House Deck building assistance needed

[5030tinkerer]

I am building a deck on the back of my house. The plans call for the post railings to be mounted in the inside corners of the joists. In two places, however, the 6x6 support posts are cut out at the outside edge to allow the joists to rest on them while still providing something to attach them to. They come up to the top level of the joists. The joists are 2x12's. I was thinking of cutting the remaining 4"x4" portion of the 6x6 that protrudes up to the top of the joists to create a half-lap joint for the 4x4 railing posts.

Given that the 4x4 posts are really 3.5"x3.5", I was going to cut the 4"x4" portion to leave 1.75" against one of the joists and just secure the half-lap with two 1/2" galvanized carriage through bolts. This will mean that I'll have a 1/2" alignment issue with the joint, but it will be under the decking boards where it won't be seen and the outer edge of my post railings will still line up nicely on the top side.

Is there a better way to do this? I don't like the reduced strength that these two posts will have, but figure that the railing system will help out quite a bit.

 

[CurlyDave]

I would consult an engineer about the strength of your railing.

Most building codes have a strength requirement, although they frequently allow residential railing to be less sturdy than industrial ones. In my opinion, this is a mistake. People give parties on residential decks, and railing leaning is is favored activity. I have seen teens wrestle with each other, putting enormous force on railings.

When my house was built, I had a wrap-around deck 14 to 18 feet above a concrete patio. A fall could be fatal. I specified the railing as no more than 1/2" deflection under a 200 lb. outward horizontal load perpendicular to the top of the railing, and no permanent deflection after the application and removal of a 1500 lb. load. This is far beyond any code requirements I know of, but I feel it is very safe.

 

[DennisArrow]

What is the overall size of your deck..........2X12 is more than adequate for LOTS of load......In my thinking run the half lap you already have lag bolted to the post......then use joist hangers on your stringers..........but not enough detail yet to get into any kind of railing issues........Curly Dave only gives specifications..........NOT HOW IT WAS DONE........

Good luck.......Dennis

 

[rtimgray]

On the decks that I have done, I've usually had the posts run continuously from the bottom of the post hole to the top of the railing. Where that was not feasible, I've used 4x4s or 6x6s (whichever matched the other posts) and carriaged bolted them to the rim joist/outside stringer/whatever (I would use through bolts with washers in lieu of lag bolts, just so there's no chance of pulling out). I would think that your plan would be more than adequate for railing.

Curly Dave makes some good points and has good ideas, but you can't engineer-out every hazard and everyway that somebody could hurt themselves. I guess that's what insurance is for.

Take care and good luck

 

[5030tinkerer]

The 2x12 joists aren't what I am concerned about having fail - it's the railing (though to answer the question, the joists are 16" OC with an 18' span from the ledger to the girder [girder not depicted in the attached drawing]).

The posts will be 4x4 pressure treat, wrapped to create a 6x6 feel without the checking that comes with 6x6 posts. The vast majority will be through bolted with two 1/2" HDG carriage bolts to the inside edge of the 2x12 rim joists. To prevent any of the PT lumber from showing through the 1x4/1x6 wrapping, the posts have been run through a planer/jointer to get them perfectly square (but this left them at only 3.38" square I am now realizing, thus weakening them a bit - not sure how 3.38" sq PT posts hold up to a 200 pound load 39" up at the top edge)).

Similar to CurlyDave, my deck is 11' off the ground with a concrete paver patio planned below - falls could potentially be fatal.

Concerns that I now have upon thinking this through more:

1. Are my 3.38" square PT posts enough to hold just generally?
2. Is having these posts extend downward 8" under the decking surface for through bolting to the 2x12 rim joists with two 1/2" carriage bolts adequate?
3. Understanding that I cannot simply cut off the 6x6 at the bottom edge of the rim joist pictured since it will leave nothing for the rim joist to connect to, I was planning on cutting them off 4" under the top edge of the joist, and then half-lapping the remaining 4". One through bolt would be on the 3.38" post to rim joist connection, another would go through the combination of the 1.69" or so half-lapped portion of the post, the 1.69" of the 6x6, and the rim joist, and a third would go though the lower bit of the joist (just connecting the 6x6 to the rim joist). Is this enough?

