[NCMtn]

Hey Everyone, I would love to hear some expert opinions on the best way for me to proceed on the below scenario.

I plan on building a 12 x 12 deck on a fairly steep slope in the NC Mountains, (6x12 pitch), where the wind gusts will reach 60 to 80 MPH every year. In addition I eventually would add a small bunk house on part of the deck in the future.

Part of my problem is in addition to being steep the ground is fairly rocky so digging the post holes in the desired locations for lining up the holes is a challenge. For this reason I have the following questions:

• Would using just 4 6x6 posts be acceptable. If not, would 4 8x8 posts work. I would rather dig 4 larger holes then 6 to 9 smaller holes which would almost be impossible to line up. I realize I would have to use larger joists etc., but that is fine.
• Would a cantilever style work better for my scenario since I could have the posts a little closer together and still achieve a 12 x 12. If so, how strong are they in the wind.

I do plan on using joist hangers and diagonal supports etc. to help with structural integrity.


Thanks for your replies.

 

[Nealtw]

Are you thinking about planting the posts in concrete? I wouldn't.
Measuring 12 ft on a slope is no rocket science.
You just need 2 straight 2x4s pre mark the distance on a 14 foot one and prop it up level for the high spot and at the lower spot plumb down to mark the hole.

 

[NCMtn]

Are you thinking about planting the posts in concrete? I wouldn't.
Measuring 12 ft on a slope is no rocket science.
You just need 2 straight 2x4s pre mark the distance on a 14 foot one and prop it up level for the high spot and at the lower spot plumb down to mark the hole.

Hey Neal, Thanks for the reply. Measuring where to put the posts is not a problem. Having trouble lining up is due to the rocky terrain. I was hoping to answer the below questions. Most 12 x 12 deck plans show at least 6 posts being used mostly with 4 x 4 posts. I am wondering if any one can confirm that using just 4 6x6 posts would be acceptable based on a 12 foot span in each direction. From my limited knowledge it appears so if the joists are at least 2 x 10's. Just looking for confirmation from more seasoned experts.

• Would using just 4 6x6 posts be acceptable. If not, would 4 8x8 posts work. I would rather dig 4 larger holes then 6 to 9 smaller holes which would almost be impossible to line up. I realize I would have to use larger joists etc., but that is fine.
• Would a cantilever style work better for my scenario since I could have the posts a little closer together and still achieve a 12 x 12. If so, how strong are they in the wind.

 

[Nealtw]

Hey Neal, Thanks for the reply. Measuring where to put the posts is not a problem. Having trouble lining up is due to the rocky terrain. I was hoping to answer the below questions. Most 12 x 12 deck plans show at least 6 posts being used mostly with 4 x 4 posts. I am wondering if any one can confirm that using just 4 6x6 posts would be acceptable based on a 12 foot span in each direction. From my limited knowledge it appears so if the joists are at least 2 x 10's. Just looking for confirmation from more seasoned experts.

• Would using just 4 6x6 posts be acceptable. If not, would 4 8x8 posts work. I would rather dig 4 larger holes then 6 to 9 smaller holes which would almost be impossible to line up. I realize I would have to use larger joists etc., but that is fine.
• Would a cantilever style work better for my scenario since I could have the posts a little closer together and still achieve a 12 x 12. If so, how strong are they in the wind.

4x4 are fence posts and normally it would 4, our normal would be 6x6 but the wind might want you to go to 8x8,

Cantilever is fine but the roof posts want to be directly above the lower posts


Finding the 3 hole is easy too.

 

[3onthetree]

Most 12 x 12 deck plans show at least 6 posts being used mostly with 4 x 4 posts. I am wondering if any one can confirm that using just 4 6x6 posts would be acceptable based on a 12 foot span in each direction. From my limited knowledge it appears so if the joists are at least 2 x 10's.

The number of posts has to do with the span they are supporting, and that will vary depending on where and how you place them (i.e. all on outer perimeter, or ledger on house with beam 10' out w/2' deck overhang, or freestanding with 2 beams, etc). A 4x4 is still sufficient to carry vertical loads, but a 6x6 allows for better beam/post connections. Per codes, AWC figures 6x6, but I believe IRC still allows 4x4, so you may have local codes that dictate the minimum size.

