Source: http://www.findmeagift.co.uk/
This post is dedicated to Clive Upton, who was surprised that I spelled Granddad with two d’s in the middle. I’m no native speaker, and just told myself: surely it must be grand + dad. Since then I looked it up in my copy of the Concise Oxford Dictionary, which allows for either: grand(d)ad, but this doesn’t help the linguistically insecure or those of us who just want to know which of the two is the correct spelling. Surely one must be better than the other? So, tell us what YOU think is better or more correct? Or if you think either is fine, in which case it wouldn’t be a usage problem?
Even if most people on this blog voted for granDDad (71.4% at the moment of writing), Penguin seems to prefer granDad. Or seemed to do so in the mid-1980s: I’ve just reread Nice Work by David Lodge (p. 373, where Grandad is spelled with a capital G as well).
Bridging the Unbridgeable 
grandmother>grandma>gramma>grama?
Grammar, clearly.
Never had this problem as one grandfather was always called ‘Pop’ and the other was Grandpa. Grandpa’s wife was Nana and Pop’s wife was Grandma. No logic to it, I agree.
I haven’t ever seen Granddad in print so suspect because it looks awkward it isn’t used much. Even though it’s an option.
In India the grandparents are named to indicate which side of the family they belong to.
Father’s parents : Dada / Dadi
Mother’s parents: Nana / Nani
All relatives have titles relative to their position to you, so my maternal aunt is Masi but my paternal aunt would be Bua. This is very useful when you are working out how people are related.
Fowler (1926) prefers ‘grand-dad’, following the OED; (1965, ed. Gowers) ignores it; (1996, ed. Burchfield) prefers ‘grandad’, but also lists ‘grand dad’; (2015, ed. Butterfield) also prefers ‘grandad’ but has lost ‘grand dad’.
Meanwhile, in the US, Garner (1998) says: ‘granddad’; ‘grandpa’. These colloquial terms for ‘grandfather’ are so spelled. ‘Grandad’ and ‘granpa’ are variant forms to be avoided. But ‘grandpa’ is sometimes shortened to ‘gramp’, ‘grampa’, or ‘gramps’, all three of which are recognized as good colloquial AmE.
By (2016) Garner has moved the entry to ‘grandpa’: ‘grandpa’; granddad’; ‘grandad’. These are colloquial terms for ‘grandfather’. ‘Grandpa’ (the most prevalent term) is sometimes shortened to ‘granpa’, ‘gramp’ ‘grampa’, or ‘gramps, all of which are recognized as good colloquial English. As for the ‘-dad’ terms, the standard spellings are ‘granddad’ in AmE and ‘grandad’ in BrE.
A Google Ngram search in ‘English’ yields ‘grandfather/grandpa/granddad/grandad/grampa/gramps/granpa/gramp’, with ‘grandfather’ way out in front, ‘grandpa’ a strong second, and the rest more or less lumped together.
Changing the search to ‘American English’ yields largely the same result, with ‘grampa’ and ‘grandad’ changing places, while changing it to ‘British English’ yields ‘grandfather/grandpa/grandad/granddad/granpa/gramp/grampa/gramps’.
There is more checking to be done!
We could do a much longer survey!