I suppose an alternative solution on #3 above would be to replace my two 6x6 posts with longer ones that extended up to the top of the railing or to use square steel tubing in lieu of the wooden core post, but both of these options are pricey. If necessary, fine, but I'd rather avoid it *IF* it will remain safe. Attached is a sketch of the deck - the red squares show the location of these more troublesome post connection locations.

 

[rtimgray]

Again, I would just install 4x4s or 6x6s or whatever combination you need to do to get the look you want and carriage bolt it to the rim joist. If a load were to slam into that railing, the point of failure could just as likely be the rim joist coming detached from the posts that go to the ground as anywhere else. Similarly, if you carriage bolt those as well, then it is likely that the point of failure would be the railing itself. Is it going to be 2x4s, 2x6s, steel, or something else entirely. As long as the vertical members are bolted to the rim joist, and the joists are bolted to the post, then I don't think you have anything to worry about.

Good luck and take care.

 

[5030tinkerer]

I don't for a second think that the rim joist will be giving way - I even plan to install blocking between the joists to ensure that no one joist has to do all the support work alone.

The real question is whether the amount of force necessary for a lap-jointed 4x4 to snap under horizontal load is greater than one could reasonably expect on a deck.

I don't want the railing itself to 'feel' spongy either, but don't know what else I could do to beef it up.

 

[CurlyDave]

I am not a licensed engineer and am legally not qualified to design your deck. For a deck this high you really need to consult with an engineer in your area.

But, if I were doing this, I would continue the 4x4 posts all the way from the ground to the top of the railing. As rtimgray points out if you attach a vertical member very firmly to the rim joist, all you really do is move the failure point to the rim joist.

If the 4x4s are continuous, I really think they will be strong enough. For the railing itself, I would go down to Lowes or Home Depot and buy the ready made steel railing, which both looks good and is engineered to meet building codes when installed as intended. The last time I looked it came in 8' panels, and they can be cut to shorter lengths. Do not try to splice them together, only cut. Use the fasteners and attachments supplied with the railings, extra ones are usually right next to them.

As far as supporting your joists goes, the 2x6 sheathing on your 4x4 posts will do a lot in that area, make certain it butts firmly and squarely against the bottom of the joist, and don't spare the nails holding it onto the post. You will not go wrong assuming a 150 lb shear load for a 16d nail.

What are you going to do for a proper post base and footer under your posts?

I have never liked carriage bolts as structural members. They are more intended as a means of holding something like a trailer deck to stringers underneath it. Go with a good machine bolt, grade 5 or 8, and use about a 2" diameter very heavy galvanized washer on each end to transfer the load to the wood. I used 5/8" machine bolts, but then I believe in overkill if I am going to be leaning on something where I could fall a long way to a concrete surface...

I am presuming that the areas where you say "no railing" are going to be attached to your house. If not, you need to think about some cross bracing to prevent the posts and deck from swaying and potentially collapsing.

Two other things.

1. Along the bottom edge of the deck in your diagram, what supports the rim joist? There is no railing, and if you just have a ledger board against your house, your design is OK. If the house is only supporting the lower left corner, I do not believe that a 2x12 is adequate for the load on that joist.

2. The 20'3" span is probably too long for 2x12s on 16" centers. I would strongly recommend turning your joists 90 degrees, so they run perpendicular to the way they are shown. This reduces the maximum span to 16' and also eliminates any questions about the rim joist at the bottom of the drawing. The joist on the right hand side of your drawing has a maximum span of ~ 4' and a 2x12 is more than adequate.

 

[5030tinkerer]

It might help if I include a more detailed and more accurate version of my deck diagram - see the attached.

Upon thinking about this more, I may well have run the joists the other way to reduce my span, but my framer already built it this way and the deck boards have already been delivered, so this way it will have to stay.

The span tables that I read show that treated southern yellow pine 2x12's can span 21'9" (Deck Joist Spans). Since I am less than that, am I not okay? I suppose I could add another girder where the house takes off on that 45, parallel to the one that's there, but I'd much rather not if I could avoid it. That said, if I am going to have a trampoline this way instead of a deck, I'll go ahead and do that.

