Strange crack pattern

[Bob Hunt]

During an inspection this morning, I came across this pattern of cracks in some old shingles on the roof. The roof is decked with roof boards and not overlaid with OSB or plywood. There is only one layer of shingles, and, yes the other side of the roof was reshingled for the sale but they did not do the back side where this problem occurs. I did not see any obvious large out of plumb, square, or level issues either.

There were at least three areas with patterns similar to this, along with the usual random cracks, curling, and other deterioraton.

I don't recall seeing anything like this in the 10 years of performing inspections. Anybody else seen this before?

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[Ted Menelly]

During an inspection this morning, I came across this pattern of cracks in some old shingles on the roof. The roof is decked with roof boards and not overlaid with OSB or plywood. There is only one layer of shingles, and, yes the other side of the roof was reshingled for the sale but they did not do the back side where this problem occurs. I did not see any obvious large out of plumb, square, or level issues either.

There were at least three areas with patterns similar to this, along with the usual random cracks, curling, and other deterioraton.

I don't recall seeing anything like this in the 10 years of performing inspections. Anybody else seen this before?

!x deck boards, not plywood/OSB. I would say the roof shifted substantially and being deck baords/1x for sheathing it was easier for the roof to shift and hence the angle crack across the roof. The inside signs or exterior side walls must have been done over so they do not show but there has been a very obvious shift to the home.

With a crack like that across a roof that is far more than you need for a reason for referrral where the contractor/engineer is going to measure until they find the concern.

I saw a crack like that on a home being demolioshed!!!!!!!!!!!!!

[Jerry Peck]

If there was no addition connection point there (and you indicated no), then with roof sheathing done with 1x6 boards (or larger boards) there will be a lot of room for movement (as Ted said).

Suppose that the end of the house to the left (bottom left in the photo) settled (foundation settlement), that would try to bring the roof with it and having used board sheathing instead of structural panel sheathing there would be a lot more movement.

Now think about the underlayment ... if those 210# singles are torn like that, that 15# or 30# underlayment is likely also torn similarly.

Definitely go with a structural engineer.

[Kevin Barre]

Any chance that the home was remodeled and that is the outline of a former hip?

[Bob Hunt]

No remodeling of any kind. House was built in mid 1950's. Small, two bedroom, one bath.

Also no indication of roof leaks through potentially torn felt. That's what is really odd. From the underneath, the roof boards are practically pristine.

Ceilings and walls are plaster board covered with plaster. All ceilings and walls are original to the house except for paint. Nothing odd, just normal minor cracks around a few doorways and window openings.

Concrete block foundation wall shows only minor (less than 1/16") cracks. Poured concrete floor in basement shows only a few minor cracks also.

I don't see any indication of a major movement in the structure.

[Ted Menelly]

Now take all those minor cracks, ceilings, over doors and windows, foundation, concrete basement floor. The total sum of movement is really quite a bit. Minor movement signs often add up to major movement. Also the home could have moved a lot at one time and settled back again. Something moved enough somewhere to crack that roof like that. You found it , turn it over to the next man and hopefully he will find the dirty little culpert.

[Frank Suchodolski]

Shingles are not supposed to be installed on deck boards, there is too much movement and as the shingles adhere to each other eventually they break apart and they have cracked along the 45 degree offset line, as the shingles do not adhere to the underlayment the underlayment is still intact. My educated guess is that they need a new roof, installing OSB or plywood sheeting prior to stiffen up the roof assembly, roof shingles are made to adhere excessively together so they don`t blow off.

Frank Suchodolski, High Profile Roof Care Inc.

[Jerry Peck]

Shingles are not supposed to be installed on deck boards, there is too much movement and as the shingles adhere to each other eventually they break apart and they have cracked along the 45 degree offset line, as the shingles do not adhere to the underlayment the underlayment is still intact.

From the GAF installation instructions: (underlining is mine)

GENERAL INSTRUCTIONS

• ROOF DECKS:

For use on new or reroofing work over well-seasoned, supported wood deck, tightly-constructed with maximum 6” (152mm) wide lumber, having adequate nail-holding capacity and smooth surface. Plywood decking as recommended by The Engineered Wood Assn. is acceptable. Plywood decks for Class A installations must be 3/8" (10mm) thick or greater with underlayments as noted below. Shingles must not be fastened directly to insulation or insulated deck unless authorized in writing by GAF Materials Corporation. Roof decks and existing surfacing material must be dry prior to application of shingles.

That states that board decking, 1x6 max., is recommended, but that "plywood ... is acceptable."

From Owens-Corning installation instructions. (underlining is mine)

- (A) Roof Deck:

Recommended roof decks are 6" maximum width, 25/32" minimum thickness wood sheathing, or 3/8" minimum thickness plywood sheathing. Use plywood decking recommended by the American Plywood Association, Underwriters Laboratories, Inc., or local building codes.

- - These Owens Corning shingles have been tested and rated as Class A by Underwriters Laboratories when these shingles are applied over recommended decks. If other decks are used, the resulting construction may not qualify as Class A.

That states that either board sheathing, 1x6 max., OR plywood is recommended, either / or equally with no emphasis on either.

[Jerry Peck]

I know that the shingles are not adhered to the underlayment.

The underlayment is NAILED down.

The shingles are NAILED down.

The underlayment is now NAILED down with more nails than the shingles.

Movement which tears the shingles surely should tear the underlayment, which is weaker than than shingles.

[Frank Suchodolski]

The only shingle recommended for application on deck boards according to the RCABC (Roofing Contractors Association of British Columbia) is Inter-locking shingles (T-locks) as they do not adhere to each other and only installed with 2 nails per shingle as they allow for some movement (4 once next course is installed). Laminates and 3-tab shingles adhere to each other and become one big sheet, very evident once you have torn one of these roofs off, and installed with double the amount of nails, not allowing for movement. It's not a good idea to install laminates or 3-tab shingles on deck boards no mater where you live! If there is no other indicators in foundation or floors or walls, then re-roofing, with plywood or OSB sheeting installed would be my advice.

[Jerry Peck]

It's not a good idea to install laminates or 3-tab shingles on deck boards no mater where you live!

Guess you need to let all of the major manufacturers of shingles in on that.

[Frank Bombardiere]

Most roofs here that were built before the mid 80s' have deck boards. I do see more buckling etc with deck boards but not a major problem.

From the looks of that pic, I agree with others about structural issues. If there were not many signs inside, it could be that the roof structure itself is shifting and not related to the foundation or structural walls. Refer an SE.

[Frank Suchodolski]

Manufacturers application don't always reflect the best practices, if one manufacturer says its OK then most will follow along not to lose the business. Its all numbers to them, not best methods. Plus they only warrant the material, if your roof starts to leak because of building movement buckling your shingles... that will not be warranted. So if your going to install shingles on deck boards, do yourself a favor, install plywood or non veneer Sheeting.

[Fred Weck]

Just a thought, you said the other side of the roof was reshingled, could have been when they were roofing a hose or extension cord was pulled and got caught under the edge of the shingles causing the damage your seeing. To repair it they resealed them down with roofing cement.