6

I am installing a wood railing post for an exterior railing and had planned to use the approach for fastening to a [new] concrete footing that is commonly used for deck posts. The post sits on a bracket that is elevated a couple of inches/few centimers above the concrete by a hefty steel screw that is embedded into the concrete when poured.

The tradesman working with me vehemently disagrees with this approach pointing out that a railing must withstand quite different - in particular shearing as a heavier person pulls on it - versus a deck post that must withstand primarily compression stress. The tradesman is concerned that a single bolt would not be sufficient to withstand the pulling on it.

I am still leaning towards the bolt embedded into the concrete due to the concern of rot when embedding the pressure treated lumber into concrete. It will rot.

So then - does the embedded bolt possess sufficient shear strength? I believe that to be the case but do not have a reference to back that up.

Note that there are mixed results when doing a web search for:

should I encase the railing stair post into concrete

The first two results I found resulted in a "split decision" on the choice.

So, is my preferred approach of using the bolt going to be safe for that pro football player grabbing onto the railing? Or do we sacrifice some longevity of the structure by burying the post into concrete so that it's definitely not going anywhere today.. but conversely will definitely be suspect in a couple to several decades?

WestCoastProjects
  • 639
  • 3
  • 15
  • Wood will rot if not maintain. Well maintain/protected wood will last for years. A post/pin in the centre of piece of will reduce the shear strength quite a bit compared to the full section of the wood. Most safety concerns are not what you will do, but what a drunken idiot will do on your land. – crip659 Jul 18 '22 at 10:37
  • Your tradesman is confused. Pushing and pulling on the top of a railing does not impose significant *sheer* force on the at the bottom end of the post. It's a *bending* force you see there. Sheer is what you would see if you sat down on the deck and push directly against the bottom of the post where it meets the deck - and you'll never apply much force that way. – brhans Jul 18 '22 at 14:41
  • 1
    @brhans The terminology [/confusion?] is _mine_ not the contractor. He described the forces but did not use the term "shear". Apparently "bending" is correct - glad to be corrected here. – WestCoastProjects Jul 18 '22 at 14:43
  • How is the lifespan due to rot not dependant on how much (gloop) the wood was soaked in? – Robbie Goodwin Jul 19 '22 at 17:50
  • @RobbieGoodwin Noone said it is not. Do you have specific info/recommendations about the preservatives? I've seen "mixed reviews" on supplementing pressure treated external ground exposure rated posts with additional protectants: some say to do so in the ground/concrete portions. – WestCoastProjects Jul 19 '22 at 17:57
  • Why not Edit the OQ to include that request… or would you rather treat SE as a substitute for a search engine? – Robbie Goodwin Jul 19 '22 at 18:01
  • 1
    Because *you* brought that up and I was responding to you directly. I can add it to the question, but the snarkiness is unnecessary – WestCoastProjects Jul 19 '22 at 18:06
  • Attachment of the bracket to concrete is the easy part, whether you use J-bolts or expanding sleeves or whatever makes hardly any difference. The most challenging aspect is affixing the wood to the bracket. A wood railing post will easily get ripped out of e.g. a Simpson ABA44Z if that is the only means of affixing it, even if the bracket is perfectly rigidly attached to the concrete. – whatsisname Jul 20 '22 at 04:56
  • Out of curiosity, why would embedding the post in concrete make it rot? (If anything, I'd expect the concrete to help _preserve_ the post, due to its extremely-high pH.) – Vikki Jul 20 '22 at 15:54
  • Apparently the concrete holds in moisture and the wooden posts do not like it. I'm just reporting what I read since I wonder the same thing – WestCoastProjects Jul 20 '22 at 18:31

4 Answers4

17

Here is a different concept, based on the vinyl fencing options where gates are going to be in the fence: at the gate posts they have metal inserts that are the same full length of the plastic post, go inside the plastic, and provide the real strength for the gate.

Use a steel post as a skeleton for a hollow wood cover post. The wood would only be the length of the visible railing. The steel post would go as deep as required into the concrete, and be almost as tall as the wood railing post.

Embed the steel post in the concrete instead, and treat it for anti-rust protection. Then set the hollow wood post over the steel post and fasten it towards the top, to not weaken the bottom of the steel post.

