Deck Slope Calculator

Deck Slope Calculator turns a slope into total drop from ledger to rim, sets how low the beam sits, and checks a built deck from a level gap, in inches per foot, percent or ratios.

Deck Slope Calculator Slopes by Surface

A wood deck should fall 1/8 to 1/4 in per foot away from the house. Where your deck lands in that range depends on what water has to cross, and the deck slope calculator recommends a slope from the deck surface.

Gapped boards laid parallel to the house shed most rain through the gaps, so 1/8 in per foot is enough. Boards that run out from the house carry water along their length to the rim, which is why they get the steeper 1/4 in per foot.

Solid surfaces have a code floor. A membrane, vinyl or tile deck over living space is a roof, and IRC Section R905 sets a minimum slope of 1/4 in per foot for built-up and modified bitumen roofs, as the California Residential Code text shows.

Remodeler Doug Horgan of BOWA puts it plainly in JLC, writing that membranes go in at a minimum 1/4 in per foot slope because "this is code, and a good idea."

Mortar-set tile on top needs the same fall, and the deck slope calculator holds every solid surface to that minimum.

A deck is not built to the steep end by default. Fine Homebuilding forum members split between 1/8 in, about 3/16 in and 1/4 in per foot in a thread on sloping a composite deck, and one decking maker told the owner the gaps alone would drain it.

So the board slopes are building practice, not code. Only solid surfaces have a code minimum, and drainage troughs follow the slope their maker calls for.

Deck Drop Formula

Total drop is the deck run multiplied by the slope per foot. The run is the horizontal distance from the ledger out to the rim, not the length of the sloped joist.

$$D=R\times s$$

D is the drop in inches, R is the run in feet and s is the slope in inches per foot. In the deck slope calculator, a 12 ft deck at 1/8 in per foot drops 12 × 0.125 = 1-1/2 in from the ledger top to the rim top.

Working the other way gives the slope of a deck that is already built. Measure how far the rim sits below a level line off the ledger, then divide by the run.

$$s=D/R$$

A 12 ft deck whose rim sits 1 in low has a slope of 1 ÷ 12 = 0.083 in per foot. That is flatter than the 1/8 in recommended for boards parallel to the house, so the deck slope calculator advises 1/2 in more drop at the rim.

Checking Deck Slope With a Level

A level gives the slope without measuring the whole deck. Lay it pointing away from the house, lift the low end until the bubble centers, and measure the gap under that end.

The deck slope calculator divides the gap by the level length to get the slope. A 5/8 in gap under a 4 ft level is 0.625 ÷ 4 = 5/32 in per foot, or 1.3%, which drops 1-7/8 in over a 12 ft deck.

At the 1/8 in slope, the deck slope calculator expects a 1/2 in gap under a 4 ft level. A 2 ft level shows 1/4 in and a 6 ft level shows 3/4 in, so any level you own can confirm the slope.

Deck Slope Chart

The chart gives total drop from ledger to rim for common runs. Each value is the run in feet times the slope, rounded to the nearest 1/16 in the way the deck slope calculator rounds it.

Deck slope in meters works the same way. The deck slope calculator uses 10.4 mm per meter for 1/8 in per foot and 20.8 mm per meter for 1/4 in per foot, so a 4 m deck at 1/4 in drops about 83 mm.

Deck Slope Ratio, Percent and Angle

Slope can be written five ways, and plans, codes and digital levels each use a different one. The deck slope calculator converts between all of them with the same rise-over-run math.

$$G=(rise/run)\times 100$$

G is the percent grade. The ratio is 1 : (run ÷ rise), and the angle is the arctangent of rise ÷ run. A 1/8 in per foot deck is 1.04%, 1 : 96 or 0.6°.

A 2% slope drops 0.24 in per foot, just under 1/4 in. Over 4 ft that is 0.02 × 48 in = 0.96 in, and over 10 ft it is 2.4 in.

A 5% slope is an angle of 2.86° and drops 0.6 in per foot. One percent slope per meter equals 10 mm of drop for every meter of run.

How to Slope Deck Joists

Keep the ledger level and set the beam lower than a level line off the ledger top. The joists then tilt from the house down to the beam, and the whole deck surface follows them.

