few collar ties were missing, the bouncy rafters would not have the support from the opposite rafters.

Anyways, when I got up into the attic, I found all the collar ties in place.   The next likely diagnosis was cracked rafters.  But… I couldn’t find any rafters that were cracked or damaged in other ways.  I was a little befuddled because from above I knew something was wrong.  As I looked around a bit closer, I noticed that in the area where the roof was overhanging a porch, the rafters were supported on a short wall that extends up from the exterior wall.  This wall was not built correctly.  The wall has to have either two top plates, or there has to be a stud below every rafter. Joints in the top plate have to fall on a stud. 
The wall in this house didn’t pass any of the requirements. This knee wall, as it is called had only one top plate and at one location the top plate joined between studs, in mid air.   To make matters worse, a rafter was sitting right there.  Right were there was no support.  That must be the problem.  That would  have satisfied me except for the fact that there was more than one bouncy spot.   Then I saw it.  The rafters were joined over the knee wall.  That is quite common.  What is not so common is the way they were joined.  Normally the joining rafters are sitting side by side but here the rafters were cut to sit on top of each other.  This was definitely not conventional framing but it would work.  However, what has happened here is that three rafters were joined backwards.  Instead of one rafter sitting on top of the other, it is actually hanging from it.   If you look at the joint pointed out by the green arrows you can see that the lower section is bearing on the wall and it is providing support for the upper section.  The joint with the yellow arrows is backwards.  The lower section of the rafter is just fine but the upper section is resting on the wall with a tiny tip.  Two of these poorly joined rafters were side by side and a third one was a bit further down.  Actually the third one was being "supported" by that non-existent stud in the picture above.  This inspection was not in my regular Kitchener Waterloo area but in an area that does not have as much snow, so not as much load on the roof.  If this home inspection had been in the Kitchener area  the roof may not have been able to hang on this long.  It surprised me that it made it as long as it did.
This example shows that a home inspector does not look at individual components but he has to look at the home as a complete system.  A symptom found at one place may indicate a problem elsewhere. 

Submitted by Bert, for Benchmark Home Inspection Services.  Your Kitchener, Waterloo, Guelph and Orangeville area home inspector.