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Old 04-10-2014, 02:10 PM   #1
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Vapor Barrier for Gravel motorhome pad

Hey folks,

I constructed a pad with crusher run to park my motorhome on. Due to the slope of the terrain, I had to build up the pad into sort of an enlongated wedge shape. It works fine.

I'm going to be bringing in a load of gravel to top it off, but want to lay down a vapor barrier first. What sort of material should I be looking at for a vapor barrier?

Thanks,
Ed
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Old 04-10-2014, 02:51 PM   #2
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Just questioning here but what benefit would a vapor barrier give you with gravel anyway. As i see it it would simply trap moisture below and possibly allow the ground below to sink with the weight. Are you trying to keep water off the base? I park mine on a roughly 4" thich pad of road base without any issues for two years now. But then i'm in Central California too.
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Old 04-10-2014, 02:55 PM   #3
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What you really want if you are trying to prevent vegetation is a garden barrier. It allows moisture to pass and go into the soil, which is where you want it, not on top. Costco often sells a good heavy duty quality product around this time of year but any home and garden shop should have what you need if Costco is not nearby.
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Old 04-10-2014, 02:57 PM   #4
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Just wonder what kind of vapor barrier won't rip, tear, or break up when you run you coach over it. I would think you would want any water to go down thru the original base???

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Old 04-10-2014, 03:09 PM   #5
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Use a heavy duty landscape cloth. I did the same thing about 4 years ago and to date have had issues.....still weed free.
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Old 04-10-2014, 03:15 PM   #6
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Is the OP talking of a vapor barrier or weed barrier
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Old 04-10-2014, 03:26 PM   #7
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In my experience, I have seen a vapor barrier specified under a concrete slab to keep moisture from ground away from concrete.
Weed barriers are just that. Not really meant to go under 4" of gravel because the dirt and seeds can still settle and grow. They are not intended to add support or spread weight.
In the road industry there are geo-textiles engineered for support of road base or surface. They also keep existing weeds from coming up.
The key to gravel is compaction. Keep in mind that the first layer of gravel over the larger stone should be compacted into the larger stone to reduce settling in the future. I would do this and then put fabric down and then 4" of 3/4" gravel or smaller crushed limestone. At least the area that the tires run over should be compacted.
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Old 04-10-2014, 07:42 PM   #8
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What will a vapor barrier achieve when there is 12-16" of open air above the stone anyway? If you were going to enclose underneath the coach down to the ground, I could see a benefit, but not with an open air space. Better to let the water run away on its own. Especially if the soil is sandy (drains well).
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Old 04-10-2014, 08:13 PM   #9
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Yes a vapor barrier is needed, as you state. It prevents ground moisture from collecting on the underside of the RV and causing corrosion. I've seen it used with success many times. Most of the people I know who've built what you propose use the heaviest plastic film they could find locally. They first graded away the top soil, then laid a base of fill dust from a rock crusher and compacted it with a machine; laid the plastic film, then topped it with small crushed rock. None of them however, parked something as heavy as a motorhome on the pad though.
I once was forced by circumstances to park my 1932 Chevrolet in a barn for a few months that had a dirt floor. I was amazed at how quickly the undercarriage began to corrode. Got my RV garage built, moved my old Chevy inside and began to repair the corrosion damage. Got it done, but it doesn't compare to the frame-off restoration it had.
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Old 04-11-2014, 05:24 AM   #10
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Thanks guys. I'll expand a bit on what and why...

I'm wanting to reduce the amount of water/moisture that sits underneath the motorhome. We get a decent amount of rain here and the soil is clay. Corrosion on the undercarriage and in the engine compartment is a constant battle.

I was thinking that a vapor barrier would prevent the moisture underneath the barrier from evaporating into the undercarriage, and it would allow the water that falls on top of it to drain away from the MH rather than collect and sit under it.

Those are my thoughts anyway. Appreciate your insights and suggestions. I am going to eventually put a carport over it which will help, I'm sure.
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Old 05-12-2021, 11:49 PM   #11
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Pad Moisture Barrier Selection and Placement

7 Years later and I'm wondering how the moisture barrier for your parking space has held up and what you ultimately selected.

I would like to emulate your set up for a park model that likely won't be moved after set-up. I'll be laying and compacting 4" of road base, would like to place a moisture barrier and then top off w/ a couple inches of 3/4" gravel.

What I don't understand is won't the gravel end up puncturing the moisture barrier when it is compacted/driven over? I've considered placing the barrier on top of the gravel, but it would still be driven over in placing the RV likely sliding and potentially subject to greater risk of puncture/tear.

I found this 20 mil barrier and it looks pretty heavy duty, but it seems kind of spendy. What do you think? https://www.americover.com/product/2...stic-sheeting/

Thanks much for your thoughts!

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Originally Posted by USMCRET View Post
Thanks guys. I'll expand a bit on what and why...

I'm wanting to reduce the amount of water/moisture that sits underneath the motorhome. We get a decent amount of rain here and the soil is clay. Corrosion on the undercarriage and in the engine compartment is a constant battle.

I was thinking that a vapor barrier would prevent the moisture underneath the barrier from evaporating into the undercarriage, and it would allow the water that falls on top of it to drain away from the MH rather than collect and sit under it.

Those are my thoughts anyway. Appreciate your insights and suggestions. I am going to eventually put a carport over it which will help, I'm sure.
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Old 05-13-2021, 04:12 AM   #12
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Holy thread resurrection!

Schult91, thanks for following up. This story ends with me giving up on the vapor barrier idea and putting in a 46’ X 16’ carport with enclosed sides. I also put a trench drain in the entrance to help keep water out. It’s not perfect, but it’s better than where I began.
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Old 05-13-2021, 04:25 AM   #13
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I'm assuming the park model will be skirted. If so do like a home on a crawl space, put the barrier on top of the ground. The skirting will hide it, you can use a lot lighter (cheaper) plastic, and I'll make another assumption, like a conventional home on a crawl there has to be venting to allow moisture to escape.
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Old 05-13-2021, 06:11 AM   #14
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I park our 5er on a built up gravel pad. That pad starts at driveway height then runs back 30+ feet with a final depth of about 4 feet. The sides and back were built up with 6x6 of .40 ground contact lumber, spiked together. At the low end I have a 4"PVC foundation drainage corrugated hose with holes. There is no vapor barrier and no weeds beyond a couple at the paved drive/gravel pad interface. I've been using this arrangement here since 1999, only enlarging it in 2014 for the current 5er. The gravel drains well. I used 3/4" (#57?) crushed stone fill. I had used crusher run at a previous home with less then great success since it compacts and wont drain well. If I was to do it again, would probably use one size smaller crushed stone. As a DIY project, lots of heavy work but doable if in reasonable physical shape - I got to hate shoveling several (5+) cubic yards of stone though.
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