Any boat dock builders here?

[dannyual767]

I've got a boat dock that is now 14 years old and it's covered in composite decking. The previous owner had it modified some time in the past. It originally had a double PWC lift that faced sideways. In other words, you launched the skis out of the side of the dock instead of off of the end facing out into the lake.

My wife and I assume that during the Houston area drought of 2011, they had the side facing lift filled in and then cut out the end of the dock and had a double ski lift installed at the end of the dock in deeper water.

Last summer, we noticed that some of the decking over the filled in side lift was starting to sag. Trying to beat the 100 degree temps, I started taking the composite decking off of the dock to see what was going on underneath. What I found was rot in the 2"X12" joists that they installed. That didn't surprise me.


What surprised me was the reason for the wood rot. The installer constructed "boxes" in between some of the joists (by adding a wood bottom and wood ends) and filled them with concrete! This absolutely blew my mind and I couldn't understand why they would do that. I called my brother-in-law who is a construction guy. He's not a dock builder but his opinion was the 6 concrete "boxes" were to add weight to the dock in the event of flooding and the dock were to be under water. The concrete weight would counteract the buoyancy of all of the lumber. Judging by the size of the "boxes," we estimate each one to weigh 250lbs. BTW, we've had flooding twice in the 3 1/2 yrs that we've been here and both times the dock was under water!

Is the concrete to counteract the buoyancy?

 

[harringtondav]

Seems likely. Concrete beams have little strength unless they are precast, pre stressed with stretched steel cable in their web.

Consider aluminum C channel as your replacement joist material. It doesn't float or rot. I don't think solid composite decking has much, or any buoyancy. Check if one of your deck boards floats.

I built an aluminum marine rail lift using 3 x .356 ASTM B221-08 6061-T6 Aluminum American Standard Channel with a 5' span between supports. I estimate the rear carriage wheels carry 1000" each. I can detect zero deflection mid span. It's tough stuff.

I used the minimum channel size that would handle my loads to keep the weight down. I remove the lift in 10' sections due to river ice. If yours is a permanent installation with longer spans you could go with a larger size web.
https://www.discountsteel.com/items/6061_Aluminum_American_Standard_Channel.cfm They have a facility in Ft. Worth, so freight shouldn't be too bad.

 

[dannyual767]

Consider aluminum C channel as your replacement joist material. It doesn't float or rot. I don't think solid composite decking has much, or any buoyancy. Check if one of your deck boards floats.

harrington, the composite doesn't have any buoyancy. I dropped one and it sank :facepalm:.

I don't think the installers meant for the concrete to have any structural purpose. I believe it was strictly for weight.

 

[Bob_VT]

IF you want to go with the weighted sections again I would suggest Gabion style boxes where the water could easily flow through them. I would think pressure treated would last.

 

[dannyual767]

Bob, I'm amazed at the amount of rot in the wood. The rot is very easy to see in the pictures that I posted. I just can't imagine that those boards aren't PT wood but the amount of rot makes me wonder!

I'm definitely not going to "mount" those cement weights the way those dock builders did. I think the way they did it was the easiest, quickest and least expensive way to get the work done and move on to their next job. The fact that everything was going to rot away because of it was of no concern to the dock builders!

 

[harringtondav]

Bob, I'm amazed at the amount of rot in the wood. The rot is very easy to see in the pictures that I posted. I just can't imagine that those boards aren't PT wood but the amount of rot makes me wonder!

If you decide to go with PT lumber use ground contact spec as a minimum. Foundation/water contact grade is better. Shop the lumber yards for this grade of board that has the treatment penetrating to the center. Unfortunately the only real way to tell is to cross cut an end off a board. They all look good in the pile where the mill cut off ends are soaked.