2x10 joists will span the 12', but you may not want the bounce. And your beam won't span the 12', so that would probably eliminate perimeter columns if that's what your plans were showing. You need a starting point, my preference for a high deck is having posts braced between each other (not just brackets back to the floor diaphragm), so that would require 2 beams, which would be 6 columns to start with. You should also design it now for the structure you are building later.

If you have rock ledge to maneuver around take locations of those so you can place your posts. If it's just rocky soil in general you feel will give you difficulty there is some leeway to slide a post over depending on how you set up your beam support.

 

[chandler48]

Not sure how the NC inspectors are, but across your southern border, if there is an incline you must dig continuous footers 12" wide and 12" deep. Sorry to rain on your parade, but it may be in effect. What part of NC are you in?

You can cantilever, but not more than 2'. Post separation is dictated by the span. I would use 6 ea 6x6 posts, secured to post bases on your footer. 2x10 will span the 12', but I would use 2x12 for outer bands and 2x10 for joists with hangers.

A little light reading : https://www.awc.org/pdf/codes-standards/publications/dca/AWC-DCA62012-DeckGuide-1405.pdf

 

[carpdad]

In nj and irc. Beam supporting the joists is usually double 2x10, supported by posts with load bearing piers every 8'. Going to triple 2x10 beam (or double 2x12) can span 12' and no mid supports.
Because 2' cantilever is ok, above beam does not have to line up. As long as the longest cantilever is less than 24". If piers are allowed (nj and I didn't know about the continuouse footing), I would start at 11' or so and see if you can line up the hole and if not, move it around within the 24" area. Beam will look skewed but still a working beam.

Your post anchor may have to be more robust than the usual post base with one anchoring bolt for the winds. You may have to use j bolt or base with flanges that are buried in the concrete. That means you need to plan and line up the anchors before pouring the concrete.

 

[NCMtn]

In nj and irc. Beam supporting the joists is usually double 2x10, supported by posts with load bearing piers every 8'. Going to triple 2x10 beam (or double 2x12) can span 12' and no mid supports.
Because 2' cantilever is ok, above beam does not have to line up. As long as the longest cantilever is less than 24". If piers are allowed (nj and I didn't know about the continuouse footing), I would start at 11' or so and see if you can line up the hole and if not, move it around within the 24" area. Beam will look skewed but still a working beam.

Your post anchor may have to be more robust than the usual post base with one anchoring bolt for the winds. You may have to use j bolt or base with flanges that are buried in the concrete. That means you need to plan and line up the anchors before pouring the concrete.

Thanks for the reply. That makes alot of sense. I'm seeing now that a cantilever is the way to go since the beams don't have to be perfectly parallel. Luckily where I am able to dig and miss the big rocks I can get down at least 30 inches so I plan on dropping the 6x6s in the hole with gravel and concrete. I really want these posts to last decades so I am thinking of ordering black locust, marine posts, or if typical pressure treated posts applying additional coating of some sort to the below ground portion. Any thoughts on the best option?

 

[carpdad]

I searched for "post flashing". Also search for ground contact 6x6 post. There are joist or beam treated lumbers for extra life. They have slots on the surface, so more chemicals can enter. Regular pt 6x6 will have decades of life, except the endgrains and if seeing constant wet/dry cycles.
Especially because chemical treatment may not reach the center and it gets exposed when they are cut. Above images show some ideas how to keep the end grains dry. Checks, splits and knots along the body also can rot but you can cover, if wanted.
Lead sheets are inert. Copper sheets (not too thin and bendable) for copper treated materials.
Flashings can be let into slots cut in the lumber, overlaps caulked and/or taped, and other ways. Physical barriers are best and not using much caulk or tape since they or the adhesion isn't for decades. OSI quad is polymer caulk with long shelf life and eternabond tape has the best adhesive I know. You can use them strategically since tape is fairly expensive.