sandwich floors and roofing tars

[Katie&Craig]

I have a few questions regarding sandwich floors and roofing tars.

1) We are going to build a sandwich floor ala the Steve Frederick plans (1/2 ply topside, ¾ framing/insulation covered in roofing tar, ¼ ply roadside. Since the ¼ ply on the roadside needs to be waterproofed anyway, is the roofing tar in between necessary?

2) If so, where do you put the tar to be most effective? Kuffel Creek & Generic Benroy plans have the tar trailerside of the insulation (1/2 ply, then roofing tar, then insulation.) SF plans call for tar to be placed roadside of the insulation. (Maybe I am mis-reading his plans – not sure if tar is used roadside only if there is no roadside ¼ ply, or if used in both cases?)

3) Once it is dry, will the tar get sticky again on hot days? The Generic Benroy plans call for the bottom of the sides to be coated in it. We are resting our sides on the frame and not the floor, so will need to coat the bottom of the sides in something. The bottom of the sides won’t be completely covered by the frame because our frame is 4’ wide, with sidebars sticking out occasionally to support the 5’wide body. We are going to have a few things mounted under the frame and I don't want to be touching sticky tar getting at them.

(Sigh - I’ve read 3 different plans and still have questions – thank goodness for this forum!)
Thanks,
Katie

[oklahomajewel]

Hey Katie... I noticed that in the Kuffel plans too.... and I wonder if adhesive is good enough or have to screw it on? It doesn't exactly say.

I didn't plan on putting another 1/4" on the roadside but that's a good idea...

julie

[mikeschn]

Since you are building a sandwich, the only place you should need tar is on the sheet of plywood facing the road.

The key to coming up with the right answer is, can water touch it. If it can, put tar on.

A while back the question was, will the inside of a sandwich collect condensation? I don't know what the answer was... but if it would collect condensation, then you want to protect those surfaces too.

On my first Benroy I used roofing tar, and once it dried, I never noticed it get sticky again. Does anyone have a different experience to report?

Mike...

[DestinDave]

I made my floor a construction also: 2x4 frame with half-lap joints. I glued and screwed 1/4 lauan to the bottom. Caulked all the inside joints, laid in 2 layers of 3/4" foamboard, and ran a bead of caulk around all the joints between foam and 2x4. Then I glued and screwed 1/4 lauan to the topside. I finished the top and sides with a coat of epoxy and coated the bottom only with rooftar. It should be waterproof all around. When I bolt it to the metal frame I will run a bead of caulk along the channel also to seal water from the boltholes on the bottom. I figfure an annual check underneath should be all that's needed to maintain watertight integrity.

[cracker39]

My floor is just 3/4" ply with foam beneath. I put the tar on the plywood, then put in the foam and sealed it well with poly sealant. But, now, I am considering putting a coat of tar on the foam after I am finished with the build and can run it up a ramp so that I have more clearance to work underneath. I don't relish the idea of dripping tar all over me, but I think it need the protection for the foam to keep water out and help keep it from deteriorating.

Doesn't the roof coating brush on better if it's warm?

[Dee Bee]

Hi All

Here is a suggestion: Instead of using roofing tar, which can be a serious mess, try using spray automotive undercoating. You can buy this in spray cans. 6-8 cans should cover the average TD bottom.

I agree that this finish may not be a thick or durable as roofing asphalt. But it isn't as heavy either. I decided to use it an see how it works. If you check out my build journal you'll see that my TD deck is built on a wood frame.

So far I have been pleased with this approach. I found that it is able to pentrate cracks as I sprayed. I was able to apply the coating thicker where I thought the bottom would catch more road weather/wetness. I believeevery two or three years I spot coat the bottom again using the same product. I'll just tip up the TD and spray away.



Here is how I built my deck...
http://www.nfdc.net/home/cbdb/Teardrop%20Deck.htm

Dee Bee

[DestinDave]

Looks good, Dee... what kind of mileage do you get with that tow vehicle ???

