You don't need any tread unless you are on a surface that only offers grip when you cut into it - dense (not deep) mud and wet grass being typical - otherwise almost all surfaces, tarmac, gravel, cinder paths, smooth rock are best using a slicks whether wet or dry - faster (often very much faster) and longer lasting for a given weight. Tread was introduced on road tyres 100 years ago to distinguish between makes and models of tyre and it's a bad habit that's continued - imagine going into a bike shop and seeing 50 different tyres all hanging up - all of them slicks - a marketing man's nightmare. Avocet did some work on this years ago and brought out some gorgeous slicks with different 'tread' thicknesses for touring, tandems etc - wish you could still get them but even something cheap like Schwalbe Spicers will convince you.