When lumber prices come back down, so probably next year, I will build a deck in my backyard. The highest off the ground will be 2.5 feet (land slopes) and I was planning to have the posts 4 feet apart.

Or, should I use 6x6 and have a little more wiggle room if they aren't perfectly aligned and last longer?

The deck will be ~50'x33'.

At that spacing, I don't see why not, but 6x6's could be spaced farther. Might actually save money on wood and concrete for setting posts. Also a lot fewer holes to dig and less work to line up your columns.

ETA Column spacing will be driven by beam and joist sizes. You need to decide on what size beams you'll use, then look at their rated span. You also need to know the joist size you'll use to determine your beam spacing. After that, it's a matter of putting your columns wherever you need to to meet those requirements. Beams are usually doubled up 2x's, and joists are typically 2x6 or 2x8.

This might help: https://www.decks.com/how-to/articles/beam-span-chart-table#:~:text=Beam%20span%20maximums%20are%20based,and%20thus%20the%20beam%20supports.&text=A%20double%202x12%20beam%20can,10%20feet%20and%20so%20on.


Or this: https://www.finehomebuilding.com/project-guides/decks/how-far-can-a-deck-beam-span

4x4 is adequate. 6x6 usually will be overkill.

More important (as mentioned above) is beam spacing and joist size to include spans so they don't sag.

Also need to decide on decking material for best joist cavity spacing.

Make sure you get ones that are treated for use in ground. UC-4B.

cavscout96 said:

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Make sure you get ones that are treated for use in ground. UC-4B.

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Or get galvanized brackets that you can set in the concrete for the post to sit on. This way, they're not exposed to the ground and less prone to rot, and if one ever needs to be replaced you can simply take it out and drop a new one in. No need to bust out the concrete and repour with a deck and beam in the way.


Something like this, but there's many kinds:

https://www.lowes.com/pd/Simpson-Strong-Tie-Steel-G185-Post-Base-Common-6-in-Actual-5-in/3044772?cm_mmc=shp-_-c-_-prd-_-hdw-_-google-_-lia-_-213-_-framinghardware-_-3044772-_-0&placeholder=null&ds_rl=1286981&gclid=CjwKCAiA9bmABhBbEiwASb35V-1RcxNP-yZ2pVdcYWZPu5CZ7SmB5AjI27jEic8jtBC-ORR-yzt2oBoCG9cQAvD_BwE&gclsrc=aw.ds

I used 4x6's when building my deck several years ago. I'm sure 4x4's would be fine but I wouldn't use them. You never know what you may want to put on your deck so I'd go big. The 4x6's were used as the ledger and bolted to my slab with 8 inch lag bolts. I also used them as all the foundational beams. When you really think about cost, it's not that much. The ledger beams are the longest run but the foundation beams are much shorter pieces cut from longer beams.

That is a massive deck btw. Not sure there is a great way to keep costs down. Also you want to plan for future use of the deck and if you would ever want to do a hot tub or something else heavy. Because I did the deck myself, I over-engineered the heck out of it.

Its a math problem. Most decks are engineered to 3x designed load. So, will it hold up? Hard to know without running some calculations. Here is a snip from something I found online...

The load capacity of a 3-foot #2 grade 44 is 17,426 pounds, and a similar 66 is 20,834 pounds or 16% better. However, an 8-foot 44 supports 6468-pounds and 2339-pounds at 14-feet, while a 66 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 66.

That is for an "empty" deck. Add equipment and if you plan to put a hot tub.. well, you get the idea.

PabloSerna said:

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Its a math problem. Most decks are engineered to 3x designed load. So, will it hold up? Hard to know without running some calculations. Here is a snip from something I found online...

The load capacity of a 3-foot #2 grade 44 is 17,426 pounds, and a similar 66 is 20,834 pounds or 16% better. However, an 8-foot 44 supports 6468-pounds and 2339-pounds at 14-feet, while a 66 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 66.

That is for an "empty" deck. Add equipment and if you plan to put a hot tub.. well, you get the idea.

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Ok, so a hot tub which would be the heaviest thing you'd probably put up there. Lets say it holds 500 gallons. That's still less than 6000 lbs with the weight of the tub. Good size hottub will still be at least 8ft x 8ft. With his posts on 4 ft centers that load will be across 9 posts. Even if that deck were at your posted 14 ft height he'd be within 3x safety limits.

Unless you're building something out of the ordinarydecks are typically built to standard details that have already been engineered. I'd suggest reading up on the "prescriptive residential wood deck construction manual" put out by the American Wood Council. It's free and it's a great resource that meets typical building codes.

https://www.awc.org/codes-standards/publications/dca6

Do you have a similar resource for post-frame / pole barn buildings?

I am trying to determine post sizing and spacing on a 30 x 30 x 14 with a full loft at 10'

It's amazing how little I can find. I assume its because no one wants liability.

Kenneth_2003 said:

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PabloSerna said:

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Its a math problem. Most decks are engineered to 3x designed load. So, will it hold up? Hard to know without running some calculations. Here is a snip from something I found online...

The load capacity of a 3-foot #2 grade 44 is 17,426 pounds, and a similar 66 is 20,834 pounds or 16% better. However, an 8-foot 44 supports 6468-pounds and 2339-pounds at 14-feet, while a 66 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 66.

That is for an "empty" deck. Add equipment and if you plan to put a hot tub.. well, you get the idea.

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Ok, so a hot tub which would be the heaviest thing you'd probably put up there. Lets say it holds 500 gallons. That's still less than 6000 lbs with the weight of the tub. Good size hottub will still be at least 8ft x 8ft. With his posts on 4 ft centers that load will be across 9 posts. Even if that deck were at your posted 14 ft height he'd be within 3x safety limits.

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That's a live load though. Since it's dynamic, you need to multiply by a safety factor. You also need to add in the weight of the deck (more than typical because you need larger/more joists and beams for the tub) and allow for people standing or moving around the hot tub during parties or gatherings because they can be in the outer columns' tributary areas because you design for max load possible, not typical. You're probably in the 225-250 PSF range, which is 5x more than typical decks are designed to handle.


4x4's that close probably have the compressive strength to hold that, but then you also need to consider column to beam connections and how you will make attachments that can support that weight and can support the dynamic lateral loads of 5000 pounds of sloshing water and people in the tub over time. This is a lot of why you want the connection I posted above. With beams on columns the load of the beam is transferred to the top of column and you can bolt to the column so the beam can't slip or walk off. If you just bolt to the side, wood shrink and swell will guarantee that connection loosens over time. Then you get uneven loading, bowed columns, and a problematic deck.



Personally, I'd go 6x4 or 6x6 wherever there is going to be a hot tub, and 6x4 anywhere else with column spacing driven by joist and beam span ratings. 4' OC is kind of overkill and drives up cost and time. If OP's deck is 50'x33', that's 117 columns. With 16" OC 2x6 joists, he could span them at 9' or just 8' to be safe (with no hot tub). At an 8' joist span, he could do an 8' beam span with a double 2x10. That wild reduce his column count to 35ish, depending on final deck size and design. Then the cost comparison is how much extra you spend on larger beans versus how much you save on posts, concrete, and time. Longer beam and joist span means you spend $350-$400 more for beams (250ft of 2-2x10 compared to 450ft of single 2x6 at 4' spacing), but you save roughly $600 on 4x4 posts (assumes you buy an 8' post and use 1/2 per post) and $328 on concrete (80# bag per hole). Plus the time it would have taken to dig another 80 holes. So the beefed up beams and spacing save maybe $600 and a few days of work.

The correct way to do this is pour a slab with the tub resting on that and the deck is built to it. Don't do it much anyone but every hot tub I've ever built a deck for was never sitting directly on the deck for this reason.


And regarding deck post, 4x4 is adequate as long as you aren't lazy and set the posts deep enough with the proper amount of concrete

Bigger posts can accommodate a bigger beam means longer span and less holes in the ground. At that height you're not saving much in cost with a smaller post. Plus decks just look silly with 4x4s.

Sorry, I don't know if OP is doing hot tub. I just conjured that to get a load on his hypothetical deck and compare them to the load limits someone else posted.

I do agree with your heaviest loads going on larger timbers so they can be notched in rather than just scabbed onto the outside.