roof sheathing: anyone ever use pressure-treated plywood?

[s219]

Sounds like you have a solid plan. The one thing you didn't mention and you haven't done on your beams yet was diagonal bracing from the post to the beam. Creating that little triangle in both directions will make a huge difference in cutting down movement. Every structure moves, the goal of every builder is to minimize that movement. Over time, movement will cause nails to come out, and screws to work their way loose. I never use nails on anything that is exposed to the elements because of how quickly they work their way back out. I also like to run my screws in at different angles for that extra bit of strength in locking everything together.

Eddie

This is an interesting case, since the bottom of the pilings are down 12-15' into the muck and don't budge, but the pilings themselves are still noodly up top. I've been told by two marine contractors here to leave as much flex in the beam framing as possible, so if a boat nudges a piling or something, it won't transfer stress to the roof. So the header beams are only bolted into the end pilings, and sit on top of the middle ones with no direct attachment other than a big 6' galvanized strap that goes up and over the header and lags into the pilings to resist uplift and contain any jiggles. So the roof ends up being semi floating. Much different than I am used to.

 

[EddieWalker]

This is an interesting case, since the bottom of the pilings are down 12-15' into the muck and don't budge, but the pilings themselves are still noodly up top. I've been told by two marine contractors here to leave as much flex in the beam framing as possible, so if a boat nudges a piling or something, it won't transfer stress to the roof. So the header beams are only bolted into the end pilings, and sit on top of the middle ones with no direct attachment other than a big 6' galvanized strap that goes up and over the header and lags into the pilings to resist uplift and contain any jiggles. So the roof ends up being semi floating. Much different than I am used to.

I have a feeling that your marine contractors are making it up as they go. While I'm sure they can drive a post into the ground better then most, my experience with docks and what they build here in East Texas is pretty consistent. They do not know framing or understand loads or span tables. I can't comment on who you have there except to restate my earlier post. Movement in the roof will lead to failure. If the wood can move, the fasteners will work themselves loose. If you go with metal, the rubber seals on the screws will fail first. If you go with shingles, the shingles will start to tear where they where nailed.

You should ask them how allowing the posts to move "MORE" will result in less movement of the roof? I'd love to hear that explained. Kind of sounds like an Abbot and Costello skit. LOL

In one of your original posts you mentioned that you where concerned with wind. How does not securing "ALL" of your roof ensure it's secure during a storm? That strap idea sounds like a nightmare waiting to happen. Kind of like a trucker only using one strap on a load and thinking that it's enough.

Sometimes we get advice that sounds good because the person saying it is a good salesman or BSer. If we are lucky, we double check what we've been told and realize that its all hookey and doesn't make any sense at all. If you are not so lucky, you do it and learn the expensive way.

Eddie

 

[s219]

Maybe I was stating it unclearly, but the idea is to isolate the roof structure from the typical flex in pilings as the react to tides, boat taps, and boat lift going up/down. So the roof essentially floats over the pilings while still being secured against uplift. Basically, we have a rigid structure setting on a nonrigid one. Same principle applies to a lot of bridge construction, where they have to build in expansion joints and other allowances. If they made the structures fully rigid, stuff would break.

 

[EddieWalker]

I understood the idea, but think the application is a mistake. Unlike a bridge that weights thousands of tons and will never be affected by the strongest winds, a roof with open sides is basically an umbrella wanting to be blown away. If you do this, it will move because that's what you are designing it to do. When it moves, it will slowly work itself loose. The question is how long until it fails?

Eddie

 

[s219]

Well, the header beams are notched and bolted into the four corner pilings, so that limits the ultimate motion (and is the uplift anchor on the ends of the headers). The header beams sit on top of the middle pilings though, via crosswise carrying beams. So those pilings can move all they want and the header beams will always ride on top.

I guess one way to look at it is that there is one lower structure formed by middle pilings and crosswise carrying beams. Then perpendicular to that and sitting on top is the main header structure. Those two can move independently in the horizontal direction even while the carrying beams support the middle of the headers. The Simpson straps prevent uplift of the middle portion of the upper structure.

Some of the pictures show the combination of beams. Actually there is a third layer of aluminum I-beams in the boat lift structure, but I am only bearing on that over the pilings so as to not reduce the capacity of the boat lift. I have seen a couple neighbors use their boat lift beams to carry the structure load, and that could potentially reduce the capacity of their lift by thousands of pounds when there is a snow load on the roof. For my 16x24 roof, the combination of dead load and snow load is almost 12,000#. The vast majority rides on the middle pilings near where boat lifts are supported.