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Old 29-01-2010, 23:28   #1
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Cement Board for Cabin Sub-Floor

Hi All,

First I'd like to thank you for the resource you provide. I just purchased my first sailboat, a 35 year old Newport 27 and have used the forum extensively.

I just ripped out the cabin floor because of soaking wet/rotten plywood under the fiberglass. I am considering using cement board (hardibacker etc. commonly used for tile mounting in residential applications) to replace the plywood. I would then glass over it to try to match the rest of the fiberglass benches etc.

What are your thoughts?

Thanks,

Tim
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Old 30-01-2010, 00:13   #2
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Hmmm.... A little heavy but all in all not a bad idea...wont Handel much flex though so be aware of potential issues there...as well as attachment screws possibly wallowing out if the sole does provide a diaphragm of structural support which most do. I think I would experiment with it awhile before glassing it in myself.
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Old 30-01-2010, 00:30   #3
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Hard spots...

Fiberglass is relatively flexible. Cement board is not, as well as being heavy. As the boat flexes any join will tend to work or pull loose, dragging the fibreglass away from the hull and bulkheads.

Just imagine if that heavy cement board sole came loose as the boat rolled, not to mention the added weight of any fibreglass. How'd you like that coming across the cabin at you.

If the sole has rotted then the floors (structural transverse beams below the sole that may help to distribute the stresses from the mast compression post, the chainplates and the keel) may also have been compromised. It may be a good idea to check and repair them as necessary. The sole can be screwed on to them if you just want a fast cheap job.

Allowing for access to the underside of the sole will make future work on the boat easier.

I'd recommend plywood. Backing strips can be glued on as necessary.

There are some plywoods available with that nice dark/light wood striping.

I've used 12mm pine "Ecoply", waterproofed with thinned epoxy. If the panels are likely to flex I glue 40mm x 20mm battens to the underside.

I tend to find that for every three days that I spend working I spend one planning what I'm doing. Sometimes more.
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Old 30-01-2010, 01:45   #4
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I'd go with Boracay's advice. You're planning to glass over things anyway so use marine plywood.

Cement board is non-loadbearing. It's intended for walls not floors.
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Old 30-01-2010, 03:50   #5
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I'd go with plywood and fix whatever caused the water damage.
Boracay your so efficient....for every day I spend working its 3 days planning...if I'm lucky!
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Old 30-01-2010, 05:12   #6
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go with plywood and it will last another 30 years ..
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Old 30-01-2010, 10:22   #7
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Sounds like a consensus in favor of plywood. Securing it to the floor may be an issue as part of the old plywood sat directly on the hull on the outside, and toward the center (longitudinal axis) was supported by irregularly placed blobs of expanding foam and a one foot length of teak (still fine).

I have located and fixed two big leaks (stuffing box and cockpit drain) and replaced faulty automatic bilge pump switch. Compression posts sits on an island of very hard material that I haven't identified yet. Two small transverse beams are located fore and aft of the removed sole, and end when they run into the cabinetry so I am dubious of their structural value.

I like the concept of a removable sole. I could use stainless brackets and screws to hold down the plywood and allow for removal.

Thanks so much for all input.

Tim
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Old 30-01-2010, 10:34   #8
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The most important thing that was not mentioned...

Cement board does not provide structural support and requires plywood or other support beneath it. If you use it alone, you will fall through in a short time as it fractures.

The cement board is meant to help the adhesion of mortar and tile. Since water passes through some tile and grout, the cement board is preferred because it doesn't provide "food" for mold. It still requires a separate vapor/water barrier/membrane behind it.

So in summary: Cement board needs plywood or other to support it and a separate water barrier. So really it is just filling for the mortar sandwich.
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Old 30-01-2010, 11:00   #9
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"I would then glass over it to try to match the rest of the fiberglass benches etc. "
I think you'd be wasting your time and money, lots of both. Better to use a marine ply with one good finished side and paint that with a good white epoxy paint, which will match up to the existing fiberglass work as well as anything you do in fiberglass. Or if you want to go deluxe, use structural fiberglass panels instead of marine ply. Or, honeycomb panels, which have much less weight. (But are still way expensive.)
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Old 30-01-2010, 12:37   #10
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I have a similar project replacing a rotted cabin sole-- has anyone had experience with plastic board products? The one I've seen mentioned is called Seaboard, a high density polyethylene, I think.

