Cut and fill

The reference house's lot as the surveyor found it, contour by contour, falling toward the street. Step through the work from stripping the topsoil to the finished grade, set the house's floor height and watch cut and fill trade places, then steepen the lot and meet it another way: stepped footings, a walkout, a retaining wall or piers. Every number is the drawing's own: cubic yards, truckloads, the steepest slope left behind.

Land is never flat, and a house wants to be. Between the two sits the work of grading: cutting ground away where it is too high, filling where it is too low, and shaping what is left so that water runs away from the house instead of toward it. The figure is the reference house’s own lot, as the surveyor found it and as the excavator leaves it. Every number in the margin comes from the same model of the ground that draws it.

Reading contours

A contour joins points of equal height. Drawn every foot of height, as here, the lines are a map of the slope: where they crowd together the ground is steep, where they spread it is gentle. The slope is simply the height between two lines divided by the distance between them. On this lot the 1 ft contours lie about 12.5 ft apart, which is a fall of 1 in 12.5, or 8 percent.

Their shape says more. Contours that bend into a V pointing uphill cross a valley, where water gathers and runs; a V pointing downhill crosses a ridge, which water leaves on both sides. The shallow swale across the back of the lot shows as exactly such a V, which is why the house is not placed in it. Every fifth contour is drawn heavier so the eye can count them. A grading plan draws the ground as found in dashed contours and the ground as designed in solid ones; where a solid line leaves its dashed partner, the ground has been cut or filled.

Heights on a site plan

Every height on a building site is measured from one point the surveyor fixes before anything is touched, the benchmark. Here it is the top of the curb at the middle of the frontage, called 100.00 ft. The builder’s level reads every other height against it, so the plan can say where the house sits in the ground:

Point	Elevation	From
Benchmark, top of curb	100.00 ft	The survey
First floor	104.62 ft	The design: the number the floor-height slider moves
Top of foundation	103.66 ft	8 in above finished grade, so the wood stays dry
Finished grade at the house	102.99 ft	8 in below the ground as found at the middle of the house
Basement slab	95.99 ft	An 8 ft basement wall
Bottom of footing	94.83 ft	Well below the 42 in frost line

Wherever the finished ground is lower than the ground as found, earth is cut; wherever it is higher, it is filled. The art is to make the two nearly equal, because earth is cheap to push a few yards and expensive to send away: every load that leaves the site is paid for twice, once to dig it and once to haul it, and buying fill back costs the same again. So the designer first moves the house up or down until the yard can absorb what the house displaces.

A house with a basement displaces a great deal. Set where it is drawn, the reference house cuts 119 cu yd from its uphill side and digs 467 cu yd (357 m³) for its basement, of which only 206 cu yd goes back against the walls: 359 cu yd has to leave, about 45 truckloads. Raise the whole house 3 ft and the yard round it takes 89 cu yd of fill, leaving almost nothing to haul. That is the trade a designer weighs: a house that sits higher on its lot, with longer fill slopes and more steps to the door, against forty-odd trucks through the neighbourhood.

Volumes are slippery. Soil in the ground, bank volume, swells by a quarter or more when it is dug and loaded, so a 10 cu yd truck carries about 8 cu yd of bank. Fill compacted in place ends up denser than it was in the bank, so it takes about a tenth more bank soil than its finished volume. The figure counts in bank cubic yards and allows for both. Topsoil is kept apart from all of it: it is stripped first, piled, and spread back at the end, because nothing grows in subsoil and nothing should be built on topsoil.

How steep is safe

Pour any loose soil into a heap and its sides settle at its angle of repose, about 34° for dry sand. Finished slopes are kept much flatter than that, because rain, frost and time work on them:

Slope, run : rise	Percent	Use
50:1 to 20:1	2 to 5	Lawns, play areas; the fall away from a house
4:1	25	Comfortable to mow and walk
3:1	33	The steepest that can be mowed safely; what the figure grades to
2:1	50	The steepest cut or fill most codes allow without an engineer; planted, not mowed
Steeper	over 50	Held by a wall, rock, or a designed reinforced slope

Fill is the weaker of the two. It goes down in lifts of 8 to 12 in (205 to 305), each wetted to the right moisture and compacted, usually to 95 percent of the density a laboratory test can reach. On ground steeper than about 5:1 it is keyed into level benches cut into the slope, so the new soil cannot slide off the old as a sheet.

