Fixing a soggy backyard in the right order: drain, raised beds, then a small deck

Sam and Priya bought a house with a back lawn that squelches from November to April. The yard falls gently from the patio toward the back fence, then flattens out, and the flat strip along the fence is where every storm ends up. They want three things out of the space: vegetable beds, a small deck to sit on, and grass that can be walked on a day after rain. This guide follows their plan from the wet ground up, using three calculators in the order the work has to happen, and ends with one combined materials order instead of three separate shopping trips.

The order matters more than any single number. Beds built on waterlogged ground drown roots, and a deck framed over soil that stays wet holds damp under the boards for years. So the drain goes in first, it gets a wet spell to prove itself, and only then do the beds and the deck go on ground that now drains.

Step one: the French drain

The wet strip runs 64 feet along the fence. At the low corner of the lot there is a grassy swale that carries water to the street, and it sits lower than anywhere along the fence, so that is where the pipe will daylight. Picking the outlet first is the design step that matters most; the French drain design post covers why, and what the alternatives are when there is no lower ground.

The trench needs fall toward that outlet. A 1% grade (about ⅛ inch per foot) is the usual minimum, which over 64 feet is roughly 7¾ inches. The yard is close to level along the fence, so all of that fall has to come from digging deeper as the trench goes. Sam and Priya start at 14 inches deep at the far end and finish at 22 inches at the outlet. That is the one place where a calculator input needs thought: the French drain calculator takes a single depth, and for a trench whose bottom slopes evenly the right figure is the average of the two ends, 18 inches. Because the volume of an evenly sloping trench is the same as a flat one at the average depth, that gives the correct stone quantity.

Input	Value	Why
Trench length	64 ft	Fence line to the swale
Trench width	10 in	Width of a rented trencher's chain
Trench depth	18 in	Average of 14 in and 22 in
Pipe diameter	4 in	Perforated corrugated pipe
Gravel cost	$52/ton	Example price for washed ¾–1½ in clean stone
Gravel density	100 lb/cu ft	Tool default; ask the yard for theirs
Fabric cost	$0.90/linear ft	Explained below
Pipe cost	$1.30/linear ft	Example price

The calculator returns a trench volume of 80.0 cu ft, of which the pipe displaces 5.6 cu ft, leaving 2.76 cu yd of stone, or 3.72 tons at 100 lb per cubic foot. Gravel costs $193.48, pipe $83.20 and fabric $57.60, for a total of $334.28.

The fabric line needs a word, because the tool prices fabric per foot of trench rather than per square foot of cloth. To wrap the stone, the fabric has to line both walls and the bottom and fold over the top: 18 + 10 + 18 inches, plus about 10 inches across the top and a foot of overlap, which is close to 6 feet of cloth for every foot of trench. Non-woven drainage fabric at an example $0.15 per square foot therefore costs about $0.90 per foot of trench, and that is the number to enter. If you type the price per square foot into that field, the fabric line comes out six times too low.

Before digging: in the US, call 811 a few working days ahead so buried utilities are marked; the service is free and the wait varies by state. And the stone must be clean, washed stone without fines. Crusher run or road base packs solid and stops water moving, which defeats the trench.

Let the drain prove itself

With the trench backfilled and the sod relaid, they wait for a proper rain, and in between they run a garden hose into the high end and watch the outlet. Water should appear at the swale within minutes and the hose end of the trench should not pond. If it does, there is a dip in the trench bottom, and it is far cheaper to find that now than after a bed sits on top of the line. They also mark the trench route with a few stakes so nothing heavy, including the deck footings, ends up on top of the pipe.

Step two: three raised beds

The beds go on the newly dry ground just uphill of the drain, at least a few feet clear of the trench so roots and future digging stay out of the fabric. Each bed is 6 ft by 3 ft, built from two courses of 2×8 boards, which stack to 14½ inches. Three beds that size would read as 6 × 3 ft in a plan, but the boards are 1½ inches thick, so the soil space inside each bed is 5¾ ft by 2¾ ft. That difference is bigger than it looks.

Dimensions entered	Total volume	Bags (1.5 cu ft at $9.50)	Bulk at $55/cu yd
Outside, 6 × 3 ft × 14.5 in, 3 beds	65.3 cu ft (2.42 cu yd)	44 bags, $418.00	$132.92
Inside, 5.75 × 2.75 ft × 14.5 in, 3 beds	57.3 cu ft (2.12 cu yd)	39 bags, $370.50	$116.76

Those are the raised garden bed soil calculator results with the Simple mix selected. Outside dimensions overstate the fill by about 12% on beds this narrow. As it happens, that is close to the 10–15% that a fresh compost-rich mix settles in its first season, so some people use outside dimensions on purpose as a built-in top-up. Sam and Priya prefer to see both numbers: the inside volume, 2.12 cu yd, plus about 15% for settling gives 2.44 cu yd, and the yard sells in half yards, so the order is 2.5 cu yd.

