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Gap Between Deck And House: Is This Ok?

If you are preparing to build a deck for the first time, you will probably have a lot of questions about its construction. One area where there might be some uncertainty is whether or not it's ok for there to be any gapping between the deck and the house. We researched this construction question in depth so that you'll know for sure if this gap is recommended or not.

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When building a deck, it is highly recommended to establish a gap between 1/8 and 1/4 of an inch between the deck and the wall of the house.

Now that we know that there is a recommended gap to maintain between the house and the deck, we'll take a closer look at why this is important. You might also be curious about how much space you should leave under decking or if decking must comply with building regulations. For the answers to these questions and more, read ahead in this post to see what our research has uncovered.

Decking above storage area with wheelbarrow and tools View of under house, below decking with no privacy screen - Gap Between Deck And House Is This Ok

Why The Gap Between The Deck And The House Is Important

Putting the lumber for your deck against the wall of the house is not advised. Multiple professionals state that you should have at least some gap between these two structures.

But there is a sweet spot that you should aim for. If the gap is too much, the deck won't be as safe. But if it's not enough, you risk damage to the deck and your home's siding.

The reason you need a gap is due to moisture. The deck is made from lumber. Wood will absorb moisture over time, making it swell and expand. If you have the deck flush against the wall of the house, the deck will crack. It can also damage your siding.

But why is there a range for this gap instead of just a singular, uniform distance? There is a range because there are multiple factors that will determine how much the decking will swell. Let's take a look at the most significant ones.

The Type Of Lumber You Use Will Make A Difference In The Decking's Expansion

We know that lumber will absorb moisture. This absorption will lead to the wood expanding, especially during periods of high humidity. Some wood is better than others at keeping moisture out.

Cedar and redwood are commonly used lumber for decking. They have lower moisture absorption rates and will not expand as much. Know that no matter what lumber you use, you will need to periodically treat and seal the decking. This will keep moisture absorption at a minimum.

The climate you live in will not only impact how much your lumber will expand but will also help you determine what lumber you should use. 

The Climate You Live In Will Impact How Much Your Decking Will Expand And Contract

Wooden terrace seen from the floor

If your climate varies greatly from season to season, it will determine how much your decking lumber will expand. The more humid the air, the more the lumber will expand. So if you live in an area that has hot, humid summers but cold winters, this knowledge should be applied to how big of a gap you should leave.

In areas that have extreme differences in climate between the seasons, it's best to leave the maximum gap allowed. That gap should be 1/4 of an inch. But if you live in a spot that has level temperatures and lower humidity year round, a gap of 1/8 of an inch will be plenty.

The Season In Which The Deck Is Built Will Determine The Spacing You Will Need

The gap you leave between the decking and the wall of the house will shrink or expand depending on what time of year you build your deck.

If you are constructing the deck in the cold of winter, the gap you leave will surely lessen as the weather begins to warm up. If you don't leave enough space, the decking might swell into the wall.

The reverse is true in the summer. The wood will noticeably shrink when the season is over, pulling even farther away from the wall. Keeping this gap at 1/8 of an inch during a humid summer installation will allow the decking to contract to a safe, 1/4-inch gap in the winter.

You should use proper fasteners to secure the decking in place. While these will not keep the wood from expanding and contracting some, they do greatly reduce the amount that the wood will shift during the seasons.

How Much Space Should I Leave Under The Deck?

Your wooden deck will need plenty of room to breathe. Though there will be plenty of air circulating over the top and sides of the deck, it still needs a certain amount of room beneath it. 

We know that moisture isn't the best for lumber. The underside of a deck is the perfect environment for moisture to build up. But if you keep a deck far enough off of the ground, you eliminate a lot of the issue.

Decks that are at least 18 inches off the ground provide ample airflow. The moisture problem is minimized, allowing your lumber to have a longer life.

Decking above storage area with wheelbarrow and tools View of under house, below decking with no privacy screen

Do Decks Have To Comply With Building Regulations?

The International Residential Code (IRC) provides minimum standards for one- and two-family residential structures that are three stories tall or less. The IRC not only applies to the dwellings but also to pools, decks, and garages.

The IRC has been adopted by all 50 U.S. states. Its applications to decks include minimum railing height for stair rails, minimum railing height for top deck rails, as well as other specifications. These guidelines are meant to not only keep the users safe but also help maximize the lifespan of the structure.

Your local government may have enacted building codes that are stricter than the IRC. Consult with the building and development department within your community for their exact code requirements before you start your project. While most will follow the IRC standards to the letter, some will be a little tighter.

Inspector or engineer is checking and inspecting the building or house by using checklist. Engineers and architects work on building the house before handing it over to the landlord.

Can Deck Joists Touch The Ground?

If you've ever been under a deck, you will have noticed that the joists are usually on slabs of concrete. This keeps the deck level and also protects it from ground-level moisture. But is this type of build always necessary?

Deck joists can be on the ground, though it's recommended that they are put into the ground to help maintain structural stability. The joists are wood, so they must be treated and allowed to cure before they are ever in contact with the ground. 

If the joists are not treated, they will be exposed to all of the moisture that is on the ground. They will also be subject to pests like termites. These factors will greatly decrease the amount of useful life your deck has, so plan carefully and use treated lumber!

Close-up of worker putting new cedar boards on deck

Final Thoughts

Decks make wonderful additions to residential property and will help increase their resale value. To avoid damaging the deck and the house, you should always leave a bit of a gap between the two structures.

This will allow for the natural expansion and contraction of the wood, keeping it from cracking and from colliding with your siding and causing damage.

We hope this post on home decks answered all of your questions. For additional helpful information, we recommend reading the following posts:

How Much Space Between House And Pool? [In-Ground And Above-Ground]

What Are The Best Types Of Wood For Outdoor Furniture?

Should The Driveway Be Higher Than The Lawn? [And What Type Of Border To Use!]