Water away from the house, and round it

The first rule of grading is that the ground next to a house falls away from it: at least 6 in (150) in the first 10 ft (3 m), a slope of 5 percent, on every side. The figure’s arrows show that fall. Roof water is carried the same way, by gutters and downspouts to splash blocks or pipes that let it out well clear of the wall. A footing drain helps, but it is the last defence; most basements stay dry because the ground was shaped right.

On a slope the uphill side is the hard one. The ground there runs toward the house, so the grading forms a shallow channel, a swale, where the fall away from the wall meets the cut slope rising beyond it. The swale itself must fall, here 2 percent, from the middle of the back round both sides to the side yards and down toward the street. And whatever the grading does, it may not send water onto the neighbours that did not go there before.

When the slope is too steep

A slope that cannot be laid back far enough is protected or held. Ground cover, mats of coir or jute pinned over new seed, and stone riprap on the steepest faces all keep the surface from washing away. Where there is no room for a slope at all, a retaining wall holds the higher ground back. A gravity wall of stone or mass concrete holds by its own weight, so it grows thick as it grows tall. A cantilevered wall of reinforced concrete stands on a footing whose heel runs back under the soil it holds, so the soil’s weight helps keep it from tipping; its base is commonly 40 to 60 percent of its height. Segmental block walls lock dry-laid units together and tie the soil behind them into the wall with layers of plastic grid.

Every kind fails the same way when it is not drained. Saturated soil pushes far harder than drained soil, and water trapped behind a wall can double the load on it. So each one has free-draining stone behind it, a perforated pipe at its heel and weep holes through it, and most codes ask for an engineer once it holds back more than 4 ft.

While the work is open

Bare ground erodes fast. Before the first machine works, a silt fence of woven fabric is trenched and staked along the downhill edge of the work, a pad of crushed stone at the entrance knocks the mud off truck tyres before they reach the street, and stockpiles are kept away from the fence and covered or seeded if they will sit. Trees that stay get a fence at the drip line of their branches: most of a tree’s roots lie in the top foot of soil, and a few passes of a loaded machine over them can kill it a few years later.

The choices

A gentle lot is met with a level pad. As the slope steepens, the pad’s cut and fill grow faster than the slope does, and the other answers start to pay. On a 20 percent lot the reference house on a level pad moves 930 cu yd of earth; turned into a walkout and set to balance, it moves 170 cu yd and gains a lower floor that opens onto the yard.

How does the house meet a sloping lot?

Decided by how steep the lot is · what the lower level is for · the soil and how deep frost goes · the cost of moving and hauling earth · where the water goes

Level pad, cut and fillThis house
The ground round the house is cut down uphill and filled downhill to one level, and the house is built as if the land were flat.
Choose it when the lot falls less than about 1 in 10 across the house; a full basement is wanted anyway, and its spoil can raise the low side; the soil makes good fill: it can be compacted and drains.
The catch earth that leaves the site is paid for twice, to dig and to haul, about one truck for every 10 cu yd; fill must go down in thin, compacted lifts or the yard and anything on it settles; the uphill cut slopes toward the house: a swale must turn the water round it.
Digging the basement No. 11 →
Stepped footings
The floor stays level while the footings step down the slope in level runs, each still below frost, with a crawl space behind short walls.
Choose it when the ground falls more than 1 in 10 under the footings, where the code requires them to step; no basement is needed; most of the ground can be left as it is.
The catch every step is another form, riser and cold joint: more labour per foot of footing; the downhill walls grow tall and must resist the soil and the wind; the crawl space still has to be kept dry, sealed or vented.
How a footing is sized No. 12 →
Walkout basement
The basement is buried on the uphill side and opens at grade on the downhill side, with a door and full windows.
Choose it when the lot falls about a storey, 8 ft or more, across the house: a slope near 1 in 3; the lower level is to be lived in, with light, air and a door to the yard; the downhill side faces the sun or the view.
The catch on a gentle slope it means cutting a yard down to the basement floor, or raising the house; the uphill wall holds back a full storey of soil and must be drained and reinforced; the framed downhill wall needs its footing below frost and clear of splashing ground.
The basement and its alternatives No. 10 →
Retaining wall and terrace
A wall holds the higher ground back so that a level terrace can run out from the house instead of a long cut slope.
Choose it when the lot is too tight for slopes at 3:1; level yard is wanted on the uphill side; the ground behind the wall drains, or can be made to.
The catch a wall holding back more than 4 ft (1.2 m) must be engineered; without drainage behind it, water pressure can double the load: stone, a pipe and weep holes are not optional; the most expensive way to deal with a foot of slope, and a wall is forever.
Piers or posts
The house stands on concrete piers or posts that reach below frost, and the ground underneath keeps its slope.
Choose it when the slope is steep, or the ground must not be disturbed: tree roots, wet ground, a flood zone; good soil lies deeper than it is worth digging a basement to; there is little money for earthwork.
The catch no basement, and a floor open to the cold underneath: insulate it and skirt it; posts alone rack: the frame needs bracing or shear walls down to the piers; pipes and ducts must run in insulated chases from the ground to the floor.
Piers among the foundations No. 10 →

