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Block Foundations for Small Additions 2

Block Foundations for Small Additions 2

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    Blockwork starts at the corners, with the blocks laid in a full bed of mortar. Strings stretched between the corner blocks keep the courses straight. Every other course is reinforced with galvanized metal truss.

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    Parging fills the pores of the block, making them more water-resistant.

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    A coat of cold asphalt supplies another layer of protection against moisture.

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    A 4-inch bed of stone.

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    Followed by a poly vapor retarder and a thin slab will help keep the new crawlspace dry.

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    Providing an access opening into the existing crawlspace completes the foundation.

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    Switching from 12-inch to 8-inch block for the top course creates a ledge for the floor joists.

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    Which rest on a flashed PT sill. Full-depth blocking between the joists takes the place of a rim joist.

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    The finished deck, ready for framing.

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    Though only 6 feet wide, the finished bump-out accommodates an elegant, functional kitchen.

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Blockwork

After the concrete has cured at least a day and we’ve stripped the forms, we move the blocks down into the hole, setting them up on sheets of plywood to keep them clean. This job was simple to lay out: We snapped a chalk line on top of the footing parallel to the existing foundation wall to represent the outside edge of the new wall, squared up the outside corners, and started laying block.

Masonry lines strung between the corner blocks keep the courses straight. DoCouto lays block more like a carpenter than a mason: He uses his 2-foot spirit level to level each block, whereas most masons check for level only occasionally. We use a story pole marked off in 8-inch increments to keep each course on track, checking its height against both the footing and our benchmark with the transit or level.

DoCouto uses a lot of mortar while laying up block, filling up the cores, and reinforcing the corners with steel. To provide lateral strength, we add Dur-O-Wal galvanized truss reinforcement (888/977-9600, dur-o-wal.com) to every other course. We also parge the walls as we go, using the same 2 1/2-to-1 sand/Type S mortar mix that we use to lay block. Splashing a little bit of water on the block before applying the mortar helps make the troweled-on mortar stick.

It took a day to lay up and parge the three lower courses of 12-inch block on this job. The following day, we switched to 8-inch block for the last course, to create a shelf for the floor framing that exactly matched the foundation on the rest of the house. We filled the cores of the top course solid with mortar and set 12-inch anchor bolts 24 inches on-center to hold the double 2x6 PT sill. Because floor sheathing would be attached directly to the top of the 3-inchthick sill, we embedded the bolts with only 2 1/2 inches exposed and recessed the holes for the washers and nuts.

Drainage and waterproofing. Most of the time, we tie a new footing drain into either the home’s existing perimeter foundation drain system or — if we can’t run a drain to daylight — a new sump well. On this job, however, the architect didn’t specify any drainage and the building inspector didn’t require it, which is typical for a crawlspace foundation in our area. Parging plus a thick coating of cold asphalt rolled onto the below-grade portion is usually sufficient dampproofing for crawlspace foundations.

Slab. This foundation was small enough that we could backfill by hand, carefully tamping as we went. We reused the excavated soil, adding a top layer of stone dust for resetting the patio pavers. We filled the hole with a yard and a half of stone and poured another yard and a half of concrete to create a 4-inch-thick “rat slab” floor. The stone and vapor barrier underneath the slab will help prevent moisture from wicking up into it.

Floor framing. Before framing the floor, we cut an access opening into the existing crawlspace, then used 3/8-inch-diameter bolts to fasten a 2x10 ledger to both the existing double sill and the block wall underneath. The joists are attached to the ledger with hangers. At the other end, they’re nailed to the 2x4 PT sill, which we fastened to the masonry ledge with powder-actuated fasteners. Blocking between the joists over the sill substitutes for a continuous rim joist. To finish up the deck, we glued and nailed down a 3/4-inch T&G plywood subfloor.

Cost

The total project budget for construction of the complete kitchen addition came to about $82,700 after changes, upgrades, and additions (not including cabinets). Construction of the foundation accounted for about 127 man-hours (excavating, 33 hours; footing, 35 hours; blockwork, 49 hours; flatwork, 10 hours), compared with a total of 805 man-hours for the entire project.

Total materials cost for the foundation was about $2,500, including $868 for concrete and $598 for backhoe rental.

In all, the cost to the client for the foundation was approximately $7,500, which is roughly equal to what the cost would have been had we subcontracted the work. Our gross profit margin for the project was 10.1 percent.

Rob Corbo is a building contractor in Elizabeth, N.J.