Cottage Q&A
QUESTIONI have a 20' x 20' frame cottage that is supported by cedar posts resting on floating concrete pads. The cottage is 35 years old and originally had a grid of nine support posts. Over the years, extra posts have been added to this grid. Two things appear to be happening: First, the cottage does its regular heaving with the winter freeze-up and thaw; and second, the cottage is slowly inching its way to the lake. The lot has a shallow slope, but the cottage's lateral movement is still in the order of 1" per five years. I wish to raise the cottage about 6" and return to a grid of nine support posts, still set upon floating concrete slabs. I do not want to dig a foundation because of the marl clay layers that exist in this area and also because of the close proximity of my dug well to the cottage. (The well is an excellent, unfailing water source) What do other cottagers do in similar circumstances? Are there lightweight support beams available that would span the 20' length?
ANSWER
Since each cottage site is different, it's difficult to give you a solution without actually seeing the setup at your place. For example you don't mention the load that the beam (and the posts and pads) will be required to support.
We're also unsure from your letter whether you plan to put posts under the beam. If so, Wayne Judges, of Judges Contracting in Gravenhurst, Ont., points out that you can build up your own 20' beams using three of four layers of, say, 8' and 12' lengths of 2" x 10" spruce or pressure-treated lumber. One advantage of built-up beams is the convenience of transporting the materials; the wood can be carried to the cottage in pieces and then nailed together when you get there. However, if your posts are positioned in such a way that the beam must span 20' unsupported, Judges recommends engineering it out of steel or aluminum (aluminum is lighter than steel, but also more expensive); an unsupported wood beam of that length will tend to sag.
The movement of your cottage down the slope is almost certainly related to its winter heaving, Judges says. In order to minimize the heaving of the foundations, you may want to reset the posts, making sure to provide adequate drainage to keep water from freezing around and underneath the pads. There should be enough crushed stone where each pad will sit that you can rake a level pile about 6" deep. The post and pad sit on the stone Andy water that comes along will flow under the stone and drain away. Be careful not to scour or otherwise dig into the ground under the stone, however, as you'll create a depression where water can accumulate, freeze, and eventually heave, leaving you with the same problem you had at the beginning.
Judges recommends using "cottage slabs" for the pads under the posts. Not to be confused with patio slabs, these are heavy, wire-reinforced, 18"-square concrete pads about 4" thick. You can buy them at building supply stores.
Though you say you don't want to dig, sinking concrete footings could be an effective solution, suggests Toronto architect D'Arcy Dunal. The basic principle is to set the piers below the frost line (about 4') and then to remove the moisture from around the posts by backfilling the holes with gravel or another granular fill so the water drains away. "Water is the evil thing here," he says. "Eighty per cent of the structural problems we deal with at cottages are the result of water."
Adding rigid insulation on top of the ground, around the piers or on the pads, may also help prevent frost from getting into the ground.
* Published in the Winter 1997 issue of Cottage Life
