I’m a do it your selfer and I’m getting together plans for a deck inside the main cabin of a fiberglass boat. The project involves building the frame and installing the floor joists and then custom fitting engine hatches along with laying the subflooring.
The length from bow to stern is 11′ and the width from port to starboard is 13′. We are installing 2″x4″ headers across the fore and aft bulkheads and attaching them with screws to the bulkheads along with construction adhesive. There are ledges built into the port and starboard bulkheads for the 13 footers support. We are also installing lolly columns to the engine mountings spaced 30″ from the port and starboard bulkheads and then 34″from each other.
My question is what is the maximum allowable span of a 2″x4″ and how far apart should I space the bridging? I’m spacing the joists at 16″ o.c. that run port to starboard.
Any tips and/or advice would be greatly appreciated.
Thanks
Senor Dorado
Replies
Don't know. Most of my houses don't float. ;-)
Seriously, your requirements are seaworthyness, not like our requirements.
You have to worry as much about center of gravity as structural integrity and the tradeoffs are different.
Maybe there are some shipwrights here?
"Let's get crack-a-lackin" --- Adam Carolla
The tables in my book Simplified Engineering for Architects and Builders doesn't even have 2x4's for joists--it starts at 2x6 and I would use nothing less. At 16" spacing, it says a 2x6 will span only 8'-7" to 10'-4" depending on its modulus of elasticity (with live load 40 psf).
If you want to span a width of 13', this book says you need at least 2x8's at 16" centers and a modulus of e. of greater than 1.7. Woods having such a high modulus of e. include select structural southern yellow pine, No. 2 dense southern yellow pine, Douglas fir-larch dense select structural and Doug. fir as "low" as no.2. In other words, you probably couldn't just grab a bunch of 2x8's from the home center and hope they will do. If you reduce the distance between centers to 12", the book says you could use just about any grade of 2x8, but even at 12" centers, a 2x6 will not span 13 feet (and hold any kind of load). And no way would I try using 2x4's.
I'm not sure what you meant about lolly columns--if you have beams running fore and aft that are supported by columns, that's a different story. Then 2x6's might work, but I would still be leary of 2x4's. I'm no engineer, but just my experience tells me to avoid 2x4's in this application. I'm sure others will chime in.
My Span Tables (San Diego)say a 2x4 at 16oc can span 5'6", and at 12oc it's 6'3".
Thanks, that's good to know. If he has one or two beams running longitudinally, maybe he could get by with 2x4's, but otherwise.... I suppose you could make the deck totally out of 2X4's glued together--use teak so it would stand up to conditions at sea. I would imagine teak is pretty pricey for that! (And teak doesn't glue all that well, from what I understand--I've heard you can wipe the oil off the surface with solvent before gluing up, but....
Thanks for the info on the 2"x4"s. How far apart would you install the bridging?
Senor Dorado
Danno
Thanks for your reply. We are also consulting and obtaining materials from a marine lumber dealer, and if there are discrepancies they will be addressed here. As someone already said I wont be going to Home Depot. The idea of a torsion box sounds interesting and I have to give that some thought.
If I did use 2"x6" or 2"x8" , how far apart would you suggest that I install the bridging?
Senor Dorado
That's a loaded question. Lots of people say that bridging is unnecessary and that it does nothing, others say it is good. Code requires it in houses--every six feet if I remember right. I really don't know--there's a web site called http://www.buildingsciences.com that may tell you something about bridging.
I was just thinking that there are lots of spiders on the waterfront (when I went sailing on White Lake in Michigan, the boats and houses on the shore were loaded with them), so the less place you can give them to live the better. Open joists and bridging seem like good places for spiders. Maybe just run two 2x4's (or maybe even 1x3's would do it) perpendicular to the joists at third points and glue and screw them to the undersides of the joists (2x4's or 1x3's laying flat). That should keep the joists from tipping and will act sort of like a stressed skin panel (you could always totally skin the bottoms of the joists with plywood too--that may actually be better--lose less headspace below decks that way (that was also some of the reasonomg behind the torsion box idea)).
I was just thinking (and that's always dangerous!)--why couldn't you make the deck from a sandwitch of marine plywood top and bottom with a grid of 1x2's (all planed to be exactly the same dimension from top to bottom) glued between? This would make a torsion box and would be very strong. You could even use thin veneers for the "bread" of the sandwitch and glue them up with epoxy ala the Gougeon Brothers (boat builders in Bay City, MI). The end result would be light (they use veneers that when laminated are only a quarter inch thick for some parts of their boats--I was shown around their yard once and there was a hollow wooden pontoon about 12 feet long and maybe a foot in depth by 8" wide and I could easily lift one end, as if the whole thinge were made of balsa wood.) and very strong as well as waterproof.
why is your boat wider than it is long?
These are the dimensions inside the main cabin, and I'm standing in the bilge within the engine compartment.
Senor Dorado