It is not necessary for a compost bin to have ground contact. Many commercial composters do not and some people purposely build a bin that is raised above the ground and harvest the finished compost from under the bins rather than dig in the bin. The material you put in for digestion already has what is needed to be digested, all you do is make conditions optimal for faster digestion. For most of us it is simply easier to build the bins with ground contact.
When using cedar or redwood (cedar is a better choice if only because of sustainability) there is no need for a sealer or protectant.

It may not be necessary for a compost bin to have ground contact, but it is a very good idea for two reasons:

1) makes it easy for worms to move in and out of the pile. They move in when the temp drops; so it is good for cold and hot composting. The hot compost phase does not last forever.

2) the compost pile does not have to be as high when the pile rests on the ground. Lets take a 4' by 4' pile as a example. If you have the bin off the ground, say by putting thick woody branches at the bottom, the pile will have to be 4' high for max heat retention. If your compost pile sits on the ground, the compost pile can be less than 4' high. Because the dirt acts to store heat.

My two piles are 4'by8' by 24" to 30" high. I can reach 150F core temp with this size pile.

So which height pile would you rather turn over or move -- 48" high, or 24 to 30" high?

This optimal pile size is for hot compost piles. But I suggest that it might be a good idea to size your bins such that you COULD make hot compost if you wish. Nothing forces you to do so.

Cold composters don't have to worry about pile size.

I notice that folks hardly ever say what they will compost. That might be important.

Suggestions for bin sizes for ingredients:
Leaves finely shredded and some sawdust 3'by3' or 3' round.
Leaves finely shredded 4'by4' or 4' round
Wood Chips 5'by5' or 5' round

Keep in mind that a optimal or larger size pile will not CAUSE a hot compost pile core temp.

Rather, it will RETAIN the the heat that a good mix/moisture range/particle size compost pile starts with. And when in the hot stage, those thermophilic bacteria really go to town and reproduce at a astonishing rate.

Earthworms are not a significant part of making compost since in a properly built compost pile the moisture level is too low for them to survive. A compost pile with enough moisture for earthworms to live in will not be conducive for the bacteria to work and generate the heat that indicates they are busy working.
That is why tumbler composters sometimes do work.

"This is when the third stage of composting, the cooling phase, takes place. During this phase, the microorganisms that were chased away by the thermophiles migrate back into the compost and get back to work digesting the more resistant organic materials. Fungi and macroorganisms such as earthworms and sowbugs that break the coarser elements down into humus also move back in."

Weblife.org "The Four Stages of Compost"

Also, I fish. When the ground is dry it is easier to look for worms in the compost pile.

Below is a link to the bin system I created last Autumn. It has a lot of good info in it. If you search by my username you'll find other threads (with pics) related to the progress of my system as I constructed it.

It's made of untreated pine and covered in tung oil for some protection from the elements. Once it falls apart my plan is to replace it with a cedar version.

It was cooking up my compost pretty nicely until we got hit with a wicked cold winter here in the Northeast. Once it became impossible to turn the compost (which I always did to cover up my kitchen scraps from vermins) all I could do was dump coffee grinds on top of it for the rest of the winter.

I was finally able to turn the compost today but it did have some pretty large chunks of frozen "compost" that need to thaw out so I made sure I laid them on top of the loose stuff.

Here is a link that might be useful: 3-bin system

Earthworms may well move into finished compost and begin to eat that material, but I would prefer they do that in the garden where my plants would benefit more.
For the purpose of converting vegetative waste into finished compost earthworms are not a significant means of doing that, except in a vermicomposting operation.

https://www.growveg.com/guides/the-pros-and-cons-of-compost-tumblers/

States a UK study where tumblers took an extra month to compost vs ground piles of the same mass. They were turned the same amount.

