WiFi to the Hub
Thanks for the shout. 
The general recommendations are very good, but for clarity we should just note first that only one of the smartthings hub models, the Samsung WiFi Connect which is also a Wi-Fi router, has Wi-Fi at all. The other models are all ethernet connected with no Wi-Fi radio.
You should still put the hub centrally in the home both vertically and horizontally regardless of the model, because it will reduce the number of hops required, but you donât have to worry about Wi-Fi to the hub because it doesnât use Wi-Fi unless you also have that particular model.
On the other hand, and this may be what @KL_Forslund was referring to because itâs a method that many community members use, you can get a Wi-Fi access point that plugs into the wall and includes an ethernet port on the side. Then you can just plug the smartthings hub into that port and that gives you the ability to put the smartthings hub anywhere you want to in the home, it doesnât have to be right next to your router. 
https://www.amazon.com/NETGEAR-N300-Range-Extender-EX2700/dp/B00L0YLRUW/
In fact weâve even had some community members use a similar method with a powerline device, one near the router where Internet comes into the home and one somewhere else in the Home that they plug the smartthings hub into. Since those signals travel over the homeâs wiring rather than through the air, they can be a really good way of getting signal into what is otherwise dead space. They wonât help directly with your zwave or zigbee devices, but they do help if you need to locate the hub someplace far away from your Internet router.
We actually use a couple of powerline devices in our own home, although not for the smartthings hub, but because one room is a converted garage with thick cement walls and we werenât getting any signal in there at all. Other people use the same method for basements which are RF dead zones. So itâs just another option to consider.
Cement and Plaster
These will block all radio signals, including Zwave, zigbee, and Wi-Fi. Itâs a common issue for homes in Asia, for example, but also in the American Southwest where Adobe is sometimes used. Thatâs why you can typically use Wi-Fi just as an indicator of where signal is getting through, even though the hub itself doesnât use Wi-Fi for most models.
Usually the only solution is more hops, and, as was noted, zwave allows for a maximum of four hops per message while the zigbee profile that Smartthings uses, zigbee home automation, allows for 15 into the hub and 15 out. So zigbee is commonly chosen for industrial buildings like warehouses where you may have a lot of cement construction. A lot of times we will bounce the signal out into the hallway and then into the next room, for example. It just gives you more options.
You may also be able to locate light switches so that they are almost back to back and get signal through the wall that way. But it does use up a lot of hops.
The biggest advantage that Z wave has over Zigbee is that itâs not in the same frequency range as Wi-Fi, but zigbee is. So very strong Wi-Fi can drown out zigbee where it wonât interfere with Z wave. The solution to that, as you may have guessed, is again, using more hops. But it is just something to be aware of if you are trying to adjust Wi-Fi reception and Zigbee reception at the same time, as you may be working against yourself.
Itâs usually best to get the Wi-Fi stable and then start laying out the zigbee backbone. It doesnât make any difference technically, it just saves you some time and inconvenience.
General Principles
See the following FAQ. Start by reading post 11, then go back up to the top of the thread and read the whole thing. It covers both Z wave and Zigbee.
Large Houses
Thereâs also a community FAQ for Z wave in a large house. Itâs not specific to the cement wall issues, but it might give you some ideas. In particular, it may be helpful to consider the use of zwave lightbulbs as repeaters as they can sometimes be the easiest way of getting signal down a hallway or up and downstairs in an area where there are no outlets.
Again, we should note here that Z wave bulbs generally are good repeaters (for zwave) while zigbee bulbs generally are not good repeaters for other zigbee devices.
What Other People Have Done
If youâd like to read about what other people have done with large house projects, you can take a look at the quick browse lists in the community â created wiki, look down at the bottom of the page for the âproject reportsâ section, and look for the âwhole houseâ list.
Weâve also had a couple of community members who live in Vietnam and have cement homes and had similar challenges. I could dig up some of those posts if youâre interested but they are also dealing with 230 V wiring so they have some additional device selection issues as well.