Wind Turbulence due to roof design ? 1

[cubexco]

Hello,
I will appreciate some input on how to control wind turbulence causing uplift and ripping out shingles in a particular area of my roof.
In this section of the roof, the 14:12 pitch roof over the main house intersects the 9:12 roof over the garage.
Strong winds over 50-60 mph causes shingles in this area to start flapping and subsequently get ripped out.
This has happened 2-3 times in the past year. Always the same area with the 14:12 pitch roof. No damage in any other area of the roof.
The strong winds typically are from S, S-SW direction.
I have attached a jpg of the roof areas with this post. Not sure if I am doing it right.

Is there anything I can try to reduce the turbulence, or is adhesion of shingles (something like roofing tar between the overlapping portion of the shingles) an option I should consider?

Thanks,
Nik

 

[cubexco]

Trying to link images again

 

[Ron]

Your attachment didn't seem to work; however, this is not uncommon.

Shingles typically have sealing tabs that require heat to properly adhere. That heat usually comes from sun exposure, but if your roof is shaded or if the roofer did not install the shingles properly, the tabs will not adhere properly. You should not be able to easily lift the leading edge of a shingle that is properly adhered.

You can use supplementary asphaltic adhesive to help this along or you can try one of the commercial constuction adhesives. Heating the area above the tabs can be done with a small torch, but be careful as you can easily burn the shingle or start a fire in your roof sheathing..Not good!

 

[cubexco]

Thanks Ron for your insight.
I understand the sealing tabs require heat.
The area has full sun during the summer and the shingles were installed during the spring of 2007, so have gone through 2 summers, which should have been sufficient.
I had talked to the roofer about improper installation and I tend to agree with his opinion about "why only in this section of the roof?" He also mentioned that even while installing, he could feel stronger winds in that area.

I think this link does work for the attachment -

http://files.engineering.com/downlo...19b-9da6-0aea9eb3f24f&file=TopAndWestView.jpg

 

[Ron]

cubexco...I see the issue. This is a turbulent uplift area similar to the "lift" created over an airplane wing. We see this condition at low parapets where the wind blows over the parapet and because of the negative pressure that's created downstream of the parapet, the uplift occurs. In your case the ridge of your roof is acting as a parapet with the intersecting gable section downstream not allowing a downward slipstream for the wind.

There are several things you can do. The first would be a better adhesive on the shingles, adhering all the way to the leading edge of the shingles so that wind can't get under the edge.

Secondly, you could provide a diverter on the windward side that would change the wind path. This could, however, just move the problem up a little higher on the roof surface. Putting an "off-ridge" vent on the windward side would probable solve the problem since it would significantly change the wind flow. Use a 36-inch wide vent, made of galvanized steel. Be sure to use a rain diverter inside the vent so that you don't get wind-blown rain in it.

Lastly, you could fasten the shingles down on the tabs with exposed, gasketed fasteners, installed through asphaltic adhesive. Since your slope is so high, the exposed fasteners, properly sealed, will hold the shingles down and likely not leak. A little risky, but workable.

 

[cubexco]

Once again Ron, Thanks!
The wierd thing is, the shingles being ripped are only on the steep roof to the West and below the ridge line of the intersecting gable downstream.
None of the shingles along the West edge of the main roof were flapping or ripped, just near the valley line.

I wonder if it is only the turbulence caused by downstream slipstream of the main roof ridge, or is it a combination of above, along with turbulence caused by the 1' overhang along the main roof gable, as there is no damage or flapping on the East side of the intersecting gable.

I will try the adhesion first. Gasketed fasteners through the asphaltic adhesive, with caulk on the fastener head might not be too risky.

If it is a combination of two turbulence uplifts, would some soffit vents along the West overhang of the main roof work?
or perhaps a divertor along the same edge?

I originally thought the uplift caused the the ridge would make the vent on the main roof quite effective. Didn't realise the uplift would be so much that the shingles wouldn't handle it!

Nik

 

[Ron]

Nik...there's a good chance that the confluence of the turbulence occurs only at that one area. It is likely a combination of the ridge turbulence, some edge turbulence,and the catalyst is likely the lower ridge/valley intersection. It would be extremely interesting to put a smoke stream in there when the wind blows to see it. If you can take a look during a rainstorm from across the street you can probably see it.

I took some photos a few years back of my neighbor's roof during a windy rainstorm....it showed textbook slipstreams across the ridge and dormers.