Craft Quantities
Plan & Takeoffs

Cross-market term

Fall and gradient

The slope built into a drain, a gutter or a flat roof so that water runs off it. Quoted as the drop divided by the horizontal distance, in whichever of three conventions the market uses.
  • 4Markets
  • NoSame thing everywhere
  • 4Calculators

What it is called, by market

One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.

  • United Kingdom

    Same word, different thing

    fall

    also gradient, 1 in 80, self-cleansing velocity, crossfall, backfall

    Stated as a RATIO, 1 in 40 or 1 in 80. BACKFALL is the defect — a length sloping the wrong way — and is not a kind of fall but the absence of one.

  • Australia

    Close, not identical

    fall

    also grade, gradient, percentage fall, crossfall, AS grade

    Commonly a PERCENTAGE, so 1 in 100 is written 1%. The arithmetic is trivial and the confusion is not: 1 in 80 is 1.25%, which reads as a smaller number for a steeper slope.

  • United Statesyours

    Same word, different thing

    slope

    also pitch, fall, 1/4 inch per foot, grade, cross slope

    Stated as INCHES PER FOOT — a quarter inch per foot is the usual drain minimum, equal to 1 in 48. PITCH here means slope, which is not the roof sense of the same word.

  • Canada

    Same word, different thing

    slope

    also grade, fall, 1:50, percent grade

    Mixes the American inches-per-foot habit with metric ratios written 1:50, so a single drawing can carry both and the units are not always stated on either.

The governing standard, by market

  • United Kingdom

    Approved Document H / BS EN 12056-2

    Drainage and waste disposal including minimum gradients for foul drains, and gravity drainage systems inside buildings.

  • United States

    IPC 704 / UPC 708

    Drainage pipe slope requirements by pipe size, and the alternative plumbing code's cleanouts and slope provisions.

Calculators for this

Each works in either measurement system, and the terminology on the page follows whichever market you have selected.

Drain Pipe Slope Calculator →HVAC Condensate Drain Pipe Slope Calculator →Low-Slope Roof Cricket Taper Calculator →Flexible Culvert Pipe Deflection Calculator (Iowa/Modified Spangler Formula) →

Frequently asked questions

Why can a drain be laid too steeply?
Because a drain has to carry solids, and solids need the water to stay with them. A foul drain works by the flow reaching a SELF-CLEANSING VELOCITY — fast enough to keep solids in suspension and scour the invert as it goes. Too shallow and the water crawls, solids settle out and the drain silts up. Too steep and the water runs away faster than the solids it was carrying, leaving them stranded on a dry pipe to dry out and block it. That is why minimum gradients are paired with maximums rather than being a floor to exceed, why a steep site uses a shallow-laid run with a backdrop manhole rather than one steep pipe, and why a length that has been re-laid steeper to clear an obstruction is a common cause of a blockage that keeps coming back at the same place.
Does a flat roof have a fall?
Always, and a roof that does not is the problem rather than the design. FLAT means low-slope, not level: a roof that is genuinely level holds water, and standing water is the enemy of almost every waterproofing system — it finds the smallest defect, it multiplies the load, it grows organic matter, and in freezing climates it lifts the membrane. Guidance therefore specifies a DESIGN fall large enough that the finished roof still drains after construction tolerances and deflection have taken their share, which is why the design figure is roughly double the minimum anyone wants to end up with. The fall can be built into the deck, into firrings on top of the joists, or into a tapered insulation scheme cut to a scheme drawing — and the last is the one that goes wrong on site, because the boards are not interchangeable and a pallet laid in the wrong order produces a roof that ponds exactly where the scheme said it would drain.
How do the three conventions convert?
By arithmetic that is easy and by a habit of mind that is not. A ratio of 1 in 80 means one unit of drop per eighty of run, which is 1.25 percent, which is 0.15 inches per foot. The trap is direction: in a RATIO the second number grows as the slope gets SHALLOWER, so 1 in 80 is gentler than 1 in 40, while in a percentage or an inches-per-foot figure a bigger number is a steeper slope. So a specification that reads *minimum 1 in 40* and one that reads *minimum 2%* are pulling in the same direction, and somebody converting carelessly can satisfy one while breaking the other. The safe practice on a drawing crossing markets is to state the drop and the run as two dimensions, which is unambiguous in every convention, and to put the ratio beside it rather than instead of it.