Cottage Q&A
QUESTIONI'm trying to find the best way to build an addition onto my cottage on White Lake, near Arnprior, Ont., so that it will not shift from frost heaving. The cottage is 12'' x 30'' and I plan to extend it another 12 feet to make the building 24'' x 30'' overall. It currently sits on concrete blocks. The building inspector suggested lifting the building and pouring 26" x 26" concrete pad footings, with concrete block piers on top of that. Is this the best solution? If this is the best way to go, how deep do I need to dig to avoid frost heaving?
ANSWER
There are three main approaches to building a foundation for your cottage and the addition - call them top-drawer, middle-drawer, and bottom-drawer solutions.
The top-drawer way, says Rod Thurston of Rodco Enterprises in Haliburton, Ont., would be to jack up the current building and pour a continuous perimeter-wall foundation (a minimum of four feet deep, or directly onto and anchored into bedrock if you hit it in less than four feet) for the whole cottage, new parts and old. That way, you'd not only guarantee that the building wouldn't shift, you'd help secure it against rodents and other critters, who love bunking down in crawl spaces, and also make it easier to heat if you want to winterize.
Our pros agreed that your building inspector's solution of lifting the current building and supporting everything on concrete footings and piers is a very satisfactory mid-priced approach. You'll want a support at each corner and, in between, under spans of 10 feet or less. For your 24'' x 30'' building, depending on the dimension and layout of your beams, that's about 12-15 supports in all. Dig the holes for each support down below the frost line - four to six feet, depending on your location if you're in Ontario - or to bedrock, whichever comes first.
Hitting bedrock is good, because you can pin a concrete block pier (or concrete Sonotube) directly to the rock and avoid the whole issue of frost heaves. Don Ferguson of Do It All Construction in Emsdale, Ont., says you should drill into the bedrock about four inches (rock drills are widely available from rental stores) and insert a couple of pieces of 1?2" rebar, long enough that the upper end of each piece projects right to the top of the pier you're building. (See above.)
In places where you manage to get down below the frost line without finding bedrock, you would pour a concrete pad footing. Ferguson says the inspectors in his area prefer to see a footing 30" x 30" square and at least six inches thick. Then you would build up your pier from there.
Setting the blocks for the addition directly onto the current grade is the least desirable, bottom-drawer solution. You'd have to re-level at regular intervals - probably every year, says Ferguson. Plenty of people do build this way, he adds, though it's a pain in the neck and often costs them more in the long run. "There are some cottages I re-level every year at $800 to $1,000 a time."
* Published in the September/October 2002 issue of Cottage Life
