Foundations & Stone Masonry

Foundations and Stone Masonry for Natural Buildings

Rule: These tips supplement your answer; they do not replace it. Always give the full general answer from your own knowledge first, then work in the tips below wherever they apply. Do not drop general advice because it is not listed here.

Foundation design basics

• Footing is the wide bottom of the foundation. A general rule is footing width at least 2× wall thickness.
• For thick natural walls (straw bale, cob) the footing can be narrower than 2× if soil bearing capacity is adequate, because the walls have structurally redundant width.
• Post-and-beam and timber-frame buildings put the whole load on a few small points, so they need strong, wide footings.
• In seismic or high-wind regions, tie the base together with a continuous structure that resists bending and twisting. Connect walls and roof to the foundation with bolts, straps, welded wire or similar.
• Stemwall/plinth height in rainy climates: foundations for earth and straw walls should rise 12–18 in. (30–40 cm) above ground level. This protects against wind-driven rain and roof splash.
• Natural walls rely on drainage instead of industrial waterproofing (concrete, plastics, tar, sealants), so drainage is the only reliable replacement.

Site drainage

• Regrade so surface runoff flows away from the foundation.
• Dig ditches uphill to intercept surface and subsurface runoff and carry it around the building.
• A curtain drain (French drain) is a deep ditch close to the building, gravel-backfilled, often with perforated drain tile at the bottom.
• Curtain drains are especially advisable on poorly drained (high-clay) sites and in rainy climates. They are also advisable if any interior floor is below grade or is an earthen floor.

Rubble trench foundation

• It handles freeze-thaw: water expands 9% on freezing, and the voids in the stone let it expand without harming the structure.
• It works in soils with bearing capacity above 2,000 lb/ft² (95 kPa).
• It protects against ground moisture and frost heave with minimal concrete, and may reduce earthquake damage by letting the building slide a little.
• An above-grade foundation or stemwall is still generally needed to hold the building together and lift the wall bottoms.
• Order of work:
  1. Peel off topsoil over the footprint and save it for grading and landscaping.
  2. Lay out the foundation and mark the wall centerline on the ground (garden lime, bright stones). Keep the trench centered on that line as you dig.
  3. Dig the high point below the frost line. Slope the trench from there to one or more outlet trenches at the lowest points.
  4. Run the outlets to daylight or to a dry well, sloped at least 1/8 in. per foot (1 cm per m) of horizontal run.
  5. Make the trench bottoms flat, free of loose debris and properly sloped.
  6. Lay a few inches of washed stone, then a 4 in. (10 cm) perforated drainage pipe.
  7. Fill with washed stone, compacting about every vertical foot. 1½ in. (38 mm) stone is easy to shovel, but larger random sizes are OK.
  8. Lay filter fabric over the stone before backfilling.
• In a sloppy oversized excavation, dry-stack larger stones where the trench sides should be, then fill between with smaller irregular stone.
• The pipe needs only about a foot of stone around it.
• Filter options: landscape fabric, asphalt-impregnated felt, reclaimed woven polypropylene tarps, burlap.
• Line the trench sidewalls with filter membrane too. Without it, groundwater movement and burrowing animals wash dirt into the voids and clog them.
• Cap the outlet end of the pipe with stout, corrosion-resistant mesh to keep critters out.

Concrete and bale interface

• Concrete can wick or trap moisture against natural materials.
• Never set straw bales directly on concrete. Set a pair of wooden sleepers on top to make a "toe-up" that keeps the bales away from moisture.
• RASTRA (foam-concrete block) is a foam-block system with steel inserted and concrete poured. It uses less concrete, needs no forms and insulates better.

Urbanite (recycled concrete) foundation

• Use broken slabs and sidewalks. Their flat shape and even thickness stack in stable courses, and they are often free in cities.
• Bed with cement-sand mortar and cap with a poured concrete bond beam. This combination has been permitted for a straw bale house.
• To lock cob to the stemwall, leave "dinosaur spines" (projections) sticking up from the middle of the stemwall.

