4x4 vs 6x6 Deck Post: Which is Better? (2024)

Building a deck is a great way to extend the outdoor living space of a home or business. Making sure it is solid and structurally sound from the ground up though, is often a matter of 4×4 vs 6×6 deck post. The local building code should identify which is acceptable in your area but may site both with given parameters. So, which is better?

The load capacity of a 3-foot #2 grade 4×4 is 17,426 pounds, and a similar 6×6 is 20,834 pounds or 16% better. However, an 8-foot 4×4 supports 6468-pounds and 2339-pounds at 14-feet, while a 6×6 is 18032 and 10550-pounds respectively – or 64% and 78% more load capacity. So, for posts 3-feet and shorter, it’s a matter of preference, above 3-feet use 6×6.

In this article, we’ll explain 4×4 and 6×6 posts, when and where to use them, how to secure them to a beam, and the spacing between post supports. We’ll also look at posts for deck stairs and deck railings. By the end of your read, you’ll be able to identify which post is better for your deck construction to ensure it is structurally sound and code compliant.

Deck Posts

Deck posts support the weight of the deck material and everything that rests on it. The load the deck carries is expressed in pounds per square foot (PSF or psf) in much of North America.

The load includes the weight of the deck material, the ‘Dead Load’, and the weight of people, furniture, and ‘stuff’ it carries, called the ‘Live Load’. The dead load is commonly considered to be 10psf and the live load 40psf, resulting in the design load of 50 psf.

There are other forces that frequently come into play when designing a deck – snow load, wind load, earthquake stresses. Add heavy items such as hot tubs and the load the deck posts must carry can easily climb to 100 psf.

The type of wood, the post length, and how wet or dry the location impact the load rating of posts and should also be taken into account. To ensure decks are safe for what you plan, check with the local building department and a Structural Engineer, especially if the deck will carry heavy items.

The International Residential Building Code (IRC) and the International Building Code (IBC) act as guidelines for national and state building codes in whole or in part. It is advisable to check your local ordinances as neighboring jurisdictions can have differences – deck posts being a common difference. Deck posts must be a minimum of 4×4 according to the IRC but many jurisdictions require a 6×6 post as a standard. To avoid code violations, many deck builders choose the 6×6 even if the 4×4 is acceptable.

Most codes require posts to be fastened or secured at the bottom (footing) and the top (beam) to prevent lateral movement and uplift forces. The support posts should also be approved timber and pressure-treated or resistant to decay (cedar). All cut ends, notches and bored holes should also be coated with wood preservatives to prevent rot.

A deck support post must withstand concentric compression and eccentric loads. Concentric compression forces are vertical or axial and pass through the axis of the post – top to bottom. Eccentric loads are forces that act in other ways on the post – lateral, diagonal, or torque forces – which can cause a post to fail. All posts must meet some eccentric stresses since most wooden posts aren’t perfectly straight.

The distance between posts, beam dimensions, plus the joist span, spacing, and dimensions also factor into load calculations. However, for most decks, 50psf is a safe load value. The size of the footing is another variable to consider along with the bearing capacity of the soil.

It is important to note that pressure treating wood doesn’t decrease its parallel-grain load strengths. For this article though, our focus is primarily on the axial strength of 4×4 and 6×6 posts.

4×4 Deck Post

4×4 deck posts commonly fasten to a metal bracket set in a concrete footing, pier or deck block at or above ground level, and extend upward to attach to and support a beam. A 4×4 post is actually 3-1/2”x 3-1/2”, which does impact its structural strength. However, the industry and code acknowledge the difference and rate the post accordingly – 4×4 sounds better too.

4×4 Post Support Capacity

Determining the support capacity of a post involves several factors. The type of wood and length of post influence how much it will support before snapping. Another factor is the tributary area of the deck being carried by a post.

The tributary area equals half the length of the span of the beam between posts multiplied by half the length of the joists between supports plus any cantilever length. Posts at the end of a beam carry less than the posts in the middle of a beam or deck, so the middle posts may need to be thicker than the end posts.

Example: A 10ft x10ft deck with 10-foot joists spanning ~8-feet between supports and a 1.5-foot cantilever on a beam supported at the ends and in the middle would have a tributary area of:

(5ft + 1.5ft) x 2.5ft = 16.25sqft for the end support posts and (5ft + 1.5ft) x 5 = 32.5sqft for the center support post.

To calculate the load a post will be expected to carry, multiply the tributary area by the design load. So, 32.5sqft x 50psf = 1,625-pounds. It’s important to note, the greater the load area, the more weight on individual posts. So, as the load area increases, the post height decreases. Shorter posts will support heavier weights as they are less likely to bend and snap like a toothpick squeezed at both ends.

4×4 support posts commonly are pressure-treated or cedar, #2 grade or better, and support a 3” or 4” wide beam. How much they support is based on the tributary area and post height. In 2015, the IRC limited 4×4 posts to a maximum length of 8-feet. The table below reflects the current code.

Note: The 2018 IRC Table R507.3 identifies Tributary Areas of 20 to 160sq.ft. for Live Loads of 40, 50, 60 and 70psf. Table R507.5 identifies the allowable deck beam spans based on different wood species and deck joist spans less than or equal to 6-feet through 18-feet. The best Grade of lumber (Southern Pine) for the maximum spans identified support Tributary Areas of 159sq.ft.

4×4 Deck Post Spacing

Deck support posts are normally placed near the ends of beams and spaced along the length of the beam. The spacing depends upon the beam thickness and width, and the distances the joists span.

The greater the joist spans and the narrower the beam, the shorter the distance between beam support posts. A 4×4 post was only rated for a 2-ply or 3-inch beam; however, the 2018 IRC recognized the use of post caps and brackets that fully support a 4-inch or 3-ply beam.

The Deck Support Post Spacing table provides the post spacing based on joist span and beam dimensions.

From 2018 IRC Table R507.5

Maximum Allowable Height for a 4×4 Deck Support Post

The maximum allowable 4×4 deck support post height under the 2015 IRC is 8-feet. The longer a post, the more likely it is to bend under load than shorter posts, thus the limit set on the height of 4×4 posts. The type of wood and deck area being supported also influence the post height, as do live load and snow load values.

The 2018 IRC altered the allowable height of a 4×4 post yet again. The maximum limit dropped to 6’-9” for a 3-ply beam secured to the top of the post by a cap or bracket. A 1 or 2-ply beam, or equivalent, can still mount atop an 8-foot 4×4 post.

Attaching Beam to 4×4 Post

Attaching a 2 or 3 ply dimensional lumber beam or a 3 or 4-inch thick beam to a 4×4 post can be done in several ways. The beam should be centered or flush to the post sides so the weight is fully supported. Toenailing the beam to the top of the post is one practice. Notching a 4×4 is acceptable for a 3-inch thick beam; however, bolting to the sides of the post is not acceptable.

Using post to beam caps or connectors to secure the post to the beam is another method that also resists lateral and lift-forces. Caps and connectors are fabricated to fit 4×4 or other sized posts, so make sure to get the correct piece.

Caps sit on the post and the beam sits on and inside the upper part of the cap. Two section brackets may be installed before or after the beam is placed on the post, and can adjust to different beam thickness without bending.

Use proper fasteners to secure caps and brackets to the post and beam. Paint the cut ends with waterproof sealer and then cover the post top end grain with self-adhesive water-proof tape to further reduce the risk of rot too. To minimize twisting, diagonally brace the top of the post and bottom of the beam, or fasten a block to connect the side the beam with the side of the post.

Connecting 4×4 Posts Together

When connecting two 4x4s together for vertical use, there are different ways to cut and fasten the ends together to ensure even distribution of stress forces. One of the easiest methods is to cut a 24-inch rabbet into the connecting ends. Use an 18” galvanized steel post sleeve or two 18-inch galvanized steel plates. Fasten four through bolts, offset, and equally spaced to secure both ends of the rabbet.

6×6 Deck Post

6×6 deck posts offer a solid, stable base for most decks. They are commonly secured at or above ground level with a metal bracket to a footing, pier, or deck block, and to the beam at the other end to prevent twisting.

The nominal 6×6 is actually 5-1/2” x 5-1/2” and offers ample support to 3-ply dimensional lumber beams, which is why many deck builders and DIYers use them for all deck support purposes.

How Much Weight Can a 6×6 Post Support

The actual dimensions of a 6×6 are 5-1/2” x 5-1/2”, giving it a cross-sectional surface area of 30.25 square inches. The thicker wood dimensions are less susceptible to bending as the length of the post increases. As a result, a 6×6 will support a greater tributary surface.