Here The lot falls 8 %, about 2 ft across the house, and the house wants a full basement: a level pad is the plain answer. The basement's spoil is more than the grading can use, so most of it is trucked away; set the house 3 ft higher and the yard would take nearly all of it instead, at the price of a house standing taller on its lot and fill slopes running down to the street.

Where on the lot does the house go, and which way does it lie?

Decided by setbacks · the slope and where water runs · trees and ground worth keeping · sun and view · the length of the drive and the service lines

Near the front, long side along the contoursThis house
The house sits close to the front setback, its long walls running across the slope, parallel to the contours.
Choose it when the lot is deeper than it needs to be and its back is steeper or wooded; the drive, water, sewer and power should run short: every foot from the street costs; the house should span the least change in height, so the least cut and fill.
The catch less privacy from the street; a smaller front yard, and the back yard is the uphill side, cut and swaled.
Long side down the slope
The house is turned so that its long walls run downhill, across the contours.
Choose it when the lot is too narrow to turn the house the other way; a split level or a long walkout is wanted, stepping with the ground.
The catch the fall across the house grows with its length: on this lot 36 ft instead of 24 ft of slope, half as much again of cut and fill for a level pad; water coming down the lot meets a long wall end-on and must be split round it.
Up the lot, toward the back
The house is pushed back from the street, up the slope.
Choose it when the view or the quiet is at the back; the front is low, wet or in a flood zone; a long front garden or a deep setback is wanted.
The catch a longer, steeper drive; longer and deeper service trenches: the sewer must still fall a quarter inch per foot to the street; more of the lot disturbed and more trees lost.

Here Its ridge runs across the lot, along the contours, 30 ft back from the street: the 24 ft depth of the house spans only about 2 ft of fall, and the oak at the back is left alone.

How the figure decides

Ground	The lot from the shared site plan, on a 2 ft grid: 8 % fall to the street, 1.5 % across, a shallow swale at the back
Lot slope	Steepens or flattens the fall, pivoting about the house; the curb, and so the benchmark, moves with it
At the house	Finished grade falls 6 in in 10 ft from every wall, plus 2 % along the swale line from the middle of the back
Beyond	Cut and fill slopes at 3:1 until they meet the ground as found; nothing graded past the lot lines
Basement	Dug to the footings’ underside, 2 ft beyond the walls, sides laid back 1:1 (stiff clay)
Crawl space	Footings 42 in below the grade beside them, stepped in 8 in risers, each run level
Piers	15 piers below frost; the floor set for 18 in under the joists at the highest ground
Volumes	Bank cubic yards; fill counts 10 % more bank soil; trucks of 10 cu yd loose, soil swelling 25 %
Floor height	Each way of meeting the slope starts where a designer would set it; the balance button finds the height where cut meets fill

The footings themselves are worked out in Spread footings, and the foundation each strategy implies in Foundation types.