New Enclosed Boathouse Construction Information Needed

[BrianKennedy91]

I am building a new fully enclosed double slip boathouse with a livable space inside. I am looking for advise on how to build the floor system in the enclosed room portion that might be the best due to being over the water. I am concerned about rot and moisture. I have the floor system already framed with 2x12 treated floor joist at 16" on center.

 

I'm wondering if I should put down a layer of tyvek on the joist, then deck with 3/4 treated plywood, then maybe some 1/2" Hardi cement board on top of that. I am getting so many different opinions from house builders but none of them have any experience with a structure directly over the water. One guy said 3/4 T&G Advantech OSB would actually be better than 3/4 treated plywood but i'm not so sure about that.

 

My plan is to frame wood walls on top of the floor and use 3/12 pitch gable roof trusses 19.2" OC designed to carry the weight of the boats with a center support between the two slips. For the roof i plan on putting 7/16" OSB then foam sheeting and seamless tin roof.

 

I am looking to start putting the flooring on top of my joist next weekend so any advise would be greatly appreciated.

 

Thanks.......

[BraceMaker]

Though about doing end vents in the joist space with a moisture barrier and a corregated metal ceiling the the bays? Then they'll be cold joists and no different than a foundation over soil. You can insulate against the floor above that moisture barrier and have cross air flow through the vents at the ends of the bay.

[Orlando76]

I wired a ~6000 sqft two story house completely over the water with the most awesome boat lift ever. I do believe it was just ply on top of joist then sub floor. Abandon Tyvek. Theres a self adhesive blue material that’s far superior and easier to work with. I’ll post a link of it when I find it. They might’ve used the blue smurf sheet between subfloor and plywood. Also don’t forget if you have exposed floor decking over water to put screen down or else chicken hawks will fly up between the gaps.

[Orlando76]

https://henry.com/residential-and-light-commercial/weather-resistive-air-barriers/blueskinvp100

 

Blue Smurf sheets

[jayski]

@BrianKennedy91 disclaimer first, have owned a Home Improvement store for over 20 years...first whomever told you OSB is better than treated plywood in that application manner is someone you should never take advice from again...if your looking to inhibit moisture you need to create a barrier between the two, which you can do either on top of the joists or the bottom, or both. Using a heavy polly, like 6mil or more stapled to the joist, BUT run a good bead of acoustical sealant on all the joists before the poly, this will help create a barrier. With that your need for treated plywood is not necessary, and THEN you could use just regular plywood or OSB.

[BrianKennedy91]

Anyone else out there got any suggestions?

[BCM]

If you were to run the barrier on top of the joists would you be worried about trapping moisture between the barrier and the wood? If the floor is exposed to wave action I would have to think the barrier would eventually end up with a leak or two. If there was not venting (joists, then barrier, then plywood) I would have to think it would be prone to rot, if there was an air gap (barrier, joists, then plywood) you may get moisture above the barrier but it likely would stay away from floor and allow for venting/drying. Most other house barriers are protected on both sides (sheathing then barrier then roof or siding).

 

I am in no way a builder or engineer, but i live in a very wet environment and have dealt with water intrusion issues in structures and vehicles, those with airflow resulted in minor issues, those without resulted in major issues.

[lpskier]

In my opinion, you are better off with a floor that is allowed to breathe rather than one that keeps moisture out. If you keep moisture out, it will end up trapping moisture and eventually rot. Also, if you use skirt boards make sure they are gapped to allow airflow under your boathouse and not trap water vapor under the boathouse with no where to go but up. That will promote rot in your flooring. I made that mistake and it caused my Trex exterior decking to expand and ultimately buckle (covered by warranty).

 

I would also not put down my deck boards and then frame your walls on top of them. If you do, when you eventually have to replace your deck boards, you will have a bit of a chore dealing with the boards you now want to replace that are supporting your walls.

 

Also, if you are putting lights in your boat storage area, don’t center the lights over your slips. Put them over your decks. Spiders will set up shop on your lights and crap all over your boats if the lights are centered over the slips.

 

My boathouse, that I sadly sold three years ago:

 

[Rednucleus]

Seems to me a project of this size requires permitting and so specific engineering guidelines to follow??

[BrianKennedy91]

Boathouse is being built on my ski lake, not public water.

[BraceMaker]

@BrianKennedy91 probably still subject to permitting depending where you are. The tax man needs to get his feed.

[BrianKennedy91]

Tax man is a different issue all together. Engineering guidelines, I can promise you if I build it it will be very well planned out and way more overkill than any building code. Most boathouse structures In the last 30 years on public lakes I have seen are restricted to guidelines set forth by the power companies that control the shoreline and their shoreline management plans. Most of the boathouses in the last 30 years I have seen that gal under shoreline management plans are restricted to open structures so finding a builder with knowledge of design dealing with moisture in floor system construction over water has been difficult.

[lpskier]

Here in Florida I know of one case where code enforcement took issue with a judges’ tower on a private lake. What BS.

[RichardDoane]

@lpskier that’s a beautiful set-up you sold.

 

@BrianKennedy91 The hot tip is venting the space underneath so moisture doesn’t accumulate

[BrianKennedy91]

Brian,

 

Thank you for your inquiry regarding AdvanTech subflooring. Although AdvanTech has superior water resistance, it is intended for interior use in conditioned spaces. AdvanTech is an exposure 1 rated panel which is not designed to be permanently exposed to the exterior environment. The water resistance is designed to prevent dimensional changes or structural degradation if exposed to weather during the construction process. For your application, we recommend the use of marine grade plywood. Marine grade plywood, or an otherwise exterior rated product, is designed to be permanently exposed to weather and moisture.

 

Please contact us with additional questions or concerns.

 

Nick Sabol

Huber Engineered Woods

Product Application Specialist

nicholas.sabol@huber.com

(704) 731-2673

 

[GaryJanzig]

What about a small garage on land with a marine rail system going to it? Might be cheaper and less complicated.