What would cause this?

This is a 14 year old home with concrete tile roof.

What would be the most likely cause for the very uniform ridges that run perpendicular to the roof’s ridge line all the way down to the eave.

The roof is 2x10 wood rafters with collar ties under waferboard sheathing (no trusses).

Could the ridges be from the edges of the sheathing being compressed and pushed up?


what is the rafter spacing?

I’m guessing water damage as an active factor, or an existing problem like sheathing too thin for wide rafter spacing. All should almost be visible in the attic, I would hope. :smiley:


Approximately 24" o.c.

Here are some pictures from inside…





Just guessing, you have sheathing sagging. Because of the weight(tile), the span 24", and because the ridges(where the rafters are) seem to line up with where the rafters are from your inside pictures.

I understand this would be defer-ed to a roofer, but I wonder why? Is it as simple as min code material was used? OR Is there a ventilation/moisture problem? Guess that would be for the roofer to check. :slight_smile:


Rafters placed too high on the ridge. Or birds mouth ,resting on the top plate,. not cut deep enough. Orrr rafters weren’t properly crowned.Orrrr a combination of all.

Rafters on 24" centers with 1/2" OSB is very common under concrete tile. In California collar ties are usually installed only on every other rafter pair and that condition might mean that rafter pairs without collar ties could sag, but the ridges look uniform up and down the roof… they dont sag more in the middle of the span, in fact they seem to bow up more in the middle of the span. That would indicate rafters with extreme crowns.

With adjacent rafters crowned opposite or moisture-damaged sheathing sagging or bowing between rafters, the roof should look sort of rolling, wavy, rather than sharp, short ridges like that. No sign of moisture on the underside of the seathing? No bowing/sagging of sheathing between rafters visible from underneath?

Did the whole roof look like this or just this section?

How can that line be so straight if the tiles are staggered so the joints don’t line up?

Weird, but

Is a good guess!



Just this section. The opposite side of the house is cathedral roof over the living/dining room, and it did not have ridges like this.

All I can think of is:

  1. A few rafters cut too long (except it looks worse in the middle)
  2. someone did something dumb like roof over some 1x2.

Wondering the same things about the ridges being so straight when tiles are staggered. Any broken tiles?

Not on this side. Only some over the front entry porch.

Jeff did you walk on the roof and look under a few tiles?

I damn sure would have.

“Improper ridges under concrete roofing tiles appear to have been created by aliens. I recommend lining your hats with tin foil”

Nope… This was two stories up and concrete tile. Both good reasons for me not to walk it. I’ve deferred some broken tiles on the other side of the roof to a licensed roofer and made a note that these ridges should be further investigated.

LOL :smiley:

Ahhhhh I see…:smiley: …I would have, like a ring tailed lemur!..:smiley:

Were the tile joints staggered???

Yes they were staggered. Here is a picture from the other side.

While the ridges in the original picture look relatively smooth, keep in mind that I am standing on a hill across the street from this house…probably about 60 - 70 yards away from the ridges, so the picture might make them look smoother than they really are.



From your picture the back side exposure appears excessive for tile and looks nothing like the tile on the opposing plane.

You were there and I wasn’t but what I see is metal impregnated shingles on the deflected back side which will warp to the underlayment pattern(s).

I’d expect to see numerous loose tile ends with this apparent deflection, and they aren’t showing up in your photo.

I’m leaning towards a crowning problem.As in those two rafters should have been culled from the rest and never used because the crown was so great.

You could find out by going in the attic and pulling a string tight along the bottom of the rafters end to end and compare the gap between string and rafter.Then compare to rafters that seem “normal”.


Meatal impregnated?

Yep, it sure looks like that shingle material is conforming closely to whatever’s causing the ridge. Are you sure that part was concrete tile?

My thoughts too…I’ve seen some pretty wild crowns over the years that needed to be culled.

Good call Cheremie. :wink: