Hi all,
Got a deck question. Have a friend with a 16×24 deck. It has 2/2×10 beams sitting on 6×6 posts at 18″ from the house and at the outer edge. It is also lag bolted to the rim joist. Lag bolts go completely through the rim joist.
Posts are set on concrete footers which are 30″ deep. The posts are not bolted to the footers but rest on them with metal cleats mounted to the bottom of the post. Worked great for a year. Solid as a rock, he says.
However, after being up for a year, it shakes when anyone walks on it. The posts closest to the house appear to have shrunk, ot the concrete settled.
It appears to be structurally sound but not braced diagonally. Before the posts near the house shrunk I’m guessing that they provided diagonal bracing which no longer exists. I think that anyone walking near the outer edge of the deck has a 16′ moment arm with nothing to stop that movement. It’s not going to fall out or down, but there’s nothing to stop some sideways motion.
I’m not a deck guy, but I’m thinking he’s got two things to try: put Simpson standoff post bases on the post which have pulled away from the concrete and put two stainless wires for diagonal bracing with turnbuckles to tighten. That’s probably overkill but he doesn’t want it to shake.
Photos attached, forum friendly sized. Looking for advice.
Replies
Ken,
From your pictures it's hard to be sure that there actually are footings under those posts nearest to the house. I see the Simpson plates; are the footings just covered with dirt?
My suggestions will come as a carpenter/woodworker - not as a deck expert: Those PT posts didn't likely shrink in the long direction. Wood shrinks tangentially (across the grain), so you will have splits and swells in the width of the 6x6 posts, but the length won't change much, if at all. It's much more likely that the footings have settled, if you can confirm that they exist at all. Perhaps digging those footings out and replacing them with a monolithic pour would help (footing and slab in one pour).
As for the sideways racking...I'm not sure what that could be caused by. I certainly wouldn't go crazy rigging up wind lasses and cables when the problem seems to center around the rear posts.
Justin Fink - FHB Editorial
Edited 9/22/2005 9:32 am ET by JFink
>"From your pictures it's hard to be sure that there actually are >footings under those posts nearest to the house. I see the Simpson >plates; are the footings just covered with dirt?"Justin, There are footings there, just covered with a very thin layer of dirt.thanks,
Ken
> There are footings there, just covered with a very thin layer of dirt.
That's not a good situation. It would be much better to have the concrete extend 6 - 8" above grade, with those metal Simpson bases on top. You want to keep the bottom of the wood dry and at least make the subterranian termites get some exercise on their way to lunch.
Is there anything protecting this from termites and dryrot? If it were mine, I'd start with a couple 5 gallon cans of Jasco Termin-8. Depending on how dry it is, it might take a couple more.
-- J.S.
The contractor that did his deck poured the concrete even with the grade. I'll tell him about the termite protection you recommended.
Forget the stainless cables- put short diagonal braces from the beams to the posts in both directions. As the wood has dried out, the holes at the carriage bolts holding the beam/post connections together have probably enlarged, reducing any rigid connection they were previously providing. Couple that with the post base problem, and you have too "pinned" connections on each column, and nothing to keep the deck from moving around.
Bob
that makes sense Bob, and it reminds me of something I came across in the McFeely's catalog yesterday describing this exact problem. If the washers between the lag bolts and lumber are too thin, they will flex and crush/compact the wood over time--leaving wiggle room as a result.
McFeely's solution is a washer that is twice as thick as normal (1/4"?) to offer a more positive tightening without crushing the wood.
Justin Fink - FHB Editorial
Bob,What to do about the existing bolts through the beam? Drill those out now and place larger bolts in them now that the wood should have stabilized?This in addition to the diagonal bracing you talked about.thanks
Edited 9/22/2005 11:47 am ET by Ken
Here it is...http://www.mcfeelys.com/subcat.asp?subcat=20.18.6
Stainless Steel Structural Washers
View Image
Ever wonder why bolted connections in pressure treated lumber often loosen? The wood fibers are not strong enough to prevent crushing, and the washers are too thin, so washers deform and joints eventually loosen. These 1/8" thick washers are just the ticket. Available in two diameters, they won’t deflect and distort like standard thickness washers. Imported.
Fits Bolt Size
Description
Inside Diameter
Outside Diameter
Part Number
5/16"
Stainless Steel Structural Washer
11/32"
3/4"
SWS-0500
3/8"
Stainless Steel Structural Washer
13/32"
7/8"
SWS-0600
1/2"
Stainless Steel Structural Washer
17/32"
1-1/2"
SWS-0800
5/16"
Stainless Steel Structural Washer
11/32"
1-1/2"
SWF-0500
3/8"
Stainless Steel Structural Washer
13/32"
1-1/2"
SWF-0600
1/2"
Stainless Steel Structural Washer
17/32"
2"
SWF-0800
Justin Fink - FHB Editorial
Edited 9/22/2005 10:59 am ET by JFink
Thanks Justin
Help fight diseases -- donate computer time here.
Bob wins. I had the same thing happen on some deck rail posts recently... installed them extremely tight but after a while they were loose. Everything had dried out and the bolts all needed tightening. In your case that deck need knee braces / diagonal braces.
I'll tell ya', I don't like that post and beam detail one bit. Water will get in between the layers and start rotting that stuff out. Depending on your climate you've got between 5 and 15 years before it's time to replace wood (5 in Portland, 15 in Phoenix).
Bob,
One more quick question. By short diagonal bracing, do you mean not going all the way to the ground like the one in the attachment diagram?
Found the same deck instructions all over the web, this one is from Ace Hardware.
I call it knee bracing - don't know what they call it in your neck of the woods.Definitely helps with deck "sway." Not exactly sure why that deck is lagged to the house AND has a support beam 18" from house. On the outer beam, run your knee bracing from post to beam at a 45' angle - the longer the knee brace, the more stability.Do as the other poster above suggested and run diagonal cross bracing under the joists as well.With the ledger board lagged to house and joists hung from it, I don't think your problem is stemming from the rear beam. There shouldn't be that much play that close to the ledger board.Mac
View Image
Edited 9/22/2005 3:08 pm ET by BuiltByMac
Why no poured footings in your picture? Those are concrete piers right?
Justin Fink - FHB Editorial
12"x12"x8" Pier blocks with metal straps - posts are through bolted to the straps.
pier blocks are sitting on 6" compacted gravel in hole that's been backfilled and tamped around the blocks.I did pour sonotube footings for the cedar 4x4 posts that support the pergola. Basically, I made a concrete sleeve, resting the cedar post bottom on the compacted gravel. That was to help solidify the top-heavy structure.
Mac
View Image
Edited 9/22/2005 8:48 pm ET by BuiltByMac
Thanks, That's what I thought he meant on the bracing, just making sure.The deck has the ledger board and support beam 18" from the house because the homeowner insisted on it. He was trying to have it exceed code so it would be sturdier.There's not that much play near the house. I was thinking that the rear beam might have initially taken care of some of the sway until the deck had aged a little.
Yup- what Mac showed. Of course, you could go all the way to the ground, but that looks pretty crummy and limits access to the area under the deck.
Bob
Run the diagonal board and you won't need the knee bracing.
Just screw/nail a piece of 1x4 diagonally across the bottom of the joists, from corner to corner, fastening at every joist. If a single piece won't reach, just use two pieces lapped side-to-side for a couple of joists. The piece doesn't have to reach all the way to the edges, but should at least reach to the next joist in from the rim joist.
A pair of steel cables in an X would also work, but since steel doesn't expand/contract like wood it won't work as well.
Good point on the expansion rates, hadn't thought of that.
Help fight diseases -- donate computer time here.
There are several thiongs going on IMO, most of them easy to fix.
The beam has a fairly long span depending whether you have much snow load or not.
The joists are held to the ledger at the house securely and when the joists shrunk ( not the poosts, but the joists themselves) up away from the posts, that joint is left loose.
First need is to add some knee braces or kickers - diagonal braces from post to beam. Each one only needs to nbe 18-24 " long. That will add sway bracing and shorten the span of the beam at the same time. Short ones like this can do the job without impinging on the view or headroom very much. Put them in cut at 45° and not alongside the post or beam like was shown in the cartoon image. That would depend entirely on the shear of the fasteneers to owrk, but fit in tight directly will make use of compression. use hot dipped galvies or SS screws to fasten them.
