Delaminated Cabin Roof - Shetland 535

[CraigV]

Has anyone had any experience with fixing a delaminated roof on their Shetland? Here's a brief bit of background.. fixing up brother in laws 535 as he is really unwell in hospital and the boat has been not used in a few years and needs a bit tlc. So the roof.. The roof was littered with stuff.. Stuff like.. 2 spotlights, 2 air horns, a small steel mast with a nav light, cb radio arial and two grab rails. most of these items were letting water through their bolt & wire holes. I have since took these all off the roof and there's a fair bit of water dripping out of some of the holes. I have since tarped over the boat so there will be no more water for now.

I have done a bit of research on how to fix this problem.. I.e YouTube videos and some blogs. All seem to say the same.. Cut out top layer of fibreglass, replace core with marine ply, and glass in top back on.

However I am curious if any of you guys has undertook this kind of job before? I have never worked with fibreglass before but It doesn't seem too complicated, I just don't know what I should expect when I peel the top layer of fibreglass off.

Cheers.

Oh Hi everyone I am new here

[dellquay13]

I’ve never taken on a glass fibre job that big, and given the value of such an old boat, most people would walk away from it.
You may get better looking results by keeping the integrity of the exterior and cutting the inside to get at the rotten core that way. It does mean you will be working underneath wet glass fibre though.
The plywood was used to save on the amount of grp needed, and to save some weight. You could try stripping out the rotten wood and glassing in a few wooden stringers for rigidity and then layers of glass mat in between the stringers to stiffen the cabin roof in general. It might be easier than trying to lay new sheets of ply into the correct shape without any air gaps.

The entire deck moulding could be separated from the hull under the rubbing strip where it is usually bolted together and flipped over so you would be working on it the way it was originally made but without being in the mould tool, upside down and working inside.

[dellquay13]

I have only replaced rotten cored deck below cleats stanchions or pulpit rails, in sections up to about 8” square, and I got better looking results working from below. I scraped out the rotten ply or balsa, and mixed a stiff pudge of chopped strand and resin and filled the gap between the two grp layers. The best results I got by cutting out a section from below, scraping the deck underside clean and dry, a thick layer of pudge then pressing the bottom layer of grp back into it.
I never bothered fitting new plywood, I just wanted maximum strength and reliability under the deck fittings. It’s also the way modern boats are made, the general deck has a balsa core, but the grp is solid under the fittings for strength and to stop any future leaky screw holes rotting the wooden core. Sometimes a metal backing plate ready tapped for bolts is glassed in too.

[dellquay13]

If you work from the inside, you can finish the job and hide any rough looking work by screwing up thin plywood panels covered in vinyl cloth. Shetland adopted this method from the early 80s onwards.

[CraigV]

Hi Dellquay13, thanks for your reply. The roof as it is just now looks warped and out of shape. Do you think if coming in from the underside would remedy the warping? Adding more support for the roof structure sounds good though.

Craig.

[dellquay13]

Hi,
Obviously only you know what it looks like so we can only surmise. If the coachroof is warped, is it also rigid or does it move with enough pressure?
If it moves it’s still damp inside, if it’s rigid it’s likely it has dried out inside.
Either way, if it needs doing, I’d look again at the YouTube guides that suggest going in from the outside, and consider the other way of keeping the factory finish outside by working from the inside. I haven’t looked at the guides you have seen so can’t comment on them.
If I was sorting a warped coachroof I would go from the inside personally, and look at temporary bracing and formers to get the correct shape back while getting some rigidity in with ribs/ stringers and sheets of woven glassfibre mat wetted in. A bit like how plywood formers are used to build brick arches and removed afterwards.

[dellquay13]

IMG_1203.jpeg

There is a 535 in my boatyard, and looking at it, the job doesn’t seem as big as I feared. There is very little curve to the coachroof and the actual size is smaller than I remembered.
I would work from inside, I probably wouldn’t bother with big sheets of ply, I would just use lots of woven glass mat with some lateral stringers glassed in.
You might need to use something to brace it back into shape while the resin cures, up to a couple of days if it quite thick.
I will be seeing a friend tomorrow who used to work moulding Pirate motorboats, and I’ll ask his advice on curing times. He replaced the drive shaft grp sterntube on my yacht last week and is helping me get it craned in to the harbour.
The stringers could be quite modest in cross section, as most of the rigidity of stringers comes from the grp you sheathe them in, not the wood itself. You might get away with section as small as 15mm x 38mm.
This 535 near me has thin ply sheeting up to hide the inside of the coachroof, and it looks like they have clad it in grey campervan carpet lining.
Fabric clad Ply like that screws up to stringers really well, and you can run all your lighting and vhf cables behind it. My Family four is made that way

[CraigV]

So I have an update. I took the plunge (or the cutter did ) and took off the top of the roof and the wood underneath was absolutely wringing wet. As for the slight camber in the roof.. My plan is to transfer that camber onto a thin sheet of ply and use it as a guide when adding a new core. I will be glassing in structural supports in the roof and then filling the rest of the void with marine ply cut into long strips, much like how the original balsa was done.

If you haven't spoken to your friend yet could you also ask him what fiberglass mat I should use, i.e. woven? and what weight? Much appreciated if you could

I will try to upload a few images of the cutout I have done. I cant view your image that you included as the site informs that I do not have the right credentials to view images.

[CraigV]

https://www.flickr.com/photos/134684491 ... ateposted/
https://www.flickr.com/photos/134684491 ... otostream/

Hopefully these links work.

[paulturner64]

HI,
Did this job a while back. Look at Roof! in the maintenance section.

Only thing to add to the story is that it's doing great, despite the fact that I never did get round to the cosmetic upgrade
Strips instead of sheets make the job easier, but you need to make sure the gaps are minimal and completely filled with resin.
Use epoxy resin with compatible glass fabrics rather than polyester: epoxy resin is an adhesive and will ensure the wood and fibreglass
adhere to each other and the surrounding exiting fibreglass. New polyester will not bond to old very well, additionally a new lay up tends to shrink during cure, so it's not unknown for an insert to fall out of its surroundings as it shrinks.

good luck
Pault