On the 12 ft example with the beam 9 ft out, the deck slope calculator sets the beam 1-1/8 in below level. With 2×10 joists, the beam top sits 10-3/8 in below the ledger top, and the rim drops another 3/8 in past the beam.

Run a string line or laser off the ledger top and mark these heights before you cut posts. Every post ends up as much shorter as the beam drops, and trimming a little less allows for footing settlement.

The beam and post layout comes from the Deck Post Spacing Calculator. The Deck Footing Calculator sizes the footings under those shorter posts.

Fixing a Deck With Too Little Slope

A deck that came out flat can be corrected two ways. Lower the beam by the missing drop at the beam line, or rip one tapered sleeper per joist, thick at the house and thin at the rim.

On a 12 ft deck with only 1 in of fall, the deck slope calculator finds that reaching 1/8 in per foot takes 3/8 in more drop at a beam 9 ft out. Sleepers on a 16 ft wide deck framed 16 in on center take 13 pieces instead.

Each sleeper starts 1/2 in thick at the house and tapers to nothing at the rim. That adds the missing 1/2 in of fall without touching the beam or posts.

The deck slope calculator treats water running back toward the house as the worst case. Rain then collects at the ledger, the spot most prone to rot and leaks, so reverse the fall before you replace boards.

For the frame itself, the Decking Calculator counts the joists, boards and screws a sloped deck needs.

Slope Limit at the Door

The IRC caps the slope at exterior door landings at 1/4 unit in 12, or 2%, under Section R311.3. The landing must be at least 36 in deep in the direction of travel.

A deck sloped 3/8 or 1/2 in per foot breaks that rule in front of the door. The deck slope calculator flags any slope over 1 : 48, and tables and chairs also start to rock past that point.

Ramp Slope on a Deck

A ramp up to a deck is limited to 1 in of rise for every 12 in of run, or 8.33%. That 1 : 12 slope equals 1 in per foot and 4.76°, so a 30 in high deck needs a 30 ft ramp.

The ADA defines a ramp as any walking surface steeper than 1 : 20, in its 2010 Standards. A 5% surface is therefore right at the line and not yet a ramp, while 1 : 12 is the steepest the ADA allows for one.

A 1 : 20 slope is 5%, 2.86° and 0.6 in per foot. Between 1 : 12 and 1 : 20, the 1 : 12 ramp is steeper, because it climbs the same inch in 12 in of run instead of 20 in.

A 1 : 10 slope is 10%, or 5.71°, which is steeper than an ADA ramp. The IRC allows up to 1 : 8 only where 1 : 12 is not feasible, so plan around 1 : 12 for a home ramp.

Ramp length, landings and rise in feet or meters come from the Ramp Slope Calculator. Car and basement ramps use the same math, and the deck slope calculator handles slopes up to 1 : 4, with limits set by local codes.

Deck Slope Calculator Answers

The answers below use the deck slope calculator defaults of a 12 ft deck run, 2×10 joists and a beam 9 ft from the house.

What is the correct slope for a deck?

Aim for 1/8 in per foot with gapped boards parallel to the house and 1/4 in per foot for everything else. The steeper slope suits boards running out from the house, solid surfaces and wet climates.

Is there a deck slope code?

The IRC sets no minimum slope for an open wood deck. It does cap door landings at 2% under R311.3, and it requires 1/4 in per foot under membrane roofing when the deck sits over living space.

Do composite decks need a slope?

Most still benefit from 1/8 in per foot, even with gaps. The slope keeps water from sitting at the ledger and in board gaps, so follow the board maker's guide and add slope unless it says otherwise.

What slope do UK and European decks use?

Fittings maker Eurotec allows about 2% for decks, which is 20 mm of fall per meter. Switch the deck slope calculator to metric for drop in millimeters over a run in meters.

How do you lay decking on a slight slope?

Lay the boards straight on the sloped joists, with the slope running the direction water should drain. Keep the gaps even, since a 1/8 in per foot fall is too slight to feel underfoot.

How do you find floor slope?

Use the same level method as a deck: gap under the level divided by its length. The Slope Per Foot Calculator and Percent Slope Calculator handle floors, patios and driveways.

Is there a deck slope calculator app?

The deck slope calculator runs in any phone browser with no install. Drops print in feet, inches and sixteenths, so you can mark posts on site from your phone.