What's a good Rule of Thumb when footings are on a slope?

The general rule for deck footings built on a slope are based on average soil conditions. So, if you have unstable soil conditions all bets are off. You will want to speak to an authority in your area. Usually a Geo-technical engineer or someone with lots of experience with building decks will know exactly what to do.

Be Safe Rather Than Sorry

The footings your deck rests upon are the most important part of your deck.

Note:  The seven foot rule is only a rule of thumb. All slopes have to be examined and understood before any building commences.  There are places in the St. Lawrence Valley, and Alaska where quick clay or leda clay, as it is sometimes called, is present.

This is potentially a very unstable soil at the best of times.  A water laden slope would be at even higher risk of literally washing away under the right conditions.

Alternatively, in mountainous terrain soil can be a mix of sedimentary layers or alluvial rock or sand.  This kind of slope is also at risk of sliding if its saturation point is reached.  The seven-foot rule may NOT apply in any of these situations.

Know Your Soil Type Before You Dig Your Footings

The topic of foundations and how to properly support a deck over a sloped grade demands local building knowledge that you must acquire before you build or plan any light structure.  On special locations like slopes or hillsides, take the time to contact your local building officials to find out what the soil is and what the common building practices are.

Obviously, a hillside deck can give you dramatic views and benefits but don't forget to consider safety in the equation.

Some Other Great Reading:

Different Styles of Footings on Flat Ground