Replacing Rotten Wood in Cabin Top

[rebel heart]

It's wood, all the way. Just simple painted wood. I'll take a picture of it to verify the type (so I get the right replacement).

[clausont]

Just my two cents (more) worth here Rebelheart:

I did basically what is said here about beveling and filling in, the thing that I did that I think may have made it easy for me (aside that there isn't much camber) is that I used three layers of 1/4" marine ply laminated in place with epoxy. Before I places each piece I saturated it with epoxy to be certain that there are no dry spots in the laminate when I was done with it.
In the pictures, I only have one layer of 28oz cloth on it, but will finish the job when the weather dries out and warms up a bit more.
Also, as mentioned before, if at all possible, it would be a good idea to make the joints between old and new above a cross beam for strength.

But then again, I certainly am no expert.

[svHyLyte]

It is, in large part, the crown or vertical curvature of the coach roof that gives the structure its strength and that should be preserved if possible. If the shape remains intact--i.e. the crown or remains--before cutting anything I suggest you make a series of temporary frames at roughly 6" to 8" centers that match the curvature of the underside of the coach roof. You can probe the deck with an ice pick to determine the extent of the damaged area that needs be replaced. The frames should begin roughly 2” outside the damaged area fore and aft and be long enough that you have at least two if not 3 screws on either side of the damaged area from side to side. Templating the frames with sheets of foam-board is not difficult. Once the templates fit the underside of the deck firmly, transfer the shapes to 1x6 clear fir boards that can be obtained inexpensively from Home Depot/Lowes or you local lumber supply. (Remember to number these so that you get the right temporary frame in the right place.) Hold them up to the underside of the deck and mark the length of each, and then drill a series of pilot holes, offset from your guide lines ½ the width of the frames, for Brass #6 x 1-1/2” flat head machine screws at roughly 3” centers in the remaining good decking on either side of the damaged area. Fit the frames and counter bore them with pilot holes for the screws and temporarily screw them in place to ensure fit.

Once the foregoing is done, you can remove the frames and cut the damaged area of the deck out and bevel the edges as previously suggested. Then, cover the tops of the temporary frames with wax paper and screw them into place. Then, rather than using a single sheet of plywood for the repair, use 1/8” thick single plys. The first or inner most layer can be sized to fit the cutout exactly. It can, but does not have to, overlap the edges of the original deck. When you are satisfied with the fit, staple it to the temporary frames with monel staples. This becomes the working base for the repair. Next, using 1/8” x 3” (or whatever) strips of timber (teak, mahogany, red ceder, whatever) fit the strips at a 45 deg angle to the coach roof, beveling the edges roughly to fit the bevel cut into the edges of the remaining good coach roof material. Once the fit is good, thoroughly coat the top of the first ply and the beveled edges of the cutout with epoxy and lay the strips in place, stapling each every few inches, again with monel staples. Repeat again with another layer of 1/8” strips, with the edges of these extending further out on the beveled edge of the cutout aligned at 90° to the first layer. The third layer can again be a solid sheet of 1/8” single ply laid crosswise or strips stapled and glued as before. Repeat the foregoing as necessary to build up the required thickness until just below the to edge of the surrounding undamaged coach roof. The remaining thickness will be made up with a layer of glass covering the entire repair. Before applying this however, it would be wise to add a series of #4 or #6 flat head SS screws at 3” OC through the overlapped bevel around the edges of the repair as “drift pins”. These should be of length sufficient to almost, but not completely, penetrate the decking, dry fitted initially, but the screw holes filled with a little epoxy before the screws are inserted for good. Then the covering layer of glass is applied. Once the foregoing has thoroughly cured, the temporary frames can be removed, and the tips of the monel staples from the first ply snipped off flush with the underside of the deck. The brass screws that held the frames in place are marked for length to flush out with the underside of the coach roof deck, snipped to length, coated with a little epoxy and reinserted to fill their holes. The entire deck around the repair can then be covered with a final layer of glass, smoothed and painted.


FWIW…

[Roy M]

This is where a wooden boat shines - in its ease of repair. Chop put the unsound wood, whether or not it goes even into stringers. Out with the bad stuff, first. Craft a butt block that, ideally, provides a 3 inch margin of bonding surface all around with the underside of the deck. If not, chop out a section of stringer, butt block, then "sister" a replacement stringer in place. If any of this stuff is visible (not covered by headliner) then you can get fancy with scarfing, etc. Otherwise, just replace the damaged area with the butt block, then layers of thinner ply in the void itself, finally doing the cosmetics. When you are done, but for the final painting, consider if now is the time for the hatch or ventilator that could have gone there. Also, consider the pathology: How did this come to be in the first place? Is this repair going to be the end of this issue?

[s/v Jedi]

The procedure described by HyLyte is the correct one for laminating with 3 pieces of 1/8" each. I don't agree on laminating without the supporting structure as you will get voids in between the layers because of the compound curves.

HyLyte's system results in a stronger structure, especially when using the strips diagonal etc. However, the question is if this is needed. It will be MUCH stronger than the original and thus the rest of the structure.

If done right, I think the method I described will already be stronger than the rest of the structure, and much quicker, although it doesn't give that shipwright feeling you get with HyLyte's procedure. I have the feeling you will probably go for the glass ;-)

About the questions: I meant just any tacks/tacker but forgot to mention to remove them after epoxy has cured. They are just temporary.

A square cut makes the bevel easier with a plane. But a round cut is the way to go for strength. After you mark the 6", you have several methods to start the bevel but for each you need to remember to NOT touch the last 1.5" at the line and the last 1/8" on the bottom of the cut. (that is the same 12:1 factor). So draw an extra line and just keep 1/8" away from the lowest (= 1 layer of veneer) of the 1/2" plywood.

Tools: these power-files are great. Basically a belt-sander but very narrow so you can use it for any shape of the cut. I'm not sure you can get course enough belts for it though. Another great system is using a router but you need unobstructed surface area around the cut. Use a side-guide to guide it around the cut and set the bit-depth so that the outside of the bit is high-up enough for the bevel. Next, use the cut it made as the next guide-position, making a stepped bevel. Now you get at it with that power-file (or manual file, rasp etc. ;-)
Remember that it doesn't have to be perfect-looking, it's not scarfing where another piece must fit it. You will only join thin 3-ply and glass so that will even fit an ugly bevel.

If you have this rough bevel you can finish the last 1/8" dept and 1.5" diameter. The angle is there, you just widen it.

If you do find rot in a beam, I agree that this must be addressed too and it's best to do that before the skin.

About documentation: that "wooden boat building" book by the Goucheon Brothers (West System) is great. They also have thin booklets, get them all.

cheers,
Nick.

[Feral Cement]

Is there, are there any large sheets of fiberglass available to use instead of plywood when rebuilding decks and cabin tops? Seems like the rot-repair-repeat routine is a lot like "Fool me once....." Why not use an inert material and be done with it?

Thx, John

[Cheechako]

cut it out for sure. Use exterior plywood and epoxy it to marinize it.
or.....How about the ventilation hatch idea?