Should A Deck Have A Slope?

A slight slope might allow water to gradually run off the boards instead of evaporating or absorb into the wood. This allows water to easily slide off deck boards more efficiently during rainstorms. It is also a small enough measurement that it won’t be visible nor will you notice it when standing or walking on the deck. If you have a freestanding deck, such as a low platform in the middle of your yard away from any structure, then a slope is not necessary since the direction of water egress doesn’t matter.

If your deck is attached to your house and the deck boards run parallel to your home, a drop of ⅛” per foot is more than adequate to shed water in the right direction – away from your home. Some might say that sloping your deck still won’t prevent water from pooling on deck boards because boards will inevitably cup and hold water regardless of slope.

However, proper installation of deck boards – ensuring the best face is facing upwards – will negate any significant cupping and allow the slight slope of the deck to shed water. If you have composite decking, which will not cup or one way or another, then a ⅛” slope is ideal and more than enough to shed water.

While you could keep a composite deck level, some water would stay and eventually evaporate.

This is dangerous because if you keep your deck level, you run the risk of water channeling down the width of the board into your house. To avoid this issue, consider sloping your deck ¼” each foot. Remember that the ⅛” suggested slope is more of a recommendation than a rule, and you should use judgment when considering the climate where you live.

Measurements of ⅛” to ¼” per foot are not noticeable to someone standing on your deck. Visually it would not be possible to notice any discernable difference unless your deck was spanning a long distance – say over 20’.

For instance, when you build a deck attached to your house, there are very specific rules for a ledger board and how it should be flashed properly. If your deck is level with an opening, then you immediately should know it wasn’t built to code. If so, then you don’t know if the builder – or previous homeowner – followed the building code. Once the deck boards are off, it is straightforward to run the planer down the length of a joist.

Reattach the joist and beam and continue down the length of the deck until the slope has been corrected to level. If all the deck boards are unlevel and slope in one direction, see the above process for leveling them all at once, which requires removing them, shimming, then re-installing.

If only a few are out of level, consider flipping them or simply buying new boards that are straighter.

These depressions can get quite deep, depending on your soil type, and sit for long periods of time.

Also, remember that frost heaves in cold climates can make slope change. Ensure the crown – which is the part of the earth closest to the house – is high enough to mitigate spring runoff issues.

Ensure the entire length of the slope beneath your deck runs away from your home, as spring runoff will need an outlet since the ground is still frozen. If you aren’t sure, know that a ⅛” slope per foot will not hurt the aesthetics or walkability of your deck.

Play it safe and give your deck a slope, and you won’t have to worry about pooling water.

I hope you found it informative and can apply some of these ideas or suggestions to your deck.

As always, don’t be afraid to drop me a line or comment on this article to let me know how it can be improved!