How do I keep a Billboard Gui from going through the ground?

I have a calculation that allows me to stick a BillboardGui onto a character's HumanoidRootPart.

local factor = 0.1
game:GetService"RunService".Stepped:connect(function()
    if enabled and Plr.Character then
        placetag.CFrame = CFrame.new(Plr.Character.HumanoidRootPart.Position+   (Plr.Character.HumanoidRootPart.Velocity*factor))

although this would be fine by itself, the Gui seems to go through the floor when somebody jumps. I don't want this happening as it messes up what I'm trying to do. I've tried turning the CanCollide option to false but to no avail. placetag.CanCollide = false

I need a calculation, or another way of handling this, that is able to take solid matter into account while also using the same calculations as the one above.