When and How Much Should You Slope A Deck?

Called many different things, deck slope, deck pitch or deck fall, but all are asking the same question.

How can you get water to drain off a deck?

How much should a deck be sloped for water to drain off?

How much should a deck be sloped to prevent water pooling?

Why slope a deck?

For years I have built decks with a 1/8″ (3mm) slope hoping to help water run off the deck, which created problems with installing deck railing. You may not be able to see a 1/8″ (3mm) slope with the naked eye on a deck surface, but the deck’s slope is multiplied over a 3′ (900mm) railing post. The top of the post being noticeable unlevel, requiring shims and micro-adjustment to plumb up the post.

Which drove me to research this question. What slope should a deck have? The answer is it depends if the deck is solid or not.

A solid deck needs to be sloped between a 1/8″ (3mm) and ¼” (6mm) per foot to encourage water drainage. Decks with properly gapped decking boards require no slope as rain will naturally run off the boards even when level.

Okay, why is that? Why does solid deck surfaces need slope, but gapped deck boards don’t?

“Slope is not of consequence for decks using spaced deck boards that allow for drainage.”

Alberta GovernmentOpens in a new tab.

There you have it, drainage. Gapped decking has sufficient drainage between the deck boards to drain excess water off the decking. Properly gapped decking will not pool water as the water will drain through the deck boards. It does not matter if the decking is treated, cedar, ipe, composite or PVCOpens in a new tab. rain will run off even if when level. Sloping a deck with gaps between the boards will not drain the rain any faster.

A deck’s flat surface is minimal in the size of puddle that can pool on it. As the rain builds, it will just run off the edge when it begins to pool. The more significant concern is water running towards the house off the deck. Here is the problem with solid decking; it acts as a giant funnel gathering together all the rain. If that water is not directed away from the house, it can go to the house, damaging the wall and foundation.

For example, if you have 1/4″ (6mm) of rain. That’s so little water it will do minimal damage. But with a solid deck12′ (3.6m) x 12′ (3.6m) gathering that water, it becomes over 20 gallons (75L) of water running into the house. With those low flush toilets, that’s the equivalent of flushing the toilet six times in your house while the drain is plugged. But with the same deck, but with gapped boards rather then the whole deck draining into the house, only one deck board is. A measly 0.8-gallon (3L) of water.

It’s a matter of volume. Gapped deck boards only collect water of one deck board with each individual deck board draining. With a solid deck, the entire deck collects rain, a much larger volume of potentially damaging water.  

Solid decks that need slope

These decks should be slope between 1/8″ (3mm) and ¼” (6mm). A 2% slope is enough for the rain to run off the deck.  But not so much as to feel unlevel while walking on. Here are some solid decks that require slope away from the house.

  • Vinyl membrane decking
  • Trex Porch Floorboards
  • Porcelain and Ceramic Tile
  • Interlocking or tongue-and-groove boards
  • Concrete patioOpens in a new tab. pads
  • Patio pavers

Any deck with a solid surface will need to be sloped to ensure run-off.

Timber Tech gapped decking boards an exception to sloping a deck

The hard and fast rule is a slope for solid decking and no slope for decking boards. But with every rule, there are always some exceptions. Timber Tech Composite decking is one of them.

“To aid water run-off, the deck should slope 1/2″ for every 8′ away from the house.”

Timber Tech Installation Guide

Roughly 1/16″ (1.5mm) per foot, a TimberTechOpens in a new tab. deck should be sloped. A standard 12′ (3.6m) deck dropping ¾” (18mm) from the house to the edge of the deck. A very minuscule drop in deck height.

I believe TimberTech concern is with water sitting on the decking. In a wet environment with little evaporation, this sitting water can encourage mould and mildew growth. But for most deck, the little water that does not drain off the boards through the gaps will quickly evaporate when the sun comes out. Quickly become a dry deck with little potential for mildew.

A shaded Timber Tech composite deck may be the one deck that will need the extra aid for water run-off. If your deck is shaded by the house or large trees preventing the sun’s heat from quickly evaporating the rain off the decking.

Another exception to deck slope, Trex Porch Floorboards

Trex floorboards should be installed under a roof in a porch situation. But if they are not under a roof they need to be sloped and treated like a solid deck even if they are installed like decking boards.

