Amount of Deck Boards Overhang; Advantages and Limitations


Overhanging decking is a good practice in decking building. The overhang not only enhances the look of the deck but also helps the deck to last longer. Protecting both the joist and the decking, especially wood decking.

Decking boards should overhang the deck substructure ¼ to 1 ¾” matching the stair nosing overhang. Overhanging deck boardsOpens in a new tab. not only make the deck look better but also helps to protect the substructure.

There are several reasons to overhang decking plus limitations in decking overhang. First, why is overhanging decking a good thing?

Overhanging Decking is good for your deck

Overhang helps to divert moisture away from damaging the deck, but appearance is the number one reason people overhang decking.

The overhang creates a nice look

Moisture concerns aside, decking overhang enhances the appearance of a deck. Like running boards on a truck. The small projecting decking aesthetically is pleasing to the sharp edge of the deck. Providing a small shadow along the top of the rim joist, which is pleasing to the eye.

Most of construction is an attempt to make things look and work well. Overhanging decking makes it easier to build a good looking deck.

Flush is not best, Intentional is

My carpentry shop teacher liked to say in regards to flushing material edges.

“If you can’t get it perfect, don’t even try.”

SAIT INSTUCTURE DAN

It is extremely hard to flush up material, especially composite or wood products that expand and contract with heat. Overhanging decking is the intentional answer. By overhanging the decking, it looks intentional, not sloppy. The eye can easily spot the small difference but becomes lost over great depth.

Joist should always be straightened before screwing down, but if the edge deck arches out a 1/8” in the middle. A straight line off flush-cut joist, you will see the proud edge of the joist sticking out in the middle. But if the decking goes from 1 ½” overhang to 1 3/8” overhang, you will never notice, especially with the fact that the difference is hidden under the decking. While you are standing on the deck looking down, you see nothing but the beautiful edge of the decking.

The overhang creates consistency with stair nosing

Most decks have stairs leading down to the yard. Tying the yard to the house, providing a smooth transition between the two. Having nosing on stairs improves the look while providing more tread in a smaller area. Stair nosing are a must.

The same should be done with all decking edges, have a consistent overhang as the stair nosing. Stair nosing is limited in Alberta to 25mm (1”). This provides a great reference for decking. Be consistent with the initial step off the deck with the remaining treads. Each thread must be 1” (25mm), and the decking around the deck should overhang the joist the same.

Overhanging deck boards improves the appearance of fascia boards

Overhanging deck boards protecting the top edge of the composite fascia. Especially composite material but all fascia will look better with overhanging decking.

Composite decking material is a wonderful product, especially the newer capped stuff. The cap enhances the decking visually while increases its durability, lasting many more years. But unfortunately, the edge of fascia boards isn’t as nicely cleaned up, becoming an unsightly part of the deck. Overhanging the decking covers and hides this unsightly part of the fascia board.

Creating a good looking deck is important, but the greatest threat to your deck is moisture. Your deck is outside and will get rain on it. That’s not the problem it what happens to that rain that is critical. Overhanging deck boards helps to solve the rain problem.

Overhanging Deck boards helps to prevent from fascia rotting

The overhang also stops water from getting behind the fascia and rotting the rim joist. It’s these areas of a deck that are most likely to rot. Where framing members are tightly joined, with just enough gap to allow water to get in, but to tight for water to escape. Overhanging decking avoids this problem by redirecting the water away from the gapping, preventing water from becoming trapped and rotting away the rim. 

Overhang protects joists from Moisture

Overhanging the decking directs the water off the rim joist, keeping the rim joist dry. Think of it as a roof with shingles. The decking is the shingle as the rain falls on the deck. The overhanging decking carries the water over and off the joist. Which is important for water is the number one killer of decks. Directing this water off and away from the joist prevents rot growing on the wet wood.

Exposed decking sitting on the top of the joist will dry quickly enough when the sun comes out. But the water that gets under the decking on top of joist is trapped. Trapped with nowhere to go except into the wood, bringing the fungi spores to life, destroying your deck’s joist. Overhanging decking keeps the water of the joist preventing it from being trapped under the decking.

