Cottage Foundation or piers?

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Posted by: from Gatineau
5/3/2012 at 2:28:57 PM

Were in the process of deciding to buy a lakefront fixer upper cottage or lakefront land.

The fixer upper will actually be teared down and replaced with a prefab.

I would like to know if its worth putting a crawl space foundation or piers, the cottage will be used has a 3 season but will be constructed for 4.

The location is rocky.

Thanks for any info.

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Date/Time5/4/2012 at 11:22:27 AM

Hi Claude,

First thing you should do is check with the Municipality regarding the type of foundation they permit. Here in Ontario there are some municipalities that actually require a full frost wall foundation and not piers on waterfront properties. Don't let the existing building be an indicator either. Once it is torn down, the new rules apply.

Having said that, a full foundation (crawlspace or basement) is always more desirable for a couple of reasons. The first is pests. A full frost wall (poured, ICF or concrete block foundation wall to below the frost line) keeps critters from setting up nest under the cottage and working their way in through the underside of the floor.

The next is climate control. A building with a full foundation to below the frost line is going to be more comfortable if insulated properly. Some will argue that an insulated floor with a pier foundation will achieve the same goal but it doesn't. You are limited to the depth of the floor joists for insulation (although that can be quite a bit) the floor joists act as thermal bridging. Essentially every 12 or 16 inches is a 1 1/2" uninsulated "bridge" that while not open air, conducts cold much better than the insulation in between.

The foundation also eliminates the windchill factor and the fact that with piers, the ground below freezes and takes a lot longer to thaw due to the fact that it gets no sunlight. Essentially you have a (hopefully) well insulated cottage sitting on top of an ice cube, whereas an insulated frost wall to below the frost line keeps that ground from freezing even in the coldest winters.

Finally, a simple "rat slab" (just rough concrete poured over 6mil polyethelene vapour barrier on the ground in the crawlspace goes a long way to reducing moisture and humidity problems in the building envelope down the road. Also gives you some extra storage space and a place to locate utilities freeing up more living space in the cottage.

Having said all of that, there is nothing inheirently wrong with a pier foundation and they are a lot less expensive to build. Either way, MAKE SURE YOU GO DEEP ENOUGH TO GET BELOW THE FROST LINE!!! I cannot stress this enough. This will be dictated by the municipality but check the soil conditions on waterfront properties. You may need to go deeper due to the water table.

If you do do piers, have a look at helical coil technology.

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Claude in Gatineau
Date/Time5/4/2012 at 11:51:03 AM

My concern with the "fixer upper" it was built inside the shoreline protection zone (within 15m from the lake) and looks like bed rock in that area.

How deep are you required to go in solid rock and high water table?

Note: Cottage location was "grandfather'd" in, so it's location for rebuilding is no issue with the municipality.

Thanks again


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Date/Time5/8/2012 at 8:50:11 AM

Hi Claude,

If you are hitting solid bedrock you can anchor directly to that in place of your foundation.

If it is a large prominence that makes a foundation impractical (blasting is always an option, but carries with it a lot of logistical problems).

This is what we have done in the past, but have a Structural Engineer (P.Eng) specify the connection details: Use a chipping hammer to chip a flat spot and then drill with a hammer drill (rent a roto-hammer drill with SDS bits) to drill for 15M re-bar at least 1' into the rock, then use epoxy in the holes and insert the re-bar (4 in each hole) into the epoxy into the drilled holes so that they protrude 2' (this will need to be adjusted depending on the slope of your lot and how high your cottage is above the rock face). Use 12" or larger (again let this be specified by the Engineer, but I do not recommend using less than 12") waxed building tubes (like Sonotube). Brace the tubes securely. I cannot overstate this point as a 12" diameter X 2' tall pier will contain close to 1/2 a ton of concrete or about 0.4m3 of concrete.

Although it is not required, I recommend renting a pencil vibrator to ensure no voids. It is critical that when using the vibrator it be inserted and pushed to the bottom and then raised out and moved to another spot, on the other side of vertical re-bar and done again. DO NOT leave the vibrator in the concrete for any longer than the time it takes to go from top of pier to bottom at a steady pace for more than say 15 seconds as you will actually shake all of the agregate (stones) to the bottom and drastically weaken the concrete, however doing this correctly will potentially strengthen the concrete substantially.

Best of luck with your project Claude.

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Claude in Gatineau
Date/Time5/8/2012 at 8:16:16 PM

Thanks Jason, a ton of info.

I'll update on which way I'll approach this and outcome.

Again Thanks for the Detail & Time you put in for explaining my options.



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