“When installing Trex Porch Floorboards in a non-covered environment, the porch structure should be slightly sloped to help allow for proper drainage. Joists should be sloped 1/8″ per foot away from the house to facilitate drainage.”

Trex 2019 Installation GuideOpens in a new tab.

This is because of there design. The edges of the decking slopes to hide and cover the boards gaps. With the lead board edge tucking under the following decking board. This close proximity and overhang of deck edges prevent rain from efficiently draining of the deck. Increasing the need for deck pitch to enhance run-off. If it looks like solid decking, then slope it like solid decking.

Preventing water pooling on a deck level deck

If you are concern about water pooling on the deck boards, there are three ways to minimize water pooling. Limiting individual boards from pooling water and rotting or slippery patches of ice on your deck. Before discussing ways to repel water off deck boards, let’s clarify the necessary gap.

Required gapping between deck boards for drainage

Decking boards on average need to be gapped 1/8 (3mm) to ¼” (6mm) to drain excess rain from the deck surface. Funny, it’s the same amount as a slope for solid decking. I guess, either gap it or slope it, but the number is 1/8″ (3mm) to ¼” (6mm).

All composite decking requires gapping for installation to allow for expansion and contraction. Trex goes as high as 3/8″ (10mm) when installed below 40°F (4.5°C), but in warmer temperatures ¼” (6mm) is required. TimberTech and most other composite decking are similar in required gapping. Most manufacturers have propriety concealed fasteners wit the gapped built into them. Providing both convenience and consistency. More expensive than screws but well worth it for its correct gapping space.

Wood decking requires gapping for drainage but is not as easy as composite decking. As treated wood decking will often shrink after installation as the boards dry. Most 5/4″ decking installed directly from the store can be installed tight but will sufficiently gap after drying. A 5 ½” board shrinking the required 1/8″ to ¼” to providing the gap for the deck boards after a season.

Cedar deckingOpens in a new tab. will shrink less than treated wood. Installing the decking with a small gap while installing will help in correctly gap the boards. CamoOpens in a new tab. hidden fasteners with their built-in spacer, helping to provide a consistent decking gap while installing the screws on the edge preventing small pools of water and sentiment from pooling in the screw head rotting out the decking.

Clean your deck regularly to aid water run-off

Mould and mildew growing on the surface of the deck will make it slippery when wet. Regular cleaning the decking will prevent the decking from becoming slippery.

Focus on removing all sediment build up between the decking boards. As with any drain, if it gets plugged, it won’t work. The small gap between boards can easily become filled with leaves or mud, requiring cleaning to allow water drainage. The sediment also acts as a sponge trapping water, further encouraging rot. A clean deck is a dry and safe deck.

Another way to ensure water run-off a deck is correct installation of the wood decking boards.

Installing wood decking bark side down will ensure rain does not pool on the decking

Wood decking cups when it dries. As the decking shrinks the larger wood cells around the edge of the tree shrink more in proportion to the inner cells. So, the bark side of the decking will shrink more, cupping the decking as the edges are pulled down. If the decking is installed bark side down it will curve edges down aiding the water to run of the board. If installed bark side up, the opposite will happen. The edges will curl up creating a small cup to hold water on the deck.

Correctly Cupped

The easiest way to remember it is, a smiling deck is a happy deck. If you install wood decking with the end grains smiling at you, the water will always run-off. As the core of tree will be in the middle and the edges pulling down towards the ground.

Seal the wood decking to enhance water run-off

Another way to aid water run off the deck is to regularly seal the decking. Wood decking that is sealedOpens in a new tab. will repel the water for it doesn’t soak into the decking, making it slippery. Water will only bead on the deck’s surface and run off even without a slope. Regular sealing of the decking will ensure water does not pool on its surface.

Summary of Deck Slope

Deck slope is for your house, not your deck. The slope diverting the water away from the house, preventing flooding and damage. Solid decking needs this slope, gapped decking drains and does not need to be sloped.

Water on individual deck boards can be minimized by cleaning the deck, especially the gap. With wood decking, the boards need to be installed to prevent cupping, along with regular sealing to repel water off the surface. Do these things, and you will enjoy your dry deck for years to come.

Ryan Nickel

A Red Seal carpenter, passionate about building decks to be enjoyed.

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