Rot in ply

[KINGFISHER 9]

Found a soft spot in the coachroof alongside the tabernacle .. poked around with a screwdriver which ended up going right through! .. now have a 5" x 3" hole where I dug around .. soft spot is maybe a foot square altogether .. banged around the rest of the roof which all sounded ok but I'm no expert. Yard reckons whole roof needs replacing ... (they're quiet at the moment!)
Deck is about an inch thick and as it's curved I suspect it's 3 sheets of ply glued to make up the thickness .. maybe 3 x 3/8ths or 2 x 1/2" ply but only guessing .. water seems to have got in around the tabernacle which is surprising as it's a massive steel thing bolted to a hefty piece of iroko (?) sitting on 2 bulkheads.
Do rot spores spread along ply? .. will the whole roof have to come off and be replaced? .. how does rot work?
Definitely a case for a second opinion!

 

[sarabande]

if tapping around the rest of the deck produces no soft sounds nor other problems by inspection, then I reckon you need to replace only the rot and the immediate area.

It sounds as if water might have entered via a bolt hole, so other bolts for the tabernacle might be suspect too.

Probably, to be sure, replace all the ply under the tabernacle, and the iroko pad, too, if it needs it.

Standard method is a scarf joint, but if there are multiple ply sheets then you could do a stepped layer replacement, with each layer sitting above a smaller cut-out. It may be worth using a doubler ply underneath the deckhead.

Previous threads apply.
http://www.ybw.com/forums/showthread.php?t=172227

http://www.ybw.com/forums/showthread.php?t=77816

(mirelle is a wooden boat man thru and thru. Much missed here.)


There will be supporting timbers / beams underneath the tabernacle, and these will also need careful assessment when the deck is opened up. Their layout will dictate the areas you replace, in order to provide proper support for the tabernacle and mast.

If you use really good, proper marine ply, there should be no need for resin/epoxy impregnation or use of wood preservative. Making good and painting up when the plies are in place will include measures to ensure that no water can enter the plies and repeat the rot process.

 

[kds]

rot in ply

Yes - I am afraid that it can spread - usually along one of the layers at a time. Fresh water under the mast foot is a favourite cause. You need to explore at distances out from the centre, every 4 - 6 inches and see if there is any solid stuff left (drill and chisel). I was lucky and found that there was quite a bit of solid left, so could router down, one or two layers at a time through the rot, giving steps of 2 inches wide to laminate back up to full thickness with veneer (or thin ply), one layer at a time. It may require a mould under the deck to keep the curve. It is VERY time consuming, but not all that skilled and a complete new deck will probably be a more realistic answer.
Best of luck.
Ken

 

[ninky]

if you are referring to rot 'spores' then this is dry rot. this is a fungus and can spread 4 metres a year (although that is quite unusual). despite its name dry rot like other fungi does require a source of water so the best means of attack is cutting out and replacing the affected timbers and ventilation. wet rot is simply where water has rotted the timbers. it only goes as far as the water has gone. did you notice any fungal signs such as a whitish bloom?

 

[Tranona]

Agree with most of sarabande on this, having done quite a lot of similar repairs on the coachroof of my boat. I would, however advocate the use of epoxy.

If the rot has been caused by water getting in, then once you have removed the obviously rotten bits the rest will dry out and can then be stabilised by saturating in epoxy. Watch out for rot getting into the beams, but again you can dig out the rotten wood, let it dry and pour in epoxy. You can then build it back up with either laminated veneers or a sandwich of veneer and glass cloth.

The stepped layer method of replacing the ply is effective, but getting the first one to take the curve in the gaps between to beams is a bit of a challenge. On one repair I did I let in battens to form a landing for the first layer of ply. They show, but when painted to match the deck head look OK.

Hope this helps

 

[oldharry]

It is possible to laminate and scarf a repair to a curved plywood deck, but as KDS says it takes a lot of time and effort. Unfortunately with plywood just 'banging' does not necessarily reveal where one or more veners are rotting or beginning to rot. If you have cut out the soft wood, you should expect to go AT LEAST a foot further all round to get back to solid sound plywood.

The easiest way to retain the curve is to laminate 6 sheets of thin flexible ply <4mm in to the repair, scarfing it in to the existing sound timber, and stepping it out on each sheet until you have the full thickness. Use epoxy, but you have to be very sure you have left no voids between the sheets, and that is where the difficulty comes as if you apply too much pressure it will bend the ply out of shape!

As others have said you need to be very careful to ensure framing timbers are unaffected.

I am inclined to agree that a new deck is really the only satisfactory way to go - certainly much simpler, and a job you should be able to tackle yourself if you are reasonably handy with woodworking tools. At the end of the day you will KNOW she is sound again! Again with that thickness you will save a huge amount of effort if you laminate it up from several sheets - been there, and found very early that 24mm ply does NOT bend as easily as 6 mm!

 

[sarabande]

in view of others' more extensive experience (I'm replacing a similar area round a cockpit at present) I'll go along with the policy of a large-scale replacement.

 

[BlueLancer]

Frank,
Excellent Chapter on Scarfing in plywood repairs in the book Wooden Boat Renovation by Jim Trefethan. I will scan a copy and e-mail to you if you wish. Send me a PM with your e-mail address if you want it.

 

[Niander]

Rain water will have got in
No water=no wet rot
Just fix the bad bits
It will not spread unless its damp
Find out where the waters getting in and stop it!
You also need good ventilation inside to keep it dry.

 

[Lakesailor]

I go along the repair route. Use epoxy without fillers so that it can penetrate the laminated layers. The glues may have failed and epoxy is good at creeping in and repairing those areas. I repaired some failed double curvature ply on a dinghy using epoxy and saved myself some difficult woodworking. Using wood filler on the actual scarf mating surfaces will accommodate any lack of precision in the joint cutting.
The repaired area will be stronger than the rest of the roof when you've finished.

 

[oldharry]

Yes, its dead easy on a dinghy. Gets much more complicated in 1" thickness of ply. That stuff doesnt bend in a hurry. In fact over a small area it doesnt bend at all, so building up laminations is the only way to go. And believe me, that is a slow tedious job if you do it properly, scarphing each layer and ensuring there are no glue holidays in the lamination.

But it can be done with a lot of time and patience.

Been there, got the Tee shirt and ended up wishing I had done the whole deck. Much easier.