Yeast over attenuation ???

I brewed a 2.75 gallon batch of SMASH two weeks ago. I racked it today and it appears that I got 87% attenuation from Wyeast American Ale yeast. I wanted a beer in the 4.5% ABV range, instead I got a beer that is 5.25%. Why did I get such over attenuation?
Recipe as follows;

3 gallon boil
1# Breiss golden light DME @30 min.
2# Breiss golden light DME @ 10 min.
.5 oz. Mosaic hops 11.6AA @ 30 min.
.25 oz. Mosaic hops 11.6AA @15 min.
.25 oz. Mosaic hops 11.6AA @ 10 min
800 ml starter of Wyeast American Ale yeast pitched at 64 degrees.
Fermented for 15 days
2.75 gallons of wort in a 5 gallon Carboy. Blow off tube.
Too much headspace?
Too much yeast?



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What were your OG and FG values? Measured how and at what temperatures?


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OG was 1.046 @ 65'
FG was 1.006 @ 70'




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I've heard of some people getting attenuation in the mid 80s% from US-05, which is the same strain as Wyeast American Ale but in dry form. It's a high number, but not unprecedented. It sounds to me like you just had happy, healthy, aggressive yeast that went to town. (Unless of course there is some unknown variable at play).

On an unrelated note, I would avoid leaving that much headspace in your fermentors in the future. It doesn't affect attenuation to have a little extra room (in other words, it's not the cause of your super-fermented beer), but it *can* cause oxidation. I had that very thing happen to a recent brew, in which I was shooting for 10 gallons but came in closer to 8 (new equipment that I hadn't tested for boil-off yet). 5 gallons went into a 6.5 gal fermentor, while the remaining 3 went into another 6 carboy. The former came out great, while the latter was oxidized and just "bleh."

 

I regularly get in the 80-85% attenuation range with the White Labs equivalent of that strain (wlp001). This holds true mainly to grists that don't consist of a lot of specialty grains (like yours above). This strain attenuates very well if the fermentation is ideal, and even with grists with more specialty grains I normally get in the high 70's. If you are using extract I wouldn't worry too much about it since you really don't have much control of the fermentability of your wort unless you add specialty grains or take extreme measures. If it really bothers you then you can add some maltodextrin to bring some body back to your beer without affecting flavor.

The only major problem that I see is an infection, but if you are 100% sure that you don't have anything else fermenting the sugars that the yeast can't eat then you can relax. You can pat yourself on the back for conducting a fermentation that is healthy enough to allow your yeast to work to their full extent.