Use an Appropriate Slate

The most important thing to consider when using slate in an outdoor project is to choose a type of slate which is appropriate for this application.

Red veins running through the surface of a slate are often an indication that iron is oxidizing in the surface of the stone. While generally harmless indoors, when placed outdoors moisture can speed this effect up drastically causing the slate to become brittle and break apart. This is a process that is similar to what happens to iron when it rusts.

If the slate is a straight red color without veins however, this may mean that it is one of many other elements that can cause the color red to appear within stone.

Low Absorption Rate

When using stone of any kind outdoors it is important to have a low absorption rate. This means that the pores found within the surface of the stone will not soak up moisture. When water gets into these pores, it can freeze, which causes it to expand, and which can shatter the stone.

Slate is generally is generally a good stone to use outdoors because it has a very low absorption rate, often as low as .2% compared to other materials such as bluestone which can be as much as 10% absorbent. However the absorption rate will vary slightly between different types of slate, so for optimal durability you may want to research the material you select.

Proper Installation

It is very important that you're the stone is installed properly when being used iun an outdoor application. If your contractor does not create a smooth subsurface for the slate to lay on, voids may appear underneath the stone, into which water can creep. This water can then freeze, expand, and destroy the entire project.

Layers

When choosing a material try and get a few samples so you cans see exactly how it behaves. If the material flakes apart along highly visible layers, it may have trouble maintaining its integrity while suffering from weathering over the course of several years. Ideally you want a slate that is relatively nondirectional in the way that it breaks, and which shatters in chunks rather than lines.

Weathering

When installing anything outdoors, it is important to be aware of the fact that this installation will be subject to the full power of the weather 24 hours a day, seven days a week. With natural slate, this means that some weathering may occur to the colors in the stone.

This is not necessarily an unattractive thing. Many people love the rustic patina that slate develops over the years, which can give a project a classic, aged look.

Chemical Protection

There are chemical coatings which should be applied to any natural slate project that you are creating outside, which will offset many of the negative effects that weather can have on your material. At the very least a water barrier sealer should be applied to the stone after installation. If you want to enhance the colors and protect the stone from weathering, then you can apply a non yellowing protective finish to the surface, every few months.