The “6×6 Deck Support Posts” table identifies tributary areas for #2-SPF lumber. High grades or stronger wood species support greater areas for similar lengths. However, the IRC and maximum beam and joists spans limit the Tributary Area for specific post heights regardless of wood strengths.

The Tributary Area is determined by the joist span between supports and beam span between posts. A center post carries 1/4 of four areas based on joist and beam span; so essentially, its Tributary Area is defined by the joist span multiplied by the beam span. A corner post supports a 1/4 of the area defined by the joist and beam span, and an edge post supports 1/4 of two areas.

A post may be able to support more but the code sets the limits. The Tributary Area multiplied by the ‘dead’ (10psf) and ‘live’ (40psf) load rating, or the design load (50psf), identifies the weight the post may support.

The maximum Center Post Tributary Area based on an 18-foot joist span on a 3-ply 2×12 beam for #2 Grade SPF posts 7’-11” apart is 141sq.ft. (18’x7’-10”).

Multiply that area by the design load of 50psf and the load on the post is 7050-pounds.

Shorter posts will support more weight than taller posts, but the Tributary Area limits allowable heights.

Note: The 2018 IRC Table R507.3 identifies Tributary Areas of 20 to 160sq.ft. for Live Loads of 40, 50, 60, and 70psf. Table R507.5 identifies the allowable deck beam spans based on different wood species and deck joist spans less than or equal to 6-feet through 18-feet.

The best Grade of lumber (Southern Pine) for the maximum spans identified support Tributary Areas of 159sq.ft, which is why the maximum Tributary Area is 160sq.ft.

6×6 Deck Post Spacing

The beam thickness determines the span between support posts. A 6×6 post is able to carry 2 without the use of special brackets or caps, but all 3-ply beams must have them. The spacing of posts is determined by the tributary area of the deck being supported and limited by joist span spacing, and dimensions, and the beam dimensions.

Thicker beams will support more and allow for wider post spacing. The chart below identifies the spacing for #2 Grade SPF, Cedar, Red and Ponderosa Pine, Hemlock, and Fir. Select, #1 Grade, and Southern Pine will support more, allowing posts to be about 14 to 18-inches further apart.

-Values from Table R507.5 of 2018 IRC

Maximum Height for 6×6 Posts

The maximum height for a 6×6 support post in the IRC is 14-feet. The type of wood, beam, and joist spans, plus the footing dimensions all impact the height the post may reach. Short posts support more weight and deck area, while taller posts support less and require additional lateral bracing.

Attaching Beam to 6×6 Post

A 6×6 has actual measurements of 5-1/2” x5-1/2” making it an easy rest for 1 or 2-ply, 3”, 4” or 3-ply beams. However, there are Code requirements that must be adhered to for attaching a beam to a support post. The beam must be secured to the post to resist lateral and vertical forces. It also must be flush with the postface for full bearing.

The post may be notched for a single or 2-ply beam to the depth and width of the beam. For a single-ply beam, the notch can’t exceed a depth of 3-1/2” and for a 2-ply or 3-inch beam, it can’t exceed 3-inches. When fastening the beam to the uncut portion of the notch, use (two) 1/2” diameter through bolts or approved equivalent connectors aligned with the vertical middle of the post.

When splicing a beam, the splice must meet in the middle of the notch. Fasten the ends of each with (two) 1/2” diameter through bolts or approved equivalent connectors. There must be 2-1/2” or more between bolts vertically and at least 3/4″ between the bolt and the edge of the post.

Additionally, there must be at least 2” between the top bolt and the top of the beam, and 2-1/4” between the bottom of the notch and the bottom bolt. The distance horizontally between bolts must be less than 5-inches.

Pros and DIYers often use a 2×6 splice block on the open face of the notch to provide additional security against vertical and lateral movement of the beam, especially on posts over 5-feet. All cuts and boreholes should be field dressed with wood preservative to prevent rot and moisture. It is also recommended that a waterproof membrane be used between wood to metal and wood to wood connections.

Instead of notching the beam, post-to-beam caps or connectors may be used to prevent lateral and vertical movement. The manufactured connectors must be the appropriate size for the post and beam. Through bolts must have a washer under the head and nut.

Alternative approved fasteners are also acceptable. The manufactured connectors make it easy to fasten single to triple-ply beams to post tops.

