Tyvek and other building wraps are designed to stop liquid water while letting water vapor through. Kind of a crude Gortex. I have used it mostly as ground cloths or bulk tarp material and in that role it seems to work pretty well. Generally it doesn't leak as long as there is some slope to keep the water moving.

The biggest advantage to this stuff over Sil-Nylon is not raw performance so much as price. A 8' by 10' Sil-nylon tarp will run about $70 ($0.875/sqft.); A 9' by 100' roll of Typar, Aces version, runs $141 ($0.156/sqft); Lowes sells Tyvek at $188 for 10' by 150' roll ($0.125/sqft.)

Prices from:
http://www.campmor.com/outdoor/gear/Product___20069

http://www.acehardwaresuperstore.com/typar-housewrap-9-x-100-ft-roll-p-39419.html?ref=42

http://www.lowes.com/pl_DuPont+Tyvek_4294925567+4294832803_4294937087_

The building wraps don't come with grommets and isn't hemmed but for a seventh the price you have to expect to lose some convenience.