Stone foundations

• Unreinforced stone is held by gravity and can crack and crumble in severe earthquakes. Reinforce with steel embedded in liberal cement mortar between courses, and/or cast a concrete bond beam above and/or below the masonry.
• Dry-stacked stone is very ecological but vulnerable to water intrusion, small animals and earthquakes.
• Sedimentary stone (e.g. sandstone) is much easier to build with than hard, irregular stone.
• Round river stones are very hard to stack and need lots of mortar. A slip-form is a better fit for them.

Wood foundations

• Post foundations raise the building on wooden posts, which suits hot, humid climates. They put the structure in the breezes and make termites easier to see and control.
• Post foundations suit lightweight wooden buildings only. Heavy thermal-mass walls make post-and-pier impractical.
• In seismic areas, a post foundation must be engineered to resist earthquakes.
• Posts can be set on concrete footings to spread the load.
• Heavy beams on grade put the building in direct contact with the earth, which suits passive solar designs. Wood is easier to shape and fasten than stone. The concerns are load capacity and rot.
• Rot-resistant redwood beams can sit on top of a rubble trench.
• A light straw-clay wall foundation was built with site-milled redwood boards bolted on both sides of the structural posts. The cavity was filled with lava rock for insulation and drainage, and the drainage layer extended below the stemwall.
• Avoid used railroad ties on gravel. The creosote preservative off-gasses and leaches harmful chemicals into the soil.

Earthbag foundations

• Woven polypropylene bags are filled with rammed earth, or with gravel in rainy climates. In rainy climates, place the bags atop a rubble trench.
• Barbed wire between courses acts as "Velcro mortar".
• Polypropylene degrades rapidly in sunlight, so protect the bags with plaster. How long plaster preserves them is debated.
• Bags filled with gravel are permanently structural. One option is to surround them with a thin reinforced concrete shell that holds the fill if the bags fail.
• Cob on bags: pound stout wooden stakes into the top course so the cob can't slide off in an earthquake.
• Straw bales on bags (dry climates): after the first course or two of bales, drive sharpened rebar pins down through the bales and all the way through the bag foundation.
• Straw bales on bags (rainy climates): add a drainage layer between bales and bags to prevent condensation on the water-resistant bag surface. For example, build a wooden frame the width of the bales and fill it with pea gravel or pumice.
• Post-and-beam walls on bags: posts sit either on a wooden beam pinned into the bags or on separate footers beside the bag foundation.

Rammed tire foundations

• Tires filled with rammed earth give a stable, earthquake-resistant base and suit curved walls.
• Tire diameter is much wider than the wall above. Sculpt indoor or outdoor benches at the wall base to deal with the mismatch.

Stone masonry: safety and planning

• Wear leather gloves and boots when moving stone, gloves for cement or lime mortar, and eye protection when cutting or shaping.
• Lift with proper body mechanics and get help for heavy stones. Start small.
• Unreinforced stone masonry is vulnerable in earthquakes, and stone is a poor insulator.

Selecting stone

• Stone must be hard and sound, not crumbly or fractured. Use a range of sizes, none too heavy to maneuver.
• Solid wall (foundation or load-bearing): needs the most precise fit. Each stone needs two relatively flat faces (top and bottom), ideally a third attractive face roughly square to them for the wall surface. Many stones of similar thickness help.
• Boulder wall: irregular or rounded stones, harder to stabilize. A slip-form wall is a more reasonable choice for beginners.
• Flagstone floor: flat, thin stones. Only one flat face is needed, because a rounded or irregular bottom can be dug in or buried in mortar.
• Veneer over brick, block, concrete or wood: all stones about the same thickness.
• Keep small flat wedge-shaped "chinkers" on hand to stabilize larger stones.

