Q: What is a frost line and do I need really need to build my BrickWood Box base slab so it touches the frost line since it sits above ground – not below. Also, I live in Michigan and we have long, cold, miserable, ugly winters – what’s the worst that would happen if we DIDN’T build to the frost line depth?  Final question – what is my frost line depth?


A: The frost line is the depth of frost penetration in soil and/or the depth at which the earth will freeze and swell. Over a period of time, this freeze / thaw / freeze / thaw could possibly cause a minute shift to your BrickWood Box and that’s the worst that could happen. It will not flip or fall over and it will not look like the Leaning Tower of Pizza (you can cook a mean pizza in your BrickWood Box). If a shift or tilt does occur, it is easily fixable by digging under the corner that is tilting, lifting the BrickWood Box to your desired height, then filling the hole you just dug with a bag of concrete mix.


The depth of your frost line varies in different parts of the country so you will need to consult with a local structural engineer or see if you can find your city’s frost line requirements on the Google Machine. You may live in a city, town, village, county or state that requires all structures be built to the frost line.  Our Frost Line Depth Map will give you a rough estimate of what your frostline may be.

Frost Line Map

If you choose to build your BrickWood Box base to your regional frost line, the diagram below shows you how to easily adjust your base slab.  The following materials ARE NOT ON THE MATERIALS LIST – you will need to calculate and purchase these items separately.  For EASY math calculations, jump on WolframAlpha.com.  It’s like Google for math!

After completing STEP A5, place the Base Slab frame into approximate position. Measure and mark 8” x 8” on all (4) INSIDE corners, then use a POSTHOLE DIGGER to dig each mark 4”-5” DEEPER than your Frost Line.

Insert a 6” or 8” Sonotube into each hole.  Align the TOP OF EACH TUBE AT GRADE or GROUND LEVEL (but not the “new” ground from STEP A4).  Insert (2) pieces of rebar into each tube.  The bottom of the rebar should touch the bottom of each hole and should protrude from the top 14”–20”.

Working (1) tube at a time – Fill each tube 2/3 full with High-Strength concrete.  Insert (2) pieces of rebar into the center of each tube, then fill the tube with concrete until full. Allow all (4) tubes a MINIMUM of 7 days to cure. Once the concrete is rock hard, place a scrap 2” x 4” on top of each concrete pillar and use a hammer to bend the rebar 90° along the sides of the base slab.  There needs to be a 1.5” distance between the top of the concrete pillar and bottom of the rebar bend.

At this point, you are ready to rejoin the class and proceed to STEP A6.


After STEP B5, you will need to attach the BASE FRAME to the rebar that is bent 90° in the (4) concrete pillars with REBAR WIRE TIES.

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Frost Line Map
Frost Line Map