When considering the cost of roofing materials, many people take into account just the shingle prices. This is incorrect, as a roof made just of roofing shingles will not work. You also need to consider all the accessories that are included in a complete roof system.
Business letters tend to have both the sender and receiver's address and the date at the top of the page and they begin with a polite salutation, such as dear Mr. and Mrs, followed by a blank line before the body or the content of the letter.
A business memorandum is an internal letter used for communication within an organization. They are an organization's internal version of the business letter. Memos can be written on any subject. Most organizations use pre-printed memo paper for their memoranda. Normally, when an organization communicates externally it uses a business letter, not a memo.