I have 2 very simple compost piles made of wire from a roll. On each side and in the middle I use space for storage of items such as coffee grinds and leaves. It is a 3 foot wire that gives me 27 cubic feet of space in each one. Direct ground contact is used with no problems.

http://s241.photobucket.com/user/blaamand/library/Compost%20Bin?sort=2&page=1

I use a floor for two reasons. First, it makes each bin independent of the others so that I can add and remove bins individually. Secondly, a floor raised above the ground will keep out rats. There are only three places on the earth where there are no rats - the Arctic, Antarctica, and Alberta. So unless you are composting in one of these places, rats can be an issue.

Most of my bins have been made with salvaged 1x4 or 1x6 wood from pallets and shipping crates. I have built bins for friends, for school composting programs and for community gardens. The bins are easily assembled and taken apart.

My own bins are close to 20 years old and are still solid. A bin can be rebuilt by swapping boards from top to bottom and replacing bad boards. When rebuilding the floor, I now use pressure treated wood. It lasts longer and is not an issue unless you are rabidly organic. If you are organic then try to find unplanned 1x4 boards that are 1" thick. Sometimes pallets use the 1" thick lumber. The 2x4 floor supports are stronger when placed on edge.

It's very important to place the bins off the soil on stones or bricks. The leg bottoms in contact with the soil rot quickly. You could use pressure treated wood for the 2x4 legs but that's more expensive than using stones. As well, a lid or carpet will keep the birds away.

Hot composting doesn't involve worms so forget about worms. I have a storage bin with no floor for finished compost. The worms don't seem to interested in the compost.

In the end, your design will depend on you needs and your location.

If you haven't had rats in your bin I don't think it has anything to do with the floor. Any self respecting rat could access that design if there was something inside it wanted. Maybe it's what's in the bin which is the deciding factor, not the floor.

It is also a good idea to have some type of bottom on your pile if it is anywhere near a tree. The tree feeder roots will grow up into your pile and suck out the nutrients and also make it very difficult to fork the compost out. I line the bottom of 2 of my hardware cloth bins that are near trees with a double layer of plastic.

"Earthworms may well move into finished compost and begin to eat that material, but I would prefer they do that in the garden where my plants would benefit more.
For the purpose of converting vegetative waste into finished compost earthworms are not a significant means of doing that, except in a vermicomposting operation."

In my experience earthworms can convert raw material into castings faster than composting. I had gathered a mountain of leaves last spring( not shredded). I left them in place and poked wholes in the bags that I could get to. To my surprise,when I went to relocate them, the bags were almost entirely castings. The compost I had started before acquiring these leaves was nowhere near finished. These were well maintained hot piles consisting of shredded leaves, coffee grounds and scraps. Yes, the bagged leaves where much wetter than I keep compost ,and this was more contained than my compost. The worms beat out the microbes, so much so, that I will be doing this with half of my stock of leaves from now on There were abundant earthworms but a far cry from a pile of composting worms.

Unless the bottom is so tightly sealed as to not allow rain to leach nutrients out - I am not so sure any bottom at all will stop roots from following those nutrients to the source.

I have learned to compost directly in my garden for that reason.

I have two wire bin types in the garden and the plants growing near them do very well. You can easily move the bins each year if you choose.

Compost shouldn't be allowed to sit in a bin long enough to get that much root growth into it anyway. Yes there will be some, but it shouldn't sit there more than one growing season (max) IMO. Use it or lose some nutrients is my approach.

I don't think a raised floor will keep out rats just because it's raised. It has to be rat proof as does the entire bin. Chicken wire comes to mind. And that could be put on the ground under the pile just as well. It will rust after a couple years though.

The 2 bins that I had root problems with were within 10' of mature oaks. Fall started piles would have small (1/8" or less) feeder roots in the bottom 10" or so of the pile by the following July when I would harvest the compost for use between spring and fall plantings. I line the bottom and about 6" up the sides of the bin and so far I haven't had any problems with the roots penetrating the plastic. I am also turning and merging piles now in early spring which may be helping to keep the roots out.

Tox, chicken wire will NOT keep rats or mice out! Both can get through amazingly small holes! Your best bet would be hardware cloth (sturdy wire maybe 1/4 x 1/2"). Living in the country, I had much better luck moving the compost bins away from the house to the other side of the garden where hawks/owls/cats had access to the vermin! Nancy