Recoring Cabin Roof Seeking advice

[SeaMoose]

Hello All
A quick background - I've recored my decks, and most of the house already over the 17+ years I've owned this boat. So at the least I am a qualified amateur.

76 Marine Trader/CHB/Taiwanese Tub. Started peeing that ugly brown water off the back of the cabin top this season.

Once in the barn for the winter, cut the top layer of glass off right behind the seats and found the usual little blocks of luan the Asians loved back then, held together by polyester resin:

Of interest the top layer glass was 2 layers of 18 oz. cloth, with typical poor saturation, something plastic like - spray on truck liner maybe, and yet another 18oz layer cloth, maybe 2, clearly done with epoxy. Very heavy and zero adhesion between the original top and the truck liner I pulled the layers apart by hand! SMH what people do to "fix" things...

But I digress. This area was dry. Leaving a strip about 2' wide in place I opened up the back and sure enough it was rotted black biomass mush.

Bear in mind these are initial cuts to get the rotted goo out before winter I'll clean these to the edges before I start recoring.

To my surprise the bottom layer of the fiberglass sandwich is a paper thin layer of perhaps 6oz cloth, so thin and so fragile I've already poked a hole in it scraping the last of the rotted goo off.

Such poor construction doesn't surprise me at this point but it's still pretty pathetic. Under that is about a 2" air gap and the composite board that's the interior roof.

I'm inclined to take the whole thing out, put foam insulation boards in the air gap to insulate things, then recreate the bottom layer with two layers of 6 oz cloth, then a proper core of divinicell, or end grain balsa, followed by a solid layer on top (probably a 6 oz initial layer, followed by any needed leveling and 2 18oz layers.

Curious to hear from others who have done this job, how it was built, and how they recored it.

Specifically:

What construction method did you find?

Did you leave the paper thin bottom layer?

Did you cut the whole top off, or do it in sections. From what I can see if I cut the whole top off the house will collapse...

What did you recore with? I've used 3/4 Marine ply everywhere else but seems a little heavy to put on top. If three or four really heavy people want up there I'll tell them no

I expected to find a proper fiberglass sandwich not some cheesy paper thin bottom layer and an way overbuilt top layer. How they managed to make such a paper thin bottom layer on the ceiling - to support the weight of the luan blocks - that was a real trick. It's not chop gun glass which is what they used on the house exterior walls.

Thanks for reading. The boat's in the barn for the winter, and I'll resume in the Spring so lots of time to decide how to proceed.

And yes, yes, I know it's a waste of money to restore one of these things but it's too late now and I no longer care...

 

[MurrayM]

Hero badge in the mail. Going to get snacks and a comfy blanket to watch the show

 

[boomerang]

Hey Sea Moose here's what we did. It isn't a Taiwanese-built boat but it shared the same crappy coring trait. The first pic is of the old coring exposed with the top layer of glass removed. The next is of the thin bottom layer of the floor which required the supports we put on the underside in the salon shown in the last pic.

 

[boomerang]

Here's a couple more of the coring we used. The wood is marine plywood used anywhere there was going to be something fastened & the white is Nidacore where nothing would be fastened. It was all bedded in epoxy. In hindsight, we probably should've gone with all marine ply (the lines you see are where I cut scarfs in the ply 1/2" deep so that the wood would conform the the crown of the deck). The small savings in weight wasn't worth all of the mudding to try to make the pieces fit together at the seams. The whole project was a major son-of-a-bitch but having my wife help made it doable...

 

[JimW]

Don’t get the luan squares, must have been plentiful, had the same on the deck and fly bridge of our DeFever 41, tracking down soft spots.

Went back on the fly bridge with coosa board with 2 layers of glass on top and 1 layer on the bottom air vacuumed, then glued and glassed down the seams and then will spray a slick coat then skid coat.

The coosa board and glass will hopefully be a 1 and done repair. Also used it on a couple of soft spots on the deck. Good stuff, solid fix.

 

[Insequent]

On my foredeck we used two 1/4" layers of marine ply, with the joints staggered. The thinner ply allows it to easily conform to the curvature. We put 1.5oz CSM on the lower skin first, which was the proper thickness. On top of the ply we put 1.5oz CSM, 2 layers of 24oz woven roving and lastly 2 layers of 1.5oz matte. This ended up with the same thickness of deck as the balsa cored original, about 7/8". I did not replace the teak on top. Overall it is thick enough, but not as stiff as it would have been with teak on top.

For the boat deck, firstly we drilled 3/8" holes to map out where the balsa was wet, and where it was still dry. This resulted in about half the deck not needing to be disturbed. We replaced the balsa core with closed cell foam. All good, but now an un-repaired area has the dreaded brown water emerging into a below deck light fitting, fortunately on the side walkway and not internally. I need to locate the water entry point, seal and re-bed the offending fittings. I believe it is just a small area and will defer deck repair for the time being.

