You have probably seen this and it is most likely due to roof deflection, sag and creep.
This roof is composed of rafters which meet a center ridge beam. The center beam is probably not terribly thick. What you see is the result of weight and time and gravity.
Such a roof looks a little bit like the saddle on a horse.
Why the sag and creep?
The sag is what happens to the center ridge board as it spans from side to side. In this case the span is about 20'. The rafters are nailed to it, forming the framing from ridge to the top of the walls on the sides.
The creep is the permanent sag that happens over time. It will stay that way as it gets used to its position.
I say "gets used to" and that's just about it. Over time the wood fibers stretch and change and adjust to gravity's effect on the span. Slowly the wood "remembers" its new shape and sticks to it.
And the weight of what it's holding up - in this case the shingles, wood sheathing and wood rafters - works with gravity, and slowly pushes and pulls downward on the beam. And it slowly, over time, takes a new shape. In this case it is a sagging, saddle shape.
If you have a book case, or shelf in a kitchen cabinet, and put a load of heavy books or plates and such onto the shelf, the wood might sag. But once the weight is removed the sagging goes back up and the shelf becomes level again.
But, if you left that load on the shelf over time, years later you might remove it only to discover that the sagging, or creep, is permanent. Even lifting up on the shelf does not change it back. It has taken a new form.
In this garage we could not actually see what is happening.
We only can tell from the outside that it is.
These rafters have been completely covered with insulation as this garage is used as a work shop.
Can anything be done? Likely not. Will it get worse? Probably.
But this is a very slow process. Like all of us, sagging happens with age.
My recommendation: gravity works! Things happen over time. Sometimes such things happen and are normal, with the evidence obvious to the eye. Is this structure frightening? Not really. But don't add any more undue weight to the roof, like another layer of shingles. That will only aggravate gravity's efforts over time. Then the roof would look really creepy.
Jay Markanich Real Estate Inspections, LLC
Based in Bristow, serving all of Northern Virginia.
Office (703) 330-6388 Cell (703) 585-7560