Ridge Vents

What are the pros and cons for using ridge vents? I'm having my house re-shingled, and got estimates from two highly recommended roofers. One recommended installing a ridge vent; the other did not. The one who did not recommend a ridge vent re-roofed our other house last year and insisted on installing a ridge vent.
The house we're re-roofing is a two story "colonial" with an unfinished attic that has both soffit vents and gable vents. The house we re-roofed last year is a bungalow with the second floor (what would normally be the attic) finished.
I will discuss this with each of the roofers before I decide which one to hire, but I would appreciate any insight anyone can give me.
Also, are there any shingles that really resist algae stains, which seem to be a big problem in my area (Maryland)? Both roofers want to use Certainteed XT25AR, and neither seemed enthusiastic about the more expensive "architectual" shingles, although both were willing to install them if that's what I want.
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JA wrote:

do you have problems with condensation or does snow melt off of the roof from the heat within? if either of these occur, then a ridge vent might help. otherwise, if you have soffit and gable vents, i would think you have adequate ventilation without adding a ridge vent.
i don't know about shingles that can resist algae stains. the only solution i know of is to put galvanized strips along the ridge. there are strips made for such a purpose.
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snipped-for-privacy@gmail.com says...

If there are gable end vents ridge vents shouldn't be added (without blocking the gables). That said, ridge vents are superior in most climates.

...or copper (wire works too).
--
Keith

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I tried the zinc strips in Florida a few years ago and was unimpressed. I wouldn't do it again, but Florida weather is pretty severe. Similarly, the Florida builder doing my renovations recommends against ridge vents for two reasons --
a. Water infiltration in severe storms b. In hurricanes last 2 years, for many homes the ridge vents were ripped off as the first point of wind damage.
Of course, neither of these may apply to Maryland -- Regards --

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this free document is 74 pages for your climate at: http://www.buildingscience.com/designsthatwork/mixedhumid/DTW_MixedHumid.pdf
JA wrote:

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wrote:

Pros: Additional ventilation, longer lasting roof, quiet, energy efficient, inexpensive and easy to install. Cons: Cost, most easily added when roof is re shingled.
I have never heard of "too much ventilation." If you notice algae growth on neighbor's roofs and plan on living there more than 5 years, get the anti-algae shingles. If you can add more soffit vents, do that too.
I have seen some poorly-done roofing jobs. Be careful who you decide to hire.
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