How To Build an Addition Using Concrete Blocks

Concrete block cutting

To create an addition to your home or office using concrete blocks, begin by preparing the building pad (the area where the room addition will be). Clear away all grass, shrubs and trees to a width and length approximately 3’ longer and wider than the size of the room addition.

Remove all underground sprinkler lines and terminate any water lines or electrical conduits that may be underground.

If overhead wires are present, contact your utility company and have these moved temporarily.

Next, strike two vertical chalk lines on the face of the building where the room will attach. The distance between these lines should be the width of the room addition, plus one foot on each side to allow for blending of the new exterior to the existing. Remove all exterior stucco, siding or brick veneer down to bare wood framing. Expose the face of the existing concrete footing at the building. Cut any roof overhangs to flush with the face of the building at this area.

Stake out the corners of the new addition and draw string lines from the building face to the stakes. Check the dimensions for accuracy and square. The string lines represent the outside wall lines of the addition.

Excavate for the concrete footings by digging 12” wide x 16” deep footings on the (3) sides of the addition. The outside edge of the footing should align with the string lines.

Install any underground utility or water/plumbing lines that may be necessary and terminate the connections outside of the room addition area.

Place (2) continuous horizontal runs of #4 steel rebar in the footings, one 4” from the top and another 4” from the bottom. Lap the runs at least 24” and secure with #4 wire. Ensure that no rebar is in contact with the soil.

Next, install vertical #4 rebar at 32” apart, extending upward to 3” below the top of the exterior walls. These pieces should have a 36” bend at the bottom and tie to the horizontal rebar in (2) places.

Pour the concrete footing using the desired mix formula as given in the approved building plans. Finish the top of the footing with a wood float to smooth, but not slick.

The size of the required concrete block or CMU will be specified on the approved plans. These block vary in width and height but are consistently 16” in length. Begin laying the first course of block in a mortar bed atop the concrete footing, using a string line to align the outside face of the blocks. Slip the block over the vertical rebar at those locations. Once the first course is complete, install steel dowels into the face of the existing concrete footing of the building sing an epoxy adhesive. These rebar dowels will be set at the height of the first course of block and extend into the new block wall a minimum of 4’.

Place a continuous horizontal run of #4 rebar atop the first course, of block, tying any laps as required. Continue placing courses of concrete block, each with a horizontal run of #4 rebar. Block out for doors or windows by using a 2’ thick wood frame.

Once the wall is constructed to the correct height strike a chalk line on the interior face of the wall at the height of the existing sub-floor and install ½” x 8” long foundation bolts approximately 6” below the line. These bolts will attach the wood floor joist ledger to the block wall. Ensure all bolts are in place as required.

Solid grout the wall by filling each of the block cells with mortar mix to the top of the wall. Install  ½” diameter foundation bolts into the mortar at the top of the wall at 48” intervals. These bolts should extend upward approximately 5” above the top of the wall elevation. Allow the mortar to cure for 2 days.

Begin framing of the sub-floor by installing a 2” thick rim joists, (ledger) to the inside face of the block. Drill 5/8” holes that align with the anchor bolts that protrude on the inside face of the block and secure the ledger with plate washers and machine nuts. The size of the floor joist and the span will be specified on the approved plans. Install joist hangers on this ledger per the required layout, which will likely be 12” or 16” apart. Cut the floor joist to length and install them, fastening with hanger nails as required. Install an additional ledger joist at the face of the existing floor joist at the building.

Before installing the subfloor make sure any plumbing or electrical risers are in place. The subfloor material should be attached using an approved exterior adhesive and screws or galvanized nails.

Next, install a 1-1/2” thick treated mudsill atop the block walls by drilling 5/8” holes at the anchor bolt locations. Fasten the mudsill with plate washers and machine nuts. This wood plate will allow for nailing attachment of the ceiling joist and roof rafters.

The block walls are now complete and the roof framing may be done according to the approved building plans.

Attach any interior wall partitions as desired by using embed anchors installed in the concrete block and nail or screw attachments at the subfloor.


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