4×4 vs 6×6 Deck Support Post: Verdict

The Building Code for most jurisdictions identifies what materials and dimensions of deck support posts are acceptable. The IRC sets the guidelines most locales follow. However, it is advisable to always check with the local building department as some only permit 6×6 and some permit both for specified heights.

Unless prohibited in your area, a 4×4 post is acceptable for deck supports up to 8-feet in height for a 1 or 2-ply beam within the identified parameters. If the deck height is greater than 2-feet, diagonal bracing at the corner posts is required parallel to the beam for lateral support – this applies to 6×6 beams too. Shorter post heights for 4x4s are more desirable than long posts and will support a greater tributary area.

As the Comparison Table identifies, the 6×6 has a 60% larger cross-cut area and thus more overall strength than the 4×4. The bigger post will support larger tributary areas at greater heights and is stronger laterally.

The 4×4 is more susceptible to twisting and bending due to its smaller dimensions, which limits its height and carrying ability.

Many deck builders and DIYers tend to use 6×6 support posts and so bypass any arguments or concerns.

Are 6×6 Deck Posts Better Than 4×4?

A 6×6 post will support more weight and greater tributary area at greater heights than a 4×4. For decks 4-feet or less off the ground, a 4×4 post is almost equal to a 6×6. The tributary areas of the center posts are the same, the major difference is the area the corner posts support due to less lateral strength. Additionally, a 6×6 has greater strength than a 4×4 of equal length.

Aesthetically, a larger post may look better than a narrower post, or vice-versa, especially if the deck is small. Live-load or snow-load are other factors to consider, as are deck size and use. Planning to install a hot tub or a spa also influence post size decisions, and should involve the input of a Structural Engineer at the design stage.

A 4×4 is less expensive than a 6×6 of equal length. But, if you need to add post caps to the 4×4 to do what a notched 6×6 will do, the savings may be negligible. For my money, I’ll use a 6×6.

Post Requirements for Deck Stairs

Post requirements for deck stairs depend on the type and span of the stringer. A cut stringer must be supported every 6-feet and a solid stringer every 13¼-feet. A 4×4 post may be used to provide support up to 14-feet. The post must rest on a 10-inch square or 12-inch diameter footing at least 6-inches thick.

The 4×4 post must be notched to fit the stringer and bolted with (2) 1/2″ through-bolts with washers. There must be 3/4″ or more of the stringer below the bottom bolt, and at least 2-inches of the stringer above the top bolt. The post should also be secured to the footing.

An intermediate landing must be built if the stairs exceed a vertical height of 12’-0” but may be used to shorten smaller stringer spans. Intermediate rectangular landings must be built as ledger-free decks, wide enough to support stairs at least 36-inches wide, and a landing width of no less than 36-inches. Based on the dimensions of the landing, either 4×4 or 6×6 posts may be used.

What is Better, 4×4 or 6×6 for Deck Railing Posts?

Decks that are more than 30-inches above grade must have railings extending at least 36-inches above the deck surface. The railing or guard posts must be nominally a minimum of 4×4 and fastened perpendicular to the rim joist with (2) 1/2″ through-bolts and washers. The 4×4 should not be notched to the deck.

A 6×6 will work as a railing post too but is over-kill unless there are other aesthetic or structural reasons for the heavier posts. The 6×6 is larger and checks and cracks will be more visible, however, it may not twist or bend like a 4×4. It can be notched by up to 2-inches to sit on the deck and fasten to the rim board.

The style of railing or desired look may require a different post, but for most deck railings 4×4 posts work well. Once they are tied in with the top and bottom rails and the railing cap board attached, they’ll resist twisting and bending.

Conclusion

The choice of 4×4 vs 6×6 deck post depends on the local codes, deck dimensions, and aesthetics. While a 4×4 under 5-feet will do the same job essentially as a 6×6, the more solid timber resists bending and twisting much better. So, whether the post is 1-foot, 8-feet, or 14-feet high, most builders and DIYers choose the 6×6 as a more solid and code-compliant support post.

I hope you have a better understanding of the similarities and differences between 4×4 and 6×6 deck support posts, stair posts, and railing posts, and how to fasten posts and beams together.

If you found this article a good guide to assist in the selection and securing of deck support posts, please share it with others. As always, your comments and suggestions are appreciated.