For the OP, I think core preference is closed cell foam then marine ply. I would avoid balsa. You really want to limit the spread of any water that gets in, and have a core that is resistant to rot as well.

 

[SeaMoose]

Thanks everyone for the responses so far. When I started repairing the rot some twelve years ago I planned on recoring everything with starboard but after doing a small section of the foredeck with it I switched to Marine Ply and never looked back.

Boomerang thanks for the pictures and explanation. I am now confident in my plan to do this in sections so I don't have to build a support structure.

 

[Rogerh]

SeaMoose, do you have a sag in the affected areas and are you trying to take the sag out with the support structure?

 

[SeaMoose]

No sag. I’m planning on doing it in sections so I don’t need a supporting structure.

I am surprised how weak the initial construction is though, I was expecting something like the decks where there are teak struts forming torsion boxes .vs. the cabin roof which so far appears to be held up by one support beam in the center.

 

[boomerang]

You're lucky. Your support beam keeps the shape of the deck consistent for you. Our deck was sagging so we had to not just brace the ceiling/flybridge floor but we also had to establish the correct amount of crown so that the bridge & seating would fit correctly when we reinstalled it.
It's a good thing that we were able to do the work ourselves because boatyard repair labor would've exceeded the worth of the boat!

 

[psneeld]

You are not kidding.

At marina labor prices, all the projects togetther probably exceed the boats initial cost easily, if not 150%.

I couldnt afford to cruise if I had to pay for repairs.

 

[boomerang]

You are not kidding.

At marina labor prices, all the projects togetther probably exceed the boats initial cost easily, if not 150%.

I couldnt afford to cruise if I had to pay for repairs.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Yep. That describe our situation quite succinctly!

 

[Mule]

You're lucky. Your support beam keeps the shape of the deck consistent for you. Our deck was sagging so we had to not just brace the ceiling/flybridge floor but we also had to establish the correct amount of crown so that the bridge & seating would fit correctly when we reinstalled it.
It's a good thing that we were able to do the work ourselves because boatyard repair labor would've exceeded the worth of the boat!

Did the same Repair. Used plywood. You are/have doing/done a better job than I. I should have done more on the first layer. I had 30 cinder blocks to put a crown on the bridge deck and numerous 4x4’s to avoid collapse. I still ended up with a void per the hammer.

Great sea boat. Weakness is rudder too small and too far forward, for me the v berth sucks. I lived aboard 4 years and NEVER had a friendly thing to say about the v berth. Galley, Head, cabin, deck space All good/great.

All Mainships I have encountered have these issues. It was my understanding that the riggers at the dealer level largely caused these messes by not preparing the attachment points properly before putting #12 and #14 screws in. Often without even 5200. I truly HATE end grain balsa to this day.

You had it good, your 1st Mate looks better and more competent than the collection of homeless substance abusers I had for help... my community outreach program

 

[boomerang]

Mule, I just read your last. Too funny! I'm glad that phase of the rehab is over with...while we were there in the summer heat on the hard in a boatyard I considered becoming a substance abuser myself. You're correct about my help looking better than most boatyard workers, though.

 

[JimW]

Finished product[emoji2]!

 

[FF]

As most have found out bending good ply is not easy.

The simplest method is to use 1/4 inch in staggered layers.

Between layers roofing tar is the perfect glue/gap filler.

3 - 4 layers of 1/4 is enough for up to a 10 ft width span, ringed SS nails for assembly and screws into the rib structure for strength.

A layer or 2 of 1 1/2 mat with polly resin makes the roof waterproof easy to paint.

If there is no foot traffic the white house trailer roof paint does a fantastic job of reflecting heat. It is far too soft for use as a sole.

Beware , only 50-70 sq ft of coverage per gallon.

 

[SeaMoose]

Roofing tar, very interesting idea. Thanks.

 

[FF]

"Roofing tar, very interesting idea. Thanks."

STOLE the concept from home boat building sites that have similar problems with new construction as we do repairing our boats.

Sadly I forget which NA presented the idea.

 

[boomerang]

I'm having a hard time warming up to the roofing tar laminate repair job. As much labor as was involved in the recoring of our bridge floor, I felt better using marine-grade plywood, marine resins & conventional marine techniques. I wanted everything to be as stout as possible & I know that was accomplished going the route I did. I realize there's more than one way to skin a cat (is that phrase no longer PC?...sorry cat lovers) but I wasn't going to use roofing tar on a boat to save $500.
The extent of materials we purchased at the home store for our repair was limited to sandpaper & rollers.