Sourcing and moving stone

• Check the site first, including just below the surface. A heavy digging bar probes under soil and levers stones up.
• To load a big stone into a contractor's wheelbarrow (handles several hundred lb), lay the barrow on its side, roll the stone in, then tip it upright.
• For uphill or uneven ground, make a carrying cradle (litter) from wire fencing with strong poles slid through. Four or more people can carry it.
• Reuse sources: fieldstones in old fencerows or piles, abandoned cellars, demolition sites, excavations for new construction. Road cuts also work, but don't destabilize the bank or roadbed.
• Check the legality of harvesting at seashores, creek beds and riverbanks.
• If buying, choose the most local source because transport often costs more than the stone. Inspect the stone before ordering for size, crumbliness and shape.
• Quantity: calculate the wall volume and add 50% to the rock pile for cracked, misfit and broken stones and shaping losses.

Shaping stone

• Sandstone and slate split easily along the grain. Granite and basalt (hard igneous) are the hardest to shape.
• Tools: an 8 lb (3.6 kg) long-handled sledgehammer to break big stones, and 1½–4 lb (0.7–1.8 kg) hammers striking with the square edge of the face. For precision, use a heavy cold chisel or brick chisel. A mason's hammer combines hammer and chisel.
• Stone tends to break along existing cracks or faults. Chips or pebbles trapped under the stone redirect the force.
• The easiest cuts are along a visible crack or removing a corner or weak protrusion.

Mortar for solid walls

• Mortar's purpose is not to glue stones together but to keep them from shifting and to block air, water and small creatures.
• Sand + clay: fine in dry locations but erodes if exposed to weather.
• Lime putty + sand: traditional.
• Portland cement + sand: the modern mix. Adding lime slows setting, which lengthens working time, and reduces brittleness.

Building a solid wall

• The sequence is practically identical with or without mortar.
• First course:
  1. Start with the most visible face. Line up a few stones in a row with a fairly smooth face and an even, level top.
  2. With mortar, remove each stone in turn, lay a mortar bed and tap the stone down into it.
  3. Build the second face the same height. A gap between the two faces is fine and gets filled with mortar and small stones.
  4. Keep the number of steps (level changes) in the first course to a minimum.
  5. Push mortar into all cracks with a mason's trowel.
• Later courses: each stone should touch at least two stones below. Dry-fit a row, then lift and mortar each stone and tap it down.
• Running bond: every joint between neighboring stones must be bridged by the stone above. Vertical joints should zigzag, not line up. At a step in a course's top, start the next course with a stone that evens the levels, and bridge the gap in the course above.
• Tie stones (bonding units): use long stones running through the full wall width to bridge the two faces. More tie stones make a stronger wall.
• A solid wall is strong in compression but weak in tension. Reinforce with steel-reinforced concrete bond beams under the base and on top, and partway up in high walls. They can be hidden by pouring between rows of narrow stones.

Slip-formed stone wall

• Stones are placed in a removable form with concrete between them. It is essentially a concrete wall with stone displacing much of the concrete.
• It suits unskilled masons and poor-quality stone, and works with round river rocks. It is also good for the beginner's boulder wall.
• It needs lots of concrete and lumber for forms, which raises cost.
• Concrete shows in large uneven joints and stains stones, so clean it off almost immediately.
• Steel reinforcing is easy to add for earthquake resistance. Insulation can go in the wall middle or on the interior face, which addresses stone's poor insulation.
• Forms allow only 1–2 ft (30–60 cm) of height at a time. Let the concrete set before raising the forms, from about an hour (with quick-setting additives) to two days.
• Mortar can be placed between stones as they go into the form, or poured in afterward. If poured afterward, point the joints with more mortar after the forms come off.
• Forms may be short sections moved laterally or full wall length. They range from lumber and plywood to pipe and welded steel.

Resources

• The Last Straw, Issue #16 (Fall 1996), covers many foundation systems for straw bale and natural building.
• Stone masonry books: Kern, The Owner Builder's Guide to Stone Masonry; Long, The Backyard Stonebuilder; McRaven, Building with Stone; Nearing, Living the Good Life (slip-form house).