A concrete-block foundation can be stronger than one made of poured concrete. Its big advantage in a cabin setting is that a concrete-block foundation can be installed in increments and does not require constructing large forms and pouring concrete for the entire foundation at one time. Also, since cabins are often remote, it is easier to get blocks to the job site than cement trucks. Once the foundation is laid, a cabin can be built with conventional wood framing.
Things You'll Need
- Tape measure
- Stakes and builder's twine
- 2-by-r-inch lumber
- Wood stakes
- Compacter (hand tamper)
- Concrete blocks
- Wheelbarrow (for mixing mortar and concrete)
- Reinforcing bar
Measure your foundation area and mark it with wood stakes and builder's twine. Make it four inches wider all around than the actual dimension of your foundation; concrete blocks are nominally 8-by-8-by-16 inches and need to be set on footings twice as wide and 1 1/2 times deeper. Dig a trench for footings 12 to 16 inches deep (depending on the height of the foundation) and 16 inches wide. Make sure corners are square. Use the hand tamper to firmly compact the soil.
Build a base of gravel four to eight inches thick, depending on the height of the foundation and soil conditions. Tamp the gravel to compact it firmly and place upright reinforcing bars spaced so holes in the blocks will go over them. Pour concrete to the depth of the ground. You may line your trench with wood forms, but if your soil is firm, this is not necessary. If any sewer or other utility lines need to pass through the foundation, install them before laying gravel and concrete. You can mix concrete in a wheelbarrow and pour footings in sections, one side at a time. Level the concrete with a trowel.
Lay your first course of concrete blocks, with mortar, on the footing. Mark the edges with twine to keep the blocks straight and level. Put mortar under the blocks and at the end joints. Lay the first course of blocks dry first to get exact measurements, then set them in mortar. Set a corner block first (these are special blocks with finished ends), then work down a side to the next corner, putting blocks over upright rebar as needed. Check the level every few blocks. Tap the blocks with a 2-by-4 to adjust the alignment. Build the corners first, keeping them always a block or two above the others. You can do the job in sections by stopping at any completed course.
Finish your concrete block foundation when it is the proper height by filling spaces with upright rebar with concrete; the rebar should be below the top of the block. Put concrete in other block spaces and set upright mounting bolts every three to four feet to fasten a wood base plate for the cabin wall. Once all concrete has set, install 2-by-4 base plates on top of the foundation (drill holes for the mounting bolts). Then proceed with the cabin construction using conventional wood framing.
Tips & Warnings
- Check continuously to make sure your concrete block foundation is level, square and plumb.
- For a nicer appearance, smooth mortar between the blocks with a metal jointer.
- Blocks are nominally 8-by-8-by-16 inches, but actually are 7 5/8-by-7 5/8-by-15 5/8 inches to allow for mortar.
- Never mix more mortar than you can use in an hour and a half, so it doesn't dry out.
- Concrete blocks are heavy; get help with lifting and setting.
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