Kirk
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Old 30-01-2010, 13:18   #11
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Sounds like a good idea. I have considered it in the past, as, if done well, it will be a trouble free floor which will outlast the boat by centuries. I'd use the standard layup for a ferro cement hull with square mesh, and cure it under wet stuff and plastic for a couple of weeks, at least. Don't use newspaper for the wet stuff, or you may never get the writing off. On second thought, that could make it an interesting surface, especially in a hundred years or so.. Anything which works for a hull won't break by being walked on. Tieing it to your keel bolts is a good idea to keep it in place during a knockdown, unless it runs under some of your interior woodwork, enough to keep it in place.
Plywood with fibreglass over , was not a good an idea, as it didn't work last time so why would it work this time?
My father could get a cement surface as smooth as a baby's ass. You could get a good cement guy to put such a finish on it, and add colour, in case you get too lazy to maintain anything over it. Then it would be a zero maintenance surface for centuries , after which you could let the next owner worry about it. The weight would be too low to matter much.
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Old 31-01-2010, 07:33   #12
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Quote:
Originally Posted by Kirok View Post
I have a similar project replacing a rotted cabin sole-- has anyone had experience with plastic board products? The one I've seen mentioned is called Seaboard, a high density polyethylene, I think.

Kirk
D'you mean Starboard? Similar to the material used for fish cutting boards?

That stuff is inert and will last forever but you'd probably need to fasten fibreglass or aluminium battens to the under side to stiffen it as it's fairly flexible. Further, it's a fairly soft surface so it'll scratch up and never clean properly after a while.

If you want a high-tech floor you could use something like Multipanel (Multipanel - Lightweight Waterproof Panel Substrate - Melbourne Australia) and glass both sides. Then you could finish the topside with a wood veneer / epoxy paint / rubber surface and you'd have a lightweight panel which is inert, flame retardant and strong. I'm currently building a hard bimini out of the stuff and it is very easy to work with --- but it is expensive!

But marine ply is way cheaper and will last as long as you're likely to own the boat (and then some!!).
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Old 02-02-2010, 10:46   #13
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US Plastic calls it Seaboard: Seaboard® High Density Polyethylene (HDPE) Sheeting | U.S. Plastic Corp.

It might be the same thing as Starboard, but I thought Starboard was a PVC product. I was hoping to replace the rotted sole without ripping out all the cabinetry (which is salvagable), so sliding the new sheeting underneath is my plan.

I don't care too much about it being scratchable-- this will be subfloor, covered with something else like teak veneer, carpet, etc.

Kirk
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Old 02-02-2010, 11:20   #14
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Hello,

Seaboard/Starboard not will bond to it, I did hear about something new that will but have not seen it yet.

If it is a subfloor I see it working but if it is the floor itself very slipery as soon as it gets wet you will be flat on your backside. It is also very costly for a subfloor.

Myself I would do it the way they have for years Marine Ply glass over, will last many of years.

I take it that you have a deck step mast, the very hard material is most likely oak,redwood or other hard wood. very common.

As for attaching the ply along the hull as you called it overlaying the glass, From what I have seen is screws in the stringers and glassed along the hull.

I have not started on my floor yet because it is very sound but needs a finish, I have been thinking about a number of things I would like to try, one is to lay 1/2" insulation board over the ply but then I am left with how to lay the veneer over it.

Most likely I will install a nice finish redwood ply then the veneer. will make the floor nice and level and most of all smoth.


Dutch
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Old 02-02-2010, 11:31   #15
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Cement board will not mold, but it will absorb moisture. when used for bathrooms there is a water proof membrane that is suppost to go over top, then the tiles.
A properly sealed cement floor woud be low maintinence, and last a very long time, but it would not be nice to fall on, anything dropped would break, and it would be cold,... unless you put infloor heating......there's an interesting topic.
I would go with epoxy sealed plywood, done right it will also last a very long time.
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