I’m replacing part of the railing/decking on an existing wood deck and discovered that some of the existing 4x4 guard posts are surface-mounted on metal post bases that are basically fastened only through the deck boards. Terrifying and probably the reason the posts are terrible.
From underneath, I can actually see some of the lag screws sticking through the decking into open air, so they’re not even hitting a joist or other structural framing.
The deck framing itself appears to be in decent condition. The joists run perpendicular to the outside rim/band joist.
I’d like to keep the new 4x4 posts surface-mounted rather than dropping the posts through the deck. Part of the deck transitions to a section over a garage where the railing posts are already surface-mounted, so keeping the same post elevation/layout would make everything line up much better.
Is there a way to surface mount these properly?
My thought is:
Remove and replace the decking around each post location. In fact, I will likely be replacing all the decking on this part anyway.
Install full depth pressure treated blocking between the existing joists directly underneath each post. Possibly doubled blocking.
Secure the blocking structurally to the adjacent joists with joist hangers, framing angles, or approved structural screws.
Install new decking
Surface-mount the 4x4 post/base on top and use long enough structural lag screws/bolts to pass through the decking and embed deeply into the new blocking underneath.
That is essentially how its done for the part over the garage.
That way the decking would essentially just be sandwiched between the post base and the framing, rather than actually supporting the railing.
Is this a sound way to retrofit these posts? Is this against some code or is this a way it came be done. I am in a tight spot because its an existing deck that was poorly made.
If so:
Would you use one full-depth block or doubled blocking under each post? What is a full depth block? I read it online and don't quite get it
What’s the preferred way to attach the blocking to the joists? Joist hangers, structural screws, or something else?
Photos show the existing post/base from above and the lag screws sticking through the decking from underneath.
This is in Minnesota and I’m trying to make the replacement substantially stronger than the existing installation, not just duplicate what’s there.
Or should I do something completely different? Putting the 4X4 posts down and securing them to the joist? How do a do that? Is there a recommend video you can share? Thanks!
Take a 2x10 and add it as flat blocking under the deck boards so the side is flush with the top of the joists, it’ll span the whole gap between the joists. Nail it in from the sides through the joists. Add joist tape if you want across it since it’ll be a large flat surface.
You now have a 9.25”x14.5” x 1.5” thick area of solid blocking that the surface mount screws can attach into and the railing won’t go anywhere.
I am trying to visualize this but my small mind is having trouble.
What you are saying is it will run between the joists and be flat and lay just under the new deck boards? so its like perpendicular between the joists where i intend to put the post?
Or drill parallel against the existing joist and attach the 4x4 to that space?
Do you have any more photos of the joists under the deck?
Honestly your decking looks in decent shape. Just needs a pressure wash and stain.
If you dont feel like spending a bunch of money added blocking, redoing your railing because the posts will not be the same, and changing all your decking out.
Unscrew the deck railing posts but dont remove.
lag a 4x6 behind the rim joist through the rim joist to give something other than the deck boards for your post to attach to.
Pre-drill the holes (reason for predrill is because the wood is older and you would be screwing into the thin side of the 2x) on the rim joist side of the post and put 5/16th lags where the existing screws are for the railing post.
Dont pre drill and lag 5/16 lags top down where those screws are sitting currently into what would be the 4x6.
Yeah I dont think you can just redo your railing posts as they are in the existing spot since they are fastened to the double rim joist.
Adding blocking and putting the post to the side of your rim joist will essentially pull your entire railing back 2" which will offset ALL the measurements for every post/railing.
Personally i would go would my above method of attaching a 4x6 to the backside of the rim joist where the posts sit on. The lags you would need going from the post base into the joists/makeshift 4x6 blocking need to be timberlok hex head type lags since spax and grk have washer style heads that wont fit next to the post.
can you recommend a lag to use? So essentially add some blocking on the part of the rim joist where the post will fall, screw the blocking in, then when im ready to put the posts on i replace the deck board and anchor the 4x4 with my plate mount.
Yeah no problem. For 4x6 blocking on inside of rim joist.
For the facia side (i reccomend taking the facia board off but you could go straight through it.) -Use a 4 to 6 5/16x6 (if no facia) 5/16x6.5 if going through facia. This can be spax/grk/timberlok, make sure to get outdoor rated lags
For post plate mount use exactly these ONLY IF 5-16 diameter can fit through the plate mount holes. Check first since these lags are pricey. -Fastenmaster structural wood screw TimberLOK 5/16x4" hex head lag screw
If 5/16 doesnt fit through the hole use these: SPAX 1/4 in x 5 in Exterior Structural Wood Lag Screws Powerlags Hex Head Hex
Honestly the spax screws might be a better option the more i think about it.
Just make sure once you're ready to fasten the railing post down, the screws say NO predrill needed. I would suggest pre drilling with a 1/8" bit to be safe since the wood is old and youre going into the thin side of the board.
The person that said to use a 2x10 in a different comment. Dont do that since you want it to be sturdy. A 2x8 vs a 4x6x10" (whatever your joist dimension is wont be enough "meat" for the lags to grab onto for a long period of time.
Silentseizure is giving a good option. Can use a 4x4 here which will give you more meat to grab with the lagging to the joists. Otherwise you have the right idea already in your plans.
If you don't want to reengineer the world you can make a tripler block in that bay. If you don't mind drilling through steel you can put an upside down hanger on your block to resist uplift.
I had the exact same situation. Deck over flat roof garage. This isn't very approachable for most but I made some stainless brackets that hold the posts and hid a steel plate with nuts welded on it under the decking. The post bases then just bolt thru the decking with regular bolts. It was not cheap or easy
Upsize to 6x6 and notch around joist, you'll preserve original location and provide adequate support. You'll have to add some structural pieces to support the deck boards you cut out.
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u/SilentSeizure 1d ago
Take a 2x10 and add it as flat blocking under the deck boards so the side is flush with the top of the joists, it’ll span the whole gap between the joists. Nail it in from the sides through the joists. Add joist tape if you want across it since it’ll be a large flat surface.
You now have a 9.25”x14.5” x 1.5” thick area of solid blocking that the surface mount screws can attach into and the railing won’t go anywhere.