Overhang works as a rain leader drawing water away

Yes, the majority of the rain will run off the deck regardless of decking overhang or not. But it’s the small drops that trapped under the decking which grows rot. Overhanging the decking gives the water a small path leading it away from the deck joist.

Remember the wick trick. Place a string in a cup of water and the water will wick up the string and out of the glass. The overhanging joist is the wick, or we can call it the rain leader. The overhang leading the water away from the joist and dropping unto the ground. A beautiful thing. 

Overhanging decking not only helps the deck’s substructure to stay dry, lasting longer but it also helps to keep the decking in pristine shape.

Overhanging decking helps wood decking to splits less

Wood deck boards splitting has weakened and destroyed many a deck. Deck boards will rarely spit from the middle, only from the ends. This is because wood’s elasticity it can open up around the screw without cracking, as long as the decking has enough material to pull it back together after the screw. Overhanging the decking provides this material. Wood fibres after the screw to hold the decking together, preventing splitting.

This principle should be applied even with decking in the field. Decking not screwed on the end will last longer. Butt joints will destroy decking as it splits from being screwed so close to the end.

If you are building a deck wider then the decking material, then set an insert board dividing the decking into consistent lengths. This will improve the appearance of the deck but also allow you to move the joist back a 1 ½” from the edge of the decking with directional blocks supporting the insert decking, which means no screwing the ends, splitting the decking.

Limitations for decking Overhang and challenges

Joist overhang should be limited to a 2-1 ratio

Especially decking overhanging on stairs, you want enough material to prevent them from being pulled off when someone steps on the edge of the decking. Being true with all decking, you don’t want someone stepping on the edge, and the decking board roll off the deck. You want sufficient material resting on the deck and fastening down.

I draw my guide from the decking joist. Joist can be cantilevered, over hanged two to one. For every two feet of support deck, it can overhang one foot. Yes, there are limits to joist overhang based on the joist depths, but still, the principle is always having more supported then overhanging. Putting a limit on how much decking can overhang. The edge of decking should never overhang more than one-third of the decking material. For your standard 5.5” decking, that is 1 ¾” overhang.

How much overhang should a picture frame deck have?

Picture frame decking should overhang the fascia consistently with the stair nosing, 1” (25mm). While initially installing composite decking when you will cover the rim board with composite fascia board, you will need to overhang the decking a 1 ½” (38mm) to account for the additional 9/16” (14mm) fascia board. Then once finished, the overhang will be a consistent 1” (25mm). Creating a safe and visually appealing consistent finish both of the steps and the edge of the deck.

Composite decking has limitations with overhang

Picture framing of composite decking is essential in cleaning up the decking ends. Replacing the unsightly decking end cuts with clean-finished decking edges. There are some challenges with overhanging composite fascia a 1 ½” (38mm) past the unfinished rim.

Scallop engineered composite decking will not be sufficiently supported if a scalloped groove lands on the edge of the rim. Creating a weak point as the decking will bend down under the weight of a foot. Frightening user with the sudden feeling of not being supported when stepping off the deck causing knee jerk responses.

One solution is to upgrade the picture frame decking to solid decking material. Composite decking without scallop grooves, being fully supported by the rim joist. Often this will mean upgrading to a more expensive product and finish. The highlighted change in decking finish will make the picture frame more distinct. Which visually is a great addition for the deck. A double win, a stronger, better-looking deck. Often, I would recommend the change even without the scallop’s challenge.

If your budget or design does not allow for a change in the picture framing decking, then another solution is with the fascia board. If you rim a 2×8, most composite fascia boards are 9 ¼” (235mm), allowing you to install them a little high into the scallop groove, providing additional material for the decking. Not as strong as dimensional wood but better than floating in the air.

Another option is playing with the overhang that the solid material part lands on the rim. Often this means you will reduce the finish overhang to ½” (12mm). Make sure to check the required decking overhang before installing the decking. That’s a lot of work moving all the decking one inch. A friend told me it was painful?.  

Ryan Nickel

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

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