No football players will bend the underlying steel, and the wood cover completes the deck's look.

wood post over steel post

(picture from: https://www.doityourself.com/forum/fences-posts-railings-gates/520417-wood-sleeves-over-metal-posts.html)

Triplefault
  • 2,746
  • 4
  • 22
  • 2
    For shear strength that idea should also put a sizable dent into a car. Should be no worries about a drunken linebacker knocking it over. – crip659 Jul 18 '22 at 12:22
  • What is the fastener / fastening technique at the top? – WestCoastProjects Jul 18 '22 at 13:27
  • @WestCoastProjects, a bolt through the wood shell and steel post, countersunk in the wood shell, covered by molding or wood plugs might do. Even a bolt shaft without head or nut, 'shear-pin-style', through the steel post and behind wood plugs would keep the wood shell on. Or wood screws, driven from the open wood shell top, through pre-drilled holes at the top of the steel post, into the wood shell, before the cap is put on. – Triplefault Jul 18 '22 at 13:38
  • 1
    providing the actual source of the image is expected. – FreeMan Jul 18 '22 at 13:50
  • fyi these hollow 4x4's are pricey: I found them starting at $125. Then there's the post itself. Price does not *necessarily* invalidate the approach but it does not work for my budget. – WestCoastProjects Jul 18 '22 at 13:59
  • 2
    @WestCoastProjects That price is for already made 4x4s. Can pick up a plain 4x4 and have a guy(wood shop) drill it out or buy a couple of 2x4s and hollow out the centres with a router or saw. This is a DIY site, not what to buy site. – crip659 Jul 18 '22 at 14:06
  • 2
    @crip659 What to buy vs crafting a part by hand is part of doing it yourself when we have both time and cost constraints. I am doing the work myself here just do not want to spend too much doing it. I'm not comparing the price of item X at Amazon vs Home Depot but rather noting that a certain approach is *generally* much more expensive. This is not only a reasonable consideration for a DIY but an important one. – WestCoastProjects Jul 18 '22 at 14:12
  • 10
    You don't buy a couple of 2x4s and hollow them out - you buy a couple of 1x4s and and a couple of 1x2 and screw them all together (and careful inspection shows this is how the post in the picture is made too). – Martin Bonner supports Monica Jul 18 '22 at 15:41
  • 3
    @FreeMan, picture credit added – Triplefault Jul 18 '22 at 16:37
  • @WestCoastProjects cheap, easy, good. Which two would you like? – Harper - Reinstate Monica Jul 20 '22 at 01:13
  • @Harper-ReinstateMonica Good quality, then moderation / balancing of price and effort. – WestCoastProjects Jul 20 '22 at 05:58
12

You could try using a different type of bracket, like the ones shown below.

A post bracket: a square metal bracket with some through holes, attached to a base plate

The bracket is attached to anchor bolts, either set in the concrete, or with expansion sleeves in drilled holes. The wooden post is inserted into the bracket and secured with screws or bolts. You could leave it floating an inch above the base plate to prevent rot.

There are similar ones that can be cast in place. See for instance this video at 1:20.

SQB
  • 858
  • 7
  • 21
  • Curious to learn why you're not going with the cast in place ones from your own video. – SQB Jul 18 '22 at 14:22
  • Thank you! OP was stuck on making it "1 bolt". They also make 4-bolt varieties that conform to the 4x4 footprint, though 6x6 is an option. – Harper - Reinstate Monica Jul 20 '22 at 01:16
  • Keep in mind these alone are unlikely sufficient to support a railing post. IRC requires railings to sustain 200lbs of force at any point along them, which is at minimum 36" high. 200lbs at 34" of torque puts an enormous force on the fasteners affixing the post to the bracket, and can rip the post right out, if not the first attempt certainly after some wiggling first. – whatsisname Jul 20 '22 at 04:51
6

After further thought I do want to either bury the post in the concrete or at least have it laid flat on the surface.

In this case how can we retard the rot? Some recommendations I have seen include:

  • placing six inches of gravel at the bottom of the post hole before pouring the concrete. Allows seepage of water How to Protect a Wooden Post from Rotting in the Ground enter image description here

  • further treating the lumber

  • Using a sonatube and raising the footing several inches above grade. I am going to use an 8-inch diameter tube and do a 20 inch pour: 4 inches above grade and 16 inches below.

enter image description here

Older Answer

This is only a qualitative answer but I'm likely to go with it. I'm fairly convinced by this J-Bolt for which the J provides significant additional shear strength.

