Messages and Online Forum Q&A

Re: Sono Tubes

From: info@builderswebsource.com
Category: Decks
Remote Name: 63.198.181.43
Date: 14 May 2001
Time: 08:13 AM

Comments

I'm glad you mentioned that your deck is in a region where there is a frost line. The simple answer is YES...you must dig your footings to extend past the frost line to suitable load-bearing earth. Otherwise, you risk having your deck become wavy over time. It's no different than building a solid foundation for a house. In fact, you should check with your local building department to see if there are codes governing minimum footing depth for your area. Be sure to tell them you're building a deck with a hot tub and that you're concerned about the stability of the foundation.

We found an excellent online resource that specializes in deck-related information. For information on footings, go to: http://www.bestdecksite.com/ and also click on the hyperlink that discusses frost heave (a fee now applies to this site). As with any pier foundation, the depth of the pier depends critically on the frost level, the nature of the soil and its load bearing properties as well as the total load expected. In your case, each pier is on a 4' grid. At a total 100 PSF load, which we calculated earlier and accounts for the weight of the loaded hot tub, this means each pier will have to absorb approximately 4*4*100 PSF, or 1,600 lbs. Therefore, engineer your footings accordingly.

Now, in your case, you discovered a few granite boulders just 1 foot down. The problem is, we don't know how deep those boulders go. If they're only 1-2 feet deep, then the boulders will heave along with the frost conditions, pushing up on your deck. On the other hand, if the boulder is very large (several feet in depth) and or is part of a native rock formation, then you'll be in better shape. The problem is, unless you excavate or take soil samples, you won't know how deep these boulders go, so you're taking your chances. Given the weight of hot but, supporting this corner is critical. You don't want the corner to sag or heave. You might try excavating a small area on one side of the boulder to see how large/deep it is. If you go well past the frost line and still haven't come to the end of the boulder, you're probably OK following the procedure you described. Try not to disturb too much native soil around the boulder, since you don't want it to shift later.

If you discover the boulder isn't very deep, we recommend removing it...or drilling through it if you have access to a drilling rig. Removal of one or two might be cheapest if their sizes are manageable. You may need to use a jack hammer to break it into smaller pieces that can be removed by hand.

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