I believe that all of the other questions are answered in the updated diagram. It's certainly easier to change things now than after the deck boards are down, so any and all ideas welcome!

 

[CurlyDave]

The span tables that I read show that treated southern yellow pine 2x12's can span 21'9"

As I read that table, the 21'9" span is for joists 12" on center. Your diagram shows the joists 16" on center, which is the next line down, a span of 18'1". If your actual is only 18'4", I wouldn't lose any sleep over it. If it is too bouncy, you can always come back and add another girder where you suggested.

That deck can't possibly be 11' off the ground with only 8 stair treads, but I presume the detail is only for the deck, not the stairs.

If you are going to bolt the railing posts to the rim joist, you want the bolts to be spread as far apart as practical so that the rim joist can better resist the torque from an outward force on the railing.

A better plan might be to place the railing posts exactly in line with your support posts and run your bolts through the support post, the rim joist and then the railing post. This way you have both the top of the support post and the rim joist resisting the torque from pushing on the railing. Machine bolts and heavy washers. The final assembly will be very strong, and sandwiching the rim joist between two posts won't hurt it at all.


You have to be careful that the deck boards stop maybe 1/8" to 1/4" short of the outside of the rim joist so there is no interference, even when the deck gets wet and expands slightly.

Think about what utilities you want on the deck: electrical outlet, lights, water spigot for a hose; and how you are going to get them there?

 

[5030tinkerer]

You are correct - 18'10'' is what the span table says for an L/360 result with a 10# dead load and 40 # live load. I'm a few inches under, so should be good to go (in that regard at least). I have to think that the IPE will help on the deflection front as well, along with the blocking. I hope to avoid the girder since I'd like to use the area under the deck.

You are also correct on the stairs - they are for illustrative purposes only. The deck top is actually 11'7" up from grade. The stairs are planned for later after I figure out what I'm going to do about a retaining wall issue (which impacts usable space). If I have the space (and it turns out we are using the deck alot), plans are to build a second, lower level deck. If not, I'll just build the stairs down as depicted, but have them either extend beyond the deck or have them return (either towards the house or back on themselves).

I remain undecided on the best way to bolt the railing posts. Originally, they were going to be bolted to the rim joist as you inferred. When I was adding in this extra detail, however, I thought about instead bolting them to the pictured blocking or possibly even using one bolt against the blocking and the other on the rim. I can't apparently exceed 6' OC for the railings, so the posts sort of have to stay location-wise about as drawn. Having railing posts ONLY where there are support posts won't be sufficient.

I will go ahead and make the migration to either 5/8" HDG machine bolts (if they make those?) or SS machine bolts with the larger washers as you suggested - thanks for that (and everything else, really). I was worried about the smaller washers compressing the wood anyway. Plans indeed are to place these as far apart as possible.

To give the deck a bit more character, plans are to run a 5.5" deck board parallel with the perimeter of the rim joists with about a 1" overhang over the 1" wide fascia wrap depicted and then run the other deck boards up to them. I've attached an even more updated drawing that depicts the extra blocking that will be necessary between the outer railing posts blocking to support the edge of these perimeter boards and the ends of the others.

Regarding the utilities, I already have a hose bib up there, an electrical outlet, a deck light, and even a gas supply from the house for the grill (no more running out mid-grill session!).

Plans are to run the railing tops at 36" or maybe a tiny bit higher (I have to see what the assembled product looks like), have the posts sit at 39", and then post caps above that.

Plans are also to run low voltage wires between the posts using a gap I designed into the railing top and then route (not looking forward to that) out a slot in the 1x wrap material to allow wire to get to railing post side lights and maybe even cap lights of some kind. I haven't yet figured out how to get the low voltage wires to the transformer, which could either be placed at the now-depicted electrical outlet or at one of the already existing outlets on the lower level beneath the deck (sort of voting for the latter on that as it seems likely that I'll want landscape lights down there, too, just anyway).

