Best Material To Use in Greenhouse Flooring

15 x 20 space, in planning stages for the new greenhouse. I am wanting to have some sort of flooring that is easily kept clean and provides a lot of thermal mass.
Live at 8500 feet in mtns of Colorado, so day time temps in even winter can quickly warm up the greenhouse. Then night temps can drop to 11 in a heartbeat.

I am planning on putting in three raised beds, one halfway down the center, and then one on each of the sides (lengthwise) of the greenhouse.   I have "portable" shelves I use for starter plants, then can remove when done with them and concentrate on the raised beds.

Was thinking of building the raised beds with rocks, paver stones, or some material that will help store the heat at night. Tho wondering if that would be too much for the summer?
Then paver stones for the walkways, and 1 1/2 inch rock to fill in the gaps.

I can get nice rock for free, just have to search to find good ones. 

Open to any and all suggestions on what you all use.   Our current greenhouse has 3/4 inch rock, but do not like it as soil gets trapped in there, and all kinds of stuff can grow in it. Too hard to keep clean!

You could consider an insulated black  concrete floor.
 

Dig out the base area . put in 2 inch thick sheets of blue building grade expanded polystyrene and pour on 4 inches of 6 to 1 concrete that has a decent amount of concrete dye well mixed in each barrow load .
Lay it inside a wooden frame that is staked every 18 inches or so and make sure the frame is level  make the slap slightly larger then the base of the green house and smooth the wet concrete off . Give it a light water mist spray every two hours to stop it drying too fast and thus cracking up onn the surfaces and on the seventh hour lay some planks over the frame and then use a wooden " float"  to lightly rub the surface so it takes off any glaze but still leaves it nice and flat, carry on with the very light misting till it gets dark & cover the base with held off  sheeting if it is likely to freeze .as this will also affect the concrete .

If you want to be posh when making the frame  do it in halves along the longest length so that you make the floor ever so slightly " V shaped down the middle ... this helps no end when you want to wash the floor down after a deep clean & disinfect session and have the water drain off to the outside .

The guys who cast my black  concrete  floor did it whilst we were on holiday in Germany & didn't mix the dye in the whole mix nor did they cast it in two halves like we'd agreed .

 Yesterday when I dug a small hole at the side of the green house I also found that they didn't put the blue insulation in first either.


Yet  for all their omissions & poor workmanship the darkened floor slab does make for a reasonable heat sink of about a one cubic yard block of concrete .
When (  WHEN   ) the sun shines it takes an hour or so for the slab to start heating and on a night it gives out some heat for a couple of hours after sunset . It's not so good in cloudy weather but so far this year I've not had a freeze up inside it .( min outside temp so far this winter ) about 24 oF
 A couple of weeks during the last growing season I laid squares of black pond liner on the floors and also on all the benches under the plant pots to try & garner a bit more heat as the weather was very poor for us ..it seemed to help .
 

Obviously you will not normally be able to get more heat out than is put in  ..in summer it is a good heat steadying factor in cloudy conditions .

A boardwalk.

Sounds interesting! But we were trying to avoid pouring concrete, too expensive where we live....

I don't know what is best, but we used about 6 inches of wood chips. I have a tent style plastic side greenhouse and I needed to raise the floor above the plastic to seal from the wind. The narrow ends had about a 2 inch gap so we put wood threshold boards on their vertical edge and then filled it up inside. It keeps it from being muddy if we ever get a large amount of rain that could seep under the walls.

I just finished putting in my beds in (2-2x5) my just finished cattle panel greenhouse this week end. All that I lack is the ground cover over the weed barrier on the ground. I was going to use pea gravel but after reading Plantoid and the dark floor I am going to look for a dark grey gravel.

johnP, do ya have pics of your new greenhouse? Would love to see them! 

I haven't mastered the post pictures yet but the frame of the greenhouse is in my Avatar. I will change the Avatar to the finished product in the next couple of days. Other than the messy job covering the frame with remnants instead of buying covering large enough to drape the whole frame, sides included, I'm really happy with the results. I filled and watered the two beds Saturday and hope to plant something this weekend. Kale and lettuce comes to mind.