 

[FF]

The 1/4 layers of ply are held together structurally by the SS ring nails.

The purpose of the tar is to obtain some bonding , but mostly to have a surface that stops rot in any air space , as no built up laminate is perfect.

Its not the bucks saved , its the longevity of the roof / deck that counts.

 

[psneeld]

FF.... wasnt it this NA that suggested the roofing tar between plywood sheets?

George Buehler Yacht Design Home Page

Probably not much different when they put red lead or something else smeared all over canvas and put it between deck layers and even hull cross planking if I recall correctly.

Here.......

https://www.duckworksmagazine.com/04/s/excerpts/buehler/index.htm

 

[Northern Spy]

View attachment 71610View attachment 71611View attachment 71612

Finished product[emoji2]!

Nice and clean. What color gel-coat or non-skid paint did you use. Looking for a similar color for my boat.

 

[Aquabelle]

I think core preference is closed cell foam then marine ply. I would avoid balsa. You really want to limit the spread of any water that gets in, and have a core that is resistant to rot as well.

Brian: could you explain why you think cc foam then marine ply...instead of 2 x 1/4" ply? And if you were doing the foredeck again, would you go cc foam + ply rather than 2 x ply layers?

thanks

 

[Aquabelle]

The coosa board and glass will hopefully be a 1 and done repair. Also used it on a couple of soft spots on the deck. Good stuff, solid fix.[/QUOTE]

JimW: I hadn't heard of Coosa board before your post, but have now google'd it. Why did you choose it over 1/4" or 1/2" scarf-cut marine ply? Was it because it is a lot lighter but still stiff/strong enough to walk over, or cost, or easier to work, or....? I'm in early days of investigating alternative approaches to a partial ( I hope) core replacement on my flybridge/boat deck.

thanks !

 

[Insequent]

Brian: could you explain why you think cc foam then marine ply...instead of 2 x 1/4" ply? And if you were doing the foredeck again, would you go cc foam + ply rather than 2 x ply layers?

thanks

Paul
Just my poor sentence construction! I was trying to give an order of preference of materials, not suggesting that you use a mix of materials!

I think multiple layers of ply works quite well. As FF has stated, use SS to ring nail the overlapping sheets down. He suggests using roofing tar between sheets, but I think I would do as the guys did on my foredeck: use thickened vinylester resin.

 

[boomerang]

If I were to go with with multiple layers of thin ply, I would much prefer using resin between the layers as opposed to roofing tar & ring nails. The reasons being 1:the resin at the edges of the plywood laminate repair will adhere to the existing laminate via more resin; roof tar? No bonding of the new wood to the existing structure on the edges.2: A boat continuously moves in different directions & movement is especially noticeable over a larger area such as a cabin roof, where there's lots of flexing & twisting. Ring nails or not, 3 separate panels of ply are going to tend to move in 3 different ways, Bonded as one panel would make it more rigid. 3: Roof tar is funky stuff to clean up! Yuck! Roof tar on a fiberglass yacht? Really? Why the hell NOT use resin?!!?

 

[FF]

Why the hell NOT use resin?!!? __________________




Polly Resin is not a glue and is brittle when thin after curing so more liable to crack than tar which will remain soft for decades if no air gets into the laminate.

Epoxy resin would have to be thickened to bond to layers that are not vacuumed together .

When finished a cabin top of ply is easier to bolt to the vessel .

Cored cabin tops are lighter , insulated and pass less sound , but more difficult to mount to the boat , and much harder to bolt items to as compression inserts have to be built in.

We built a replacement roof/deck for a Californian that was about 12x10, at 3 lbs per sq ft (to stand crowd traffic) it took a bunch of folks to get it up in place.

Laminating thin ply makes replacement a 1 person task, although a cored roof is simplest if thee is no structure left.

 

[boomerang]

FF please don't construe my posts as trying to argue your method of roof tar as being an incorrect method. This isn't an arm wrestling contest! The whole reason for this forums existence is to share knowledge & experience. I've shared mine with the op & he/she can do what they want with it. They can use cardboard & peanut butter to recore the deck if that's what they choose because it's their boat. My opinion and yours is just another tool for them to use in the decision making.

 

[psneeld]

"3: Roof tar is funky stuff to clean up! Yuck! Roof tar on a fiberglass yacht? Really? Why the hell NOT use resin?!!?"

hmmmmmm

 

[timb]

Jim W how did you form the Coosa board to the arch of the roof? I didn't think Coosa board flexed any (I think that is what the transom of my Carolina skiff is made of).I was thinking it would be perfect for decks . but I was wondering about anything with an arch.