Replies

1. 
   Jim_Allen | Apr 13, 2008 05:12pm | #1

   You need either a rat wall or a full frost foundation. It depends on your local codes.

   Bob's next test date: 12/10/07

1. 
   bhackford | Apr 13, 2008 05:19pm | #2

   What do you mean by a rat wall?  Thanks

1. 
   Jim_Allen | Apr 13, 2008 05:54pm | #4

   In MI, they require all detached buildings to have a concrete barrier to prevent rats from tunneling under the warmer buildings and survive the winter. It's been a very long time since I built a detached garage but I think the rat wall was only 24" deep and 4" wide. It might only require 12" deep. I just cant remember. If you are building a small detached brick fireplace, with no gas connections, it theoretically will float up and down when the frost heaves your slab. I'd want the slab to have some steel in it. Bob's next test date: 12/10/07

2. 
   peteshlagor | Apr 13, 2008 05:24pm | #3

   I had to have one in Michigan.

   Same depth.

   Your local building department will be happy to tell you.

   And, you'll find there are other restrictions.  The height of the chimney relative to the house - if within 10', gas lines and high tension going to the unit, and whatever else they can think of to force a building permit onto you.

    

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   Edited 4/13/2008 10:24 am ET by peteshlagor

   
   

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   Edited 4/13/2008 10:27 am ET by peteshlagor

3. 
   renosteinke | Apr 14, 2008 02:36am | #5

   Whether required or not, you really need to consider the weight of all that brick / stone / concrete ... and the height. Just how far will it fall, and what will it hit on the way down?

   If your design is lightweight enough, a simple slab that is considerably larger than the footprint of the fireplace might suffice to make it all "float" as the ground heaves. There is also reason to pay extra attention to making sure the site has exceptional drainage.

   The materials that you use also make a difference. Insulating fire brick, while expensive and needing surface protection, is a lot less heavy than usual fired brick. You might consider using it, or materials such as perlite and vermiculite, to fill hollows.

4. 
   WayneL5 | Apr 14, 2008 05:12am | #6

   I would not put masonry that tall on a slab.  True, a slab that is thick enough to not crack will float up and down with frost heaves.  The problem is that frost heaves are not uniform.  The sun will warm and heat up one side of the fireplace and thaw the ground on that side while the opposite side remains frozen.  That will happen over and over again when you have warmish days and cold nights.  Over time your masonry is likely to lean unpredictably.  In some soils you may be able to get away with it, but if you are putting that much money into it already it's nothing to dig a hole and put in a proper foundation.  The underground walls can be made of inexpensive concrete block, they don't need to be brick.

1. 
   Jim_Allen | Apr 14, 2008 05:24am | #7

   Good point Wayne! Bob's next test date: 12/10/07

5. 
   MFournier | Apr 14, 2008 08:38am | #8

   Might not be required but highly recommended.

   The thing is frost heaves are frozen water (dirt it self does not freeze it is the water in the dirt). So if you have enough gravel under a slab and it is thick enough to support the structure that will be on it then you would not technically need concrete down 4 feet. but you would need to still dig below frost line then fill with gravel compact that gravel and provide good drainage. So you can see you might as well go down below frost line pour a footing and foundation.

   PS Precast foundation panels rest on compacted gravel no footings so in theory anyway you could build on a concrete block foundation stacked on compacted gravel

   
   

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   Edited 4/14/2008 1:43 am ET by MFournier

1. 
   bhackford | Apr 14, 2008 07:16pm | #9

   Thanks.  I was thinking that digging a 4x4x4 foot hole and filling it with concrete would expensive.  I never thought to just to 8" walls in a square.  Thanks again