[Dee Bee]

Looks good, Dee... what kind of mileage do you get with that tow vehicle ??? :lol:

I tow with a Honda CRV and usually don't know the TD is there. I haven't really check mileage. I travel at a slower pace and leave room for braking.

Thanks!!

Dee Bee

[Artificer]

My floor is just 3/4" ply with foam beneath. I put the tar on the plywood, then put in the foam and sealed it well with poly sealant. But, now, I am considering putting a coat of tar on the foam after I am finished with the build and can run it up a ramp so that I have more clearance to work underneath. I don't relish the idea of dripping tar all over me, but I think it need the protection for the foam to keep water out and help keep it from deteriorating.

Doesn't the roof coating brush on better if it's warm?

Test the tar on a scrap piece of insulation. If you apply it directly to the foam, it might disovle it, since its petroleum based. If the foam is expanded polystyrene (white bead board) you need to seal it. If its extruded polystyrene (pink or blue foam board) or iscocyanurate (has aluminum or fiberglass covering) then its waterproof. You still need to protect the extruded polystyrene from UV by painting or covering it. The isocyanurate is alread proteced by its coating. You also might want to consider the damage it might take from road debris, like stones being kicked up as you travel.

[cracker39]

All of my foam under the floor is the foil-backed and any raw edges are coated with poly sealant, so the roof coating won't dissolve anything. I think it would add protection to the foam from road debris.

[Sonetpro]

I covered mine with Acrylic Resin Lacquer. This is boat bottom paint. I think if it seals a boat in the water it should seal the bottom of the TD.

[Steve Frederick]

I have a few questions regarding sandwich floors and roofing tars.

1) We are going to build a sandwich floor ala the Steve Frederick plans (1/2 ply topside, ¾ framing/insulation covered in roofing tar, ¼ ply roadside. Since the ¼ ply on the roadside needs to be waterproofed anyway, is the roofing tar in between necessary?

2) If so, where do you put the tar to be most effective? Kuffel Creek & Generic Benroy plans have the tar trailerside of the insulation (1/2 ply, then roofing tar, then insulation.) SF plans call for tar to be placed roadside of the insulation. (Maybe I am mis-reading his plans – not sure if tar is used roadside only if there is no roadside ¼ ply, or if used in both cases?)

3) Once it is dry, will the tar get sticky again on hot days? The Generic Benroy plans call for the bottom of the sides to be coated in it. We are resting our sides on the frame and not the floor, so will need to coat the bottom of the sides in something. The bottom of the sides won’t be completely covered by the frame because our frame is 4’ wide, with sidebars sticking out occasionally to support the 5’wide body. We are going to have a few things mounted under the frame and I don't want to be touching sticky tar getting at them.

(Sigh - I’ve read 3 different plans and still have questions – thank goodness for this forum!)
Thanks,
Katie

I think you are mixing three trailers together! I did the first, a Kuffel Creek design, with tar on the road-side of a 1/4" layer of ply. On My 'Diner, I used ply with epoxy and varnish.
Mike's 'Diner got tar over bare insulation. I'm not a fan of tar at all! It takes too long, if ever, to completely dry. It does get tacky in the heat, even after curing. I'm using epoxy on my projects. It looks nice, and is waterproof.

[Katie&Craig]

The key to coming up with the right answer is, can water touch it. If it can, put tar on.

Mike...

A good reminder!
Thanks all for the suggestions. I will definately seal the bottom ply (and caulking in the interior too). I think I will look into some of the other options listed - I know the tar will probably be the cheapest but I am still wary of the potential stickyness.

Steve,
Yes I thought I might have muddled it. Thanks for the clarification. I haven't built anything for 15 years and find I have to re-read things several times before I can visualize it.
Thanks,
Katie

[Ira]

What's the big deal if it gets sticky in hot weather? You're not going to be going under there to play with it.

Those of us who have lived in big cities have roads done in HEAVY tar. It gets hot in the summer, and you can scratch your name in it with a nail.

But the asphalt roof emulsion that a lot of us use doesn't go on that thick. More like a thick coat of paint. Plus, it's not even exposed to sunlight, so I don't know if that kind of worry about is all that justified.