The calculator's Simple mix is 60% topsoil and 40% compost, which on 57.3 cu ft is 34.4 cu ft of topsoil and 22.9 cu ft of compost. On a 2.5-yard order that becomes 1½ yards of topsoil and 1 yard of compost, or a single pre-blended garden mix if the yard has one. Bulk wins easily at this volume; the bags versus bulk post explains where that crossover sits for smaller jobs. What the tool's bulk figure leaves out is delivery, which is why the soil should travel on the same truck as the drain stone, as described below.

Step three: a 10 × 11.5 ft deck

The deck sits beside the patio, low to the ground, with 5/4 × 6 pressure-treated boards running parallel to the house. The board run is 11.5 ft, so 12-foot boards cover each row in one piece with a little to trim, and the deck projects 10 ft into the yard. Joists are 16 inches on center. That spacing is the maximum for 5/4 wood decking laid square to the joists over multiple spans under IRC Table R507.7 (12 inches if the boards run diagonally); composite boards follow the manufacturer's own span table instead.

In the deck board calculator: length 11.5 ft, width 10 ft, board width 5.5 in, gap 3/16 in, board length 12 ft, joist spacing 16 in, screws, $2.10 per linear foot for boards and $0.12 per screw (both example prices). It returns 22 boards in 22 rows, 10 joists, 440 screws, 264 linear feet of decking and a total of $607.20.

The board count is worth checking by hand, because the tool always rounds rows up. Ten feet is 120 inches, and each board plus its gap covers 5.6875 inches, so 21 rows cover 119.4 inches. The 22nd row exists only to cover the last 0.6 inch, which no one would rip a board for. In practice the builder either opens each gap by a hair (21 boards with gaps of about 7/32 inch fill 120 inches exactly) or lets the outer board overhang the rim slightly. Either way, 21 boards are laid and the 22nd becomes the spare. They buy one more on top as cover for a twisted or split board, which is the kind of allowance the deck lumber ordering post goes into. Two screws per board at every joist, which the tool assumes, is also the code minimum for wood decking.

What the deck calculator does not touch is the structure. Joists, rim boards, beams, posts, footings and connectors come from a framing plan, not a surface-area tool, and they are left out of the budget below rather than guessed. Two site points do matter here, though. Framing this close to soil, with little airflow beneath, should be lumber rated for ground contact (the end tag says which). And many towns using the IRC exempt a freestanding deck under 200 sq ft and no more than 30 inches above grade from a permit, but not all do, so ask before ordering.

One combined order

Three calculators, three suppliers' worth of materials, but only two deliveries. The stone and the garden mix both come from the same stone and soil yard, and most yards charge per truckload, so ordering them together halves the delivery cost. The catch is timing: the stone is needed in week one and the soil weeks later. Yards will usually split a load if the two piles are dumped in different spots on the drive, and the soil keeps under a tarp. The deck boards and screws come from the lumberyard with the framing.

Gravel rounds up too. The calculator says 3.72 tons; the yard sells by the half ton, and running short of stone with an open trench is worse than having a few barrow-loads left over for a splash pad at the outlet, so the order is 4 tons.

Item	Quantity	Example price	Cost
Washed clean stone	4 tons (tool: 3.72)	$52/ton	$208.00
4 in perforated pipe	64 ft	$1.30/ft	$83.20
Non-woven drainage fabric	6 ft × 64 ft	$0.90/ft of trench	$57.60
Couplers and end cap	1 set	allowance	$15.00
Garden mix (60/40)	2.5 cu yd (tool: 2.12 + settling)	$55/cu yd	$137.50
Bed lumber, 2×8 × 12 ft, untreated cedar	9 boards	$30 each	$270.00
Bed corner brackets and screws	3 beds	allowance	$30.00
Shared delivery (stone and soil)	1 load	$85	$85.00
Deck boards, 5/4 × 6 × 12 ft	23 boards (tool: 22)	$2.10/linear ft	$579.60
Deck screws	box of 500 (tool: 440)	$0.12 each	$60.00
Total, excluding deck framing			$1,525.90

Every price in that table is an example to show the method, not a quote. Stone, soil and lumber prices vary widely by region and season, and delivery fees depend on distance, so put your own supplier's numbers into the calculators and the table before you commit. Each 6 × 3 ft bed with two courses takes three 12-foot boards (two cut in half for the long sides, one cut into four 3-foot ends).

Where each tool's model stops

• French drain: one length, width and depth, so a trench with branches or a change of width is best run as separate sections and added up. Fabric and pipe are per foot of trench, with no allowance for overlap, solid outlet pipe or fittings. The density default of 100 lb/cu ft suits many clean stones, but the yard's own weight per yard should replace it.
• Raised bed soil: volume is length × width × height of whatever you enter, with no wall thickness and no settling allowance, and bulk is priced by exact fraction of a yard with no delivery charge; real yards round up to a half or quarter yard and charge per load.
• Deck boards: rows always round up, even for a fraction of an inch; boards per row round up to whole lengths with no cut waste; there is no picture-frame border, stair tread or framing; and fasteners assume every row crosses every joist.

None of that makes the numbers wrong. Each tool answers a narrow question exactly, and the judgment calls (average depth, inside dimensions, the phantom 22nd row, one truck instead of two) are where a real order gets cheaper and more accurate.