The GMC Interior Rear Floor

This section details some of what I'm going through in dealing with a rotted floor in the rear of our 1976 Palm Beach. Last summer I installed a permanent rear bed before we left for a driving trip from California to Pennsylvania and back. Because the rear false floor was stubborn, I decided to forgo the added storage space until I had a chance to spend a little time coaxing the old floor out.

As you have probably noticed, there is a ramp in the rear of the GMC that adds a few inches to the floor height. According to GMC legend, this false floor was added after a GMC executive's wife complained about the distance from the rear settee to the floor. Remove the ramp and you have more storage space.

So, when my wife and I went to work on the coach today, she started disassembling the permanent bed so I could remove the false floor. Soon I heard the dreaded "you have to come see this." "This" was a molded mattress and soggy wood. New wood. New mattress. Now useless junk. Out came the bed.

The top of the water tank area looked like it did when I first bought the coach last year. A little water stained but OK. There had been a roof leak in the past, but it was fixed. The curb rear corner doesn't look as bad as it is. The stain on the wall is what happens when the PO uses the wrong adhesive - it runs.

The genset area looked stained but no worse than originally.

Next, up comes the false floor. I unscrewed the large wood screws and pried the thing up. Ahhh. Nice flat floor. Just right for a rolling storage box. But what's this??? The floor is damp. Out comes the trusty awl to test the corner between the tank enclosure and the rear of the coach.. You can see the damp spot to the right of the awl. No I didn't hammer the awl into the floor. That's the result of pretty light pressure. The awl sunk in until it hit the aluminum on the bottom of the insulation.

OK. This means that the tank has to come out for more inspection. But at least I know what happened to my last partial tank of water. The tank comes out by removing a few screws and loosening the fittings. Drain your water tank and lines before you do this. The water pump to the rear includes a number of fittings. The gray fittings are the fresh water connections. A water filter and hose clamp fittings. The water drain is just below the bottom of this picture. Open the drain valve and the water, or most of it, drains to the ground.

My tank had been "repaired" to fix a few leaks. One leak was along a seam and opened up slightly. As you can see, the sender unit is hopelessly potted into the tank. Looks like a new water tank gets ordered tomorrow. And a new sender.

When the tank is gone, it's easier to see the damage. But what a lot of damage. The side closest to the bath module had almost no intact plywood. Even the patch put in place by some was delaminated and the end towards the rear wasn't any better.

The plywood running across the rear is completely rotted from one side to the other. My plan is to replace the rotted sections in pieces. But first the old stuff has to come out. Before I do, I measured the placement of the mounting hardware. Light scraping with a flat bladed screwdriver left large sections of the plywood torn up. You can see the edge of the rotted wood as a light tan/yellow on the driver's side of the coach. The curb side is worse. The light green in the middle of the picture is the rear floor joist.

Now I cut out the rear rot. The gray area is insulation that has been discolored by road grime (some insulation was cut away earlier to mount hangers.) The light gray/green is the insulation. To make the cut I used a skill saw with a cut down blade. I drilled a few staring holes in solid plywood and then cut just to the joists and then along them. I had to cut down a blade so that it didn't pierce the aluminum on the bottom of the insulation. Here's a view of the whole cutout area.

What's next? I have to remove the propane tank in order to cut out the rotted wood there. And the same on the battery side. Then I'll replace the rotted wood with new plywood. Joints will be half lapped where I can reach with my router. This is primarily important near the joists since I chose not to chisel off the plywood to the middle of the joists. The overlap will be joined with a construction adhesive (I'm not sure which one yet) and screwed. The water tank area will have an added layer of adhesive-joined plywood. The area for under bed storage will also have a second layer of plywood to spread the loads.

Oh. While it's open ... I'll replace the water fill, maybe the vent hose, probably the heater hose. Also, since I don't need the frame for the settees, I'll redesign the frame around the water tank to amkle access easier once the fold up permanent bed is installed.

And that's what I did today.


Continued Wednesday

The skill saw cuts were pretty good, but not as neat as I'd like. I used a plunge router with a 3/8" straight bit with a bearing tip so I could follow the floor joists. I set the depth so that the bearing was on the joist but the bit was slightly above the joist surface. In theory the bearing will keep the bit from hitting the aluminum but I wanted to make absolutely certain that the bit wouldn't hit the metal. The carbide bit would probably mill off some metal, but the "grab" when the bit hit the metal could be a problem. The router trimmed up the edges of all but one side without a problem. I had handwork in a few spots, but it was minimal.

Next came making the half laps in the floor. I used the router with a 1/2" straight bit. Parts of the edge could be routed using a normal router fence attached to the bottom of the router. The trick is that some parts of the floor can't be routed easily. So, I attached a temporary router fence made from a scrap of wood to the floor. I found this to work faster and easier than the router-mounted fence. Check to make sure that the temporary fence is square to the other cuts to make fabricating the floor patch easier and that the router will cut the half lap where you want. The router leaves some material to be worked with hand tools. The right hand side of the picture shows about 4-5" of flooring that needs to be chiseled out.

Once the floor is prepared and cleaned, It's time to make the patch. I cut the patch so that it would be a snug fit between the two aluminum casements. The odd shape was transferred from the floor to the plywood by using several measurements along each edge. With the exception of one edge, all corners on my floor cuts were square, so the creation of the patch was fairly straight forward. Measure several times and then cut. Remember that you need to keep the same orientation when laying out the patch - decide which view (bottom or top) you will use and stick with it. I mark the waste areas with an "X" to help me make the cuts where they belong. The underside of the patch is routed to make half laps corresponding to those on the floor. One short section of the floor was very difficult to cut the lap in, so I decided to leave it butted. The section is less than a foot long and has the other two sides lapped with the last side running across a floor joist. Here you can see the half lap joints routed out, and an additional relief cut into the floor. The relief is to accommodate room to remove the fresh water drain if I ever need to. The nut that holds the drain was very close to the flooring and was difficult (OK it was impossible) to turn from inside the coach. It looked like the plumbing was replaced by a Previous Owner and they finger tightened the fittings - which became frozen to finger removal with time.

I cut the patch slightly oversized so that I could make final adjustments on the coach. The half lap joint was 1/8" oversized as well, giving room to adjust the top piece of the joint while assuring clearance underneath. I trimmed all edges to fit closely using several trial fittings.

The patch was installed using urethane construction adhesive and screwed several places along the joints to hold the patch in place while the glue cures. Since I don't have the time now to remove the propane tank to complete the repair, I blocked the gap between the new floor and the aluminum casement on the propane side with "foam in a can insulation." It's a polycel urethane that gets hard when it's cured, but you can easily work it with a knife. The final patch looks pretty good.



The whole job took about 12 hours: removing bits and pieces of the coach, cutting the floor, routing out the lap joints, making a patch, adjusting it to the floor, and installing the patch. If I put the old furniture pieces back in, I'd add another hour or two putting the water tank enclosure bach together and installing the settees. The job was not too hard, but a little frustrating at times since I kept discovering things that I couldn't just leave alone while I was doing the work. Now I have an enclosure to make since the fiber board pillow locker is beyond salvage (but looks great from the front). And the same goes for the driver's side enclosure. When you redo any significant portion of the rear area, you'll want to consider making the curb and driver's side tail light interior access panels more accessible.