I have a 42 ft. by about 10 ft I am landscaping, we are starting with half moon rock steps to the front of the house and working from there. It will be one step high with a planter in the center, big heavy rocks for the base and flat slate type for the steps. Do I have to use mortor or will some other method hold the top steps together. They are all 20 lb's or more.
I'm not sure if I understand your question correctly; not clear on what "it" is that will be one step high, since you've said "steps". If the question is whether 20 lb slate pieces will stay in place on top of bigger rocks without mortar, that depends - on how flat the base is, and what you plan to put on the steps. If it's slate, it could snap or flip if you put a whole bunch of weight on the outside edge (say, when a new fridge is delivered) depending on whether it is anchored at the inside edge or not. We just built some small stone steps using fairly thick flat rock (big pieces of wallstone), about 4-6 inches thick, with the pieces weighing some 150 lbs or equivalent (see bottom piece in attached photo). Even then, when we roll something heavy up or down, we brace with some scrap wood so that the rocks are less likely to dislodge. In other words, I don't think it's mortar that makes the difference but rather the stability and suitability of the material you choose. Don't know if this helps, but if it doesn't, try explaining further or posting a photo of your work in progress.