Then you need to tap a cedar shim in each joint between joist and beam to take up the slack where the joists left space when the shrunk. Do it judiciously or you will add too much to one and leave others loose. This will load the beam again and transfer some of that load to the piers and ground.
Welcome to the
Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!
Hey Mac, what is that bottom Post to Footer connection I see in your pic ? I see a thru bolt (?) but no post base hardware. Is there some sort of interior tongue let in to the post bottom ?
On that deck, I used pier blocks with a metal strap. Set the post next to the strap, drill through and bolt with 1/2" carriage bolts. Once the pier block's been buried, it's a pretty solid footing. See the picture in my above post for a close up of the pier block.Mac
Thanks Piffin.I need to send you pictures of my own deck. I actually found Piffin screws in it the other day.
Help fight diseases -- donate computer time here.
I tack my knee bracing on with a couple of screws, then predrill and use an impact wrench to run 1/2" galv. lag screws into both connections. If the client is paying for it, I'll run them deep and plug the hole.Mac
View Image
Edited 9/22/2005 8:48 pm ET by BuiltByMac
Good looking detail
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Ah. I've seen some older decks where they used blocks like that (with an embedded redwood block for nailing) but I've never seen embedded straps like that.
I wonder if it's a regional thing - what part of the country are you in?Mac
'very' southern California ..
I hit that reply, then thought, hmmm, wonder if he filled out his profile. sure enough! high end decks, huh? Cool! I'll have to check out your website - I can never get enough of nice looking decks!Edited to add...
After looking at it just now, I realized I've seen your website before. You do some seriously kick-a$$ work, man! Very nice looking structures -Mac
Edited 9/22/2005 9:23 pm ET by BuiltByMac
Ken.
I agree with the idea of lateral "Wind Shear" supports. I would use a 1X6 so to get three nails per joist. Not sure of the joist spacing, but looks like 2X10's. What is the span of the deck? I would also add blocking at least mid-span will help prevent the side sway. Could also be added above the last beam since the cantilever is quite large.
Curious why the first set of posts so close to the house. Increasing the joist size or decreasing the joist spacing might have eliminated them all together. Then again not sure of all the specs from the Pic's.
"What is the span of the deck?"Working from memory here, but it's 16' from the house to the outer rail. Outer beam was about 18" from the edge and the inner beam was about 18" from the house. So I'm guessing widest span between any two beams was about 13'. Width was 24'.
Sorry..Working off my short term memory. I quess that was in your opening post.
Why did your friend put a post/beam @ 18" to the house? 2X12" would span 14.5'.
He had this built by a contractor. He was told Md code (or the county) required the deck to be freestanding. Can't rely on the hangars and ledger lagged to the house -- but nothing against doing that, too. Seemed even stronger to have both so that's what they did.
thanks Ken, I haven't seen that done, Probably a result of all the failing wall connections lately. Sure can't hurt
Hi....Am I wrong or did your pal forget to install blocking 1/2 way into the joist spans? I think that might be something to look at doing...it's relatively low-tech and won't cost much. It should go a long way towards reducing the sway. Might also consider some let-in diagonal bracing. Both these fixes are at the cost of a few lengths of p/t, the real cost is working over your head under the deck. Get buddy to load the beer fridge !
Edited 9/22/2005 9:53 pm ET by en2ohguy
Edited 9/22/2005 9:55 pm ET by en2ohguy
No, there's no blocking. The contractor didn't put it in, he's definitely going to add that.
I agree with Bob, use wood lateral crossbracing. I don't buy the washer sales pitch but do agree on checking out the hole diameter on the bolts. What is with the posts and beam 18" from the house? I have never seen that. Do you not trust the load bearing connection at the house or did you need it because of a long span?
Thanks, see 63493.34 for the "extra" beam explanation.
By the way, just wanted to say thanks for all the help.
Friend has done most of the suggestions and says he sees big improvement already.
Add two more tight fitting bolts to each post and beam connection. The bolts have to be tapped thru ,wire brush the dust off the threads at exit hole. Use thick washers and tighten the nuts.
mike