enter image description here

Taken from this video: Options for Fastening Deck Posts to Footings

WestCoastProjects
  • 639
  • 3
  • 15
  • 1
    Similar terminology issue here in your 'Old Answer'. The `J` provides no sheer strength at all to this method (and it doesn't need to). It resists the bolt being pulled out of the concrete by tension - and even that is only to prevent the entire railing being lifted straight up. Preventing the railing from falling over when your pro football player leans against it is up to the *bending* strength of the bolt and the *compression* strength of the concrete it's embedded in (and has nothing to do with the `J` on the end). – brhans Jul 18 '22 at 15:27
  • @brhans Fair enough. I read the J has the most effectiveness against _uplift_ such as for covered decks (wind lift) (is that called a _pergola_?) – WestCoastProjects Jul 18 '22 at 16:01
  • @brhans: and don't forget, also the strength of the attachments holding the wood to the bracket. – whatsisname Jul 20 '22 at 04:53
  • Posts that support a deck can be railing posts, because they'll have multiple 3/4" bolts into an edge beam and the two outer posts also have bolts into rim joists, holding the whole thing square, with the other end +4' in concrete. That link, way above, isn't going to be any stiffer than a fence in my mind (which is what it's to; a fencing company). – Mazura Jul 20 '22 at 06:43
  • If it's a *railing* then it *definitely* needs to not go anywhere. "cheap, easy, good. Which two would you like?" - "Good quality, then moderation / balancing of price and effort" - that's a **pressure treated 4x4 in a 10" or larger sono tube, at 3' depth *plus* w/e your climate demands.** 8" tubes at 16" ? Frowny face.... If there's a right angle kink to it, maybe that's fine. If not, that isn't. – Mazura Jul 20 '22 at 06:44
  • @WestCoastProjects: a pergola is a structure with an open lattice of wood providing shade and definition of space, it isn't necessarily over a deck. If it protects you from rain it basically isn't a pergola. – whatsisname Jul 20 '22 at 13:55
5

Use a concrete spur post

Wood will definitely rot at ground level - there's no question about that. So take steps to not only keep the structure intact when it does rot, but also make subsequent repairs easier.

Concrete spur post

Attach a spur post to the lower section of your wooden post, with 3 bolts through the holes as shown. Both the wooden post and concrete spur then go in the hole, and are concreted in place. You end up with about 50cm of spur below-ground and 50cm (the half with the bolt holes) above-ground.

Eventually the wood will rot at ground level, but it's firmly bolted to the spur so it still can't go anywhere. The wood above ground level remains sound, the spur is strong enough to stop a truck, and 3x M10 steel bolts will hold a lot, so the structure remains solid. And when (eventually) the post completely rots and needs replacing, all you need to do is bolt the new post to the concrete spur. The rotting remnants of the old post are still set in concrete, of course, but you can ignore them.

If you really wanted then you could even start with the wooden post slightly above-ground so that it doesn't rot off at ground level in the first place. This tends to cause problems with balancing the post-and-spur assembly when you're setting them in the ground though, so generally it's easier just to add 50cm to the post length you want and have the bases of the post and spur flush with each other.

Hint: You're much better using hex-head bolts instead of coach bolts into the wood, because you can't ever do up a coach bolt tight. Also use washers both sides to spread the load on the concrete and on the wood.

Graham
  • 1,949
  • 9
  • 13
  • This still has the wood below ground and encased in concrete. I don't see how this prevents rot or even how it makes it easier to repair after it rots. You still have to get the rotten wood out of the hole, remove the concrete around the "spur post", remove the bolts (embedded in the surrounding concrete), then line up and through bolt a new post, then concrete it again, knowing your going to have to go through the same thing in another `x` years... – FreeMan Jul 20 '22 at 13:23
  • "*...You still have to get the rotten wood out of the hole...*" You don't have to, because it doesn't matter. Just leave it be. And I'm not sure why you think there are any bolts embedded in the concrete - as I said, the top half of the spur post (the half with the bolts) is above ground. You simply rest the new post on the ground (on the ground-level remains of the old post that rotted off), drill 3 holes through the new post in line with the holes in the stub post, and bolt it back up. – Graham Jul 20 '22 at 15:04
  • _Both the wooden post and concrete spur then go in the hole, and are concreted in place."_ You're still putting wood in the ground. I don't see why. On my initial reading, I understood that the whole concrete spur post was to be buried. I guess I read that wrong. Still doesn't make sense to put any wood in the ground with this method. – FreeMan Jul 20 '22 at 15:31
  • @FreeMan Like I said in the second-to-last-paragraph, you could keep the wood entirely above-ground if you wanted - but the weight of the post is off to one side of the spur, so it's always unbalanced to that side, and that makes it hard to set up in the hole. Sure you can work around that (wedge it in the hole with old bricks or something), but it's much easier to have the bases of the post and spur level. Then it sits flat at the bottom of the hole and doesn't want to fall over (or not more than any regular post anyway). – Graham Jul 20 '22 at 15:49