Lastly, as I am a bit concerned about the outer left edge of the deck being dark given the location of the house deck light and the stairs not being illuminated adequately (though embedded riser lights are planned), the outer corner rail post of the deck will sit 8' off of the deck surface with a switchable 110V light atop. To avoid concerns about that inner core wood twisting (as PT wood loves to do), plans are to use a 4x4x10 piece of cedar for that and wrap it with the 1x IPE. I'm a bit uncomfortable running 110V wire with so little protection as the low voltage stuff will have so am apt to route out the cedar post to accept that and cover the resultant channel with a 1" or so strip of 16 gauge galvanized plate to avoid the possibility/likelihood of hitting it later if someone at some point gets the grand idea to hang a flower pot or something off of that post.

What do you think?

 

[CurlyDave]

Somehow I was thinking that the red squares were your support posts, and you were asking about where & how to place the railing posts.

Are the red squares your railing posts, and if they are, where are your support posts?

I agree, the stairs will be dark, but cross that bridge when you come to it. Low voltage lights sound like a good solution there.

I made my deck surface tile instead of wood because I was not comfortable with the prospect of grilling on a wooden deck. Even with a gas grill.

In CA, it is illegal to use a grill on a wooden deck. I don't know about other states.

I haven't looked at low voltage lights in 3 or 4 years, but my experience has been that the low voltage bulbs are very costly, and have very poor quality control. I have had lifetimes of a week or less on some of the bulbs. If they make fixtures with LED bulbs, think about using them.

 

[5030tinkerer]

The red squares only indicate only the locations of the post railings. The blue squares, five in total (three on the girder, two at the outer edge by the stairs), represent the 6x6 support posts. This thread began with a question of how I could place a sturdy post solution in the two corners where my framer put 6x6 support posts flush with the top edge of my joists. It's long since deviated from that topic. I was really, really hoping to not have to replace those 6x6's to make them go all the way up, but that may well be what I end up doing.

I don't know much about low voltage lighting and cannot speak to the quality differences across brands. All I've determined thus far is that they can be pricey, the LED variety being even more so.

The fire issue was one of the reasons that I choose the ipe material - it has a class A fire rating, the same that is given to concrete and steel. You can check out Ipe As A Deck Wood? pros and cons of Ipe Deck Material for more information. That said, prior to your mention, I was not aware that grilling on a wooden deck was illegal ANYWHERE. It sure is common in the midwest!

Other reasons for selecting ipe were the hardness (and therefore the longevity), the fact that it wasn't a plastic material that would end up in a landfill someday, and its natural beauty. Cost was certainly a challenge, but not as bad as I expected - it was only maybe 4% higher than the top shelf composites.

 

[CurlyDave]

I see the corners you are concerned about, and it looks very difficult to get a good solution there. I might be worthwhile to just replace those two deck support posts.

As for the other ones, I think your blocking will make them sturdy enough. I would put both bolts through the rim joists. The reason being that resisting outward force is the most important function of those posts. Outward force on the railing will put the bottom bolt in tension. If someone pulls in on the railing the upper bolt will be in tension.

Nailing the blocking to the bottom of the railing posts is a good idea -- if you can get your framer to drill pilot holes so the nails won't split the blocking. I have never had any success with getting carpenters or framers to drill pilot holes -- all of the ones I have hired have never seen the use for them, and consider drilling pilot holes an insult to their manhood. If I make a big deal of it, they will drill them while I am watching, and will not the minute my attention is diverted.

I just don't see a good solution for those two posts at the top of your drawing.

 

[5030tinkerer]

Those two upper posts will likely be replaced with 6x6x16 lumber, trimmed down at the bottom edge of the rim joists to 4x4's and then likely trimmed again to 3.5x3.5 at the top edge of the decking to allow them to be wrapped, and then cut to length. If I move forward with the lower deck level at some point, I'll be able to re-use the current posts. Regardless, I can sleep better knowing that they are safer.

Another option I am considering is just forgetting the whole PT core concept altogether and just going to wrapped steel, either just for these two posts or for the whole enchilada. The steel is hollow to make my wiring materially easier and materially stronger than the 4x4's will ever be. I'm going to get prices tomorrow, though the (slight) concern then would be the hollow feeling you might get when you tap on them. I'm looking for solid.

While the framer installed what I have now, I AM the framer moving forward. Drilling pilot holes is no problem.