This is a test

A little brain teaser for y’all on the subject of attics/roofs. Here are the givens. Read closely.

An 8 year old home.

Roof covering in good condition, except for a few nail pops.

All flashing in good condition and properly installed.

Drip edge with kickout installed properly on all rakes and eaves.

Attic construction is an engineered truss, no deflection worth mentioning, with no cut members.

Sheathing is exposure 1 plywood.

Heavy felt underlayment.

More than adequate ventilation with soffit and gable vents, plus turbine assist.

No exhaust fans from baths, etc vented into attic.

No moisture problem in attic.

The roof has never leaked.

Despite all this, the roof has “waves”, or humps and bumps.

What’s the problem? Have fun!!

what is the span on the trusses and do you have H clips?

Stupid roofers?

That’s all I got.

I can think of a few reasons:

  1. Shingle ridging happens when plywood expands into the gap and the shingles attached over the gap are forced to ridge up. Generally appears as vertical ridges over the ends of staggered sheathing. Can happen with OSB too.
  2. Expansion of the roofing felt, especially heavy felts can make ridges under composition shingles in a random manner. You’ve seen felt wrinkling I’m sure, it happens when the felt gains moisture.
  3. Your description seems to indicate random humps and bumps so I’d choose #2 before #1 but would not rule out warping and twisting of the plywood itself as a possible cause. Can happen as wood gains moisture even if not directly wetted.
  4. Might it be typical panel buckling either along or across the panel’s strength axis? Sometimes looks “random.”
  5. Are the trusses aligned? Generally out of alignment trusses would not give “random” humps and bumps though.

Finally, to figure this out with some certainty you’d probably have to remove roofing and felt to see what’s going on.

What he said.


Is there an answer to your question, or are we just hypothisizing here for fun.
I would guess that the house decking may have started warping due to above average exposure during construction, unless you have a truss design or spacing or other installation flaw visible.
In our area, typically they (Bldr’s.) have been using a rated OSB type product, for the past 10-15 yrs. That being said, if the OSB or Plywood sheathing isn’t T&G, glued and nailed/screwed, using H-clips and covered before excessive exposure, I’d think you would develop this type of problem(s) over a few short years.

I’ll be looking for the answer to your riddle!

I have been in the construction industry for over ten years now, and with out seeing what you are talking about I cant be sure but from what you said this is what I have seen happen in the past. You said “1 plywood” and i assume you mean one layer of plywood and with that said, People in the past have been using 1/2" plywood on truss roofs witch is fine in the form of “my local codes” but not how I build. The problem with 1/2" plywood we have seen on remodels we do is that the 2’ span on a typical truss roof is to much over time with the heat of the roof and the span for the plywood, with the weight of the shingles the plywood sags ( have seen this even with plywood clips in place. So to defeat this purpose I have been installing 5/8" plywood with plywood clips in between trusses and the last roof we did hasnt to this day had any visual signs if this problem. Its been 8 years. With that said I agree with some of the others that wrote before me they seem to have the same ideas as this i just wanted to give my 2 cents. (also i am guessing that the bumps are located over the truss areas so they are probably similar distance apart)

There is also something to be done about this without reframing the roof… Even though the problem is there it dosent mean is strucually unstable. Mostly just cosmetic, I would 1. strip roof make sure it is nailed properly and then 2. Install a achitect type shinge and with these the pattern is random and the is shingle thicker and you are less apt to notice the humps and bumps so bad. Like I said its just a cosmetic fix, I have used these on some 100 year old barns around here and they had the old barn board roofs and the made that old roof look like a miliion bucks.
Hope thats helpfull and like I said without seeing it that is my guess of what is happining,

Good luck
Mike Martell
Martell Home Inspections

Hey guys, just got back on. Let me clarify. And yes, there is an answer to the question.
First of all, yes, the h-clips are there.

The plywood is ***esposure ***1

Sorry, asphalt shingles are on the roof. I should have mentioned that.

No wrinkling, it’s not an underlayment problem. 30 lb. felt

There is no chimney. No step flashing needed.

Will–We don’t do ice shield in Mis’sip’i!!:wink: Soffit vents are continuous, but I know what you mean about the turbines. Baths back to back, vented through roof.

The trusses are aligned. No deflection worth mentioning.

A couple of you are really close. I’ll add a hint. Salvage 2x4 blocking was installed as extra nailers for the sheathing.

sheathing layed down in the wrong direction or short nails

Good try Ken. Not quite.

Well, Jimmy I am running out of possibilities myself.

Wild guess, if it is not the 30# felt exposed to long, it must be that they used a CDX plywood.

Panels intended to exposure to the weather. D grade veneer may not be used in Exterior panels.
Contrary to a common misconception, CDX is not an exterior panel.
Exterior plywood is not necessarily the best choice where plywood will be constantly exposed to the weather.

Exposure 1

The glue is the same as that used for Exterior plywood, but other characteristics that affect bonding are not. These panels are for use in high moisture condition or where, during construction, long delays may be expected before the panel is protected from the elements.

Plywood de-lamination.

I can guess too to these trick questions, ha. ha. Pictures would help you know.

Marcel :slight_smile: :stuck_out_tongue:


Ok, guys, I’ll end the agony. Thanks for the input. The water stains on the sheathing, without any stain patterns at all on the trusses gave it away, not to mention the extra 2x4 nailers for the sheathing. 1st thing I noticed was that the 2x4s were salvage—not really a problem there at all. I determined that the sheathing had been pretty much saturated prior to installation, and the salvaged 2x4s raised suspicions that the sheathing was salvage also. The homeowner eventually admitted as much to his agent. Not too much of a stink, they worked it out, though the listing agent was a bit miffed (go figure, huh?). The builder had nailed the heck out of it, I guess thinking (or hoping) that the extra fasteners and nailers would hold it all together. Amazingly, it has actually worked over 90% of the surface.

Jimmy, you stated that the roof never leaked, and you forgot to mention the water stains, but we tried, it made us think so no harm done and it was fun. Thanks, ken