My name was added to my fathers house deeds as a joint tenancy, all done through a solicitor, house is mortgage free.
Does the house automatically become mine on my fathers passing ?
(have one sibling)

Are you in the UK?
Wording on deeds can have varying meanings depending on where you are. Does it say 'as joint tenants' or 'as joint tenants with right of survivorship?' (Or tenants in common, etc.)

You might want to ask your dad or his solicitor. Something that big and important doesn't stand on ceremony. Your sibling might have some different compensation in your dad's will, which is no one's business and a will can be changed on the proverbial deathbed anyway.
But being clear about his intentions on the deed is a good idea. Being on the deed gives you other rights and obligations, before he dies.

Thanks,
Will have to check wording, but do know agreement was joint tenancy and not tenancy in common.

In estate law, joint tenancy is a special form of ownership by two or more persons of the same property. The individuals, who are called joint tenants, share equal ownership of the property and have the equal, undivided right to keep or dispose of the property. Joint tenancy creates a Right of Survivorship. This right provides that if any one of the joint tenants dies, the remainder of the property is transferred to the survivors.
Joint tenancy n. a crucial relationship in the ownership of real property, which provides that each party owns an undivided interest in the entire parcel, with both having the right to use all of it and the right of survivorship, which means that upon the death of one joint tenant, the other has title to it all. Procedurally, on the death of one joint tenant, title in the survivor is completed by recording an "affidavit of death of joint tenant," describing the property and the deceased tenant, with a death certificate attached, all of which is sworn to by the surviving joint tenant. This process avoids probate of the property, but may have some tax consequences which should be explored with an accountant at the time of recording the original deed. If the owners do not want full title to the property to pass to the survivor, then joint tenancy should not be used. Joint tenancy (as well as any other common ownership) between a parent and a minor child should be avoided since the property cannot be transferred in the future without the parent becoming appointed a guardian of the child's estate by court petition, and the property and the proceeds therefrom will be under court control until the child is 18. In community property states, some courts have found that joint tenancy presumes that the property is not community property, (which could result in loss of estate tax limitation on the death of the first spouse to die), but proof of community interests can be established. A bank account held in joint tenancy also presumes a right of survivorship, but this presumption can be overcome by evidence that the account was really the property of only one, and the joint tenancy was for convenience.

http://legal-dictionary.thefreedictionary.com/Joint+Tenancy

... You might want to ask your dad or his solicitor. Something that big and important doesn't stand on ceremony. ...

No, what he intends is not important. What he did is. So, for example, if he Intended that you get the property after he dies but the solicitor mis-understood and wrote something different, what was written would normally control. So double-check with the solicitor that the effect of the deed is as your father intended.

Yes, and given that his father is still alive, he can clarify his intentions, and they can both check with the solicitor. That's what I meant.