When you need to determine the size of the expansion gap you have to keep around the whole perimeter of the floor, there are a few "rules of thumb". Specially with Solid Oak flooring: 3 - 4 mm gap per meter width of the room. Why? Because that is how much per meter wide Solid Oak can expand during the seasonal changes in air humidity.
It does sound like a simple and easy to follow "rule". Until we received a phone call last week from a desperate DIY-er. He had kept himself to the rule, his room was 4 meters wide and had kept an expansion gaps all around of 18mm but the Solid Oak boards (secretly nailed directly on to joists) had started to lift up in two areas. What could be the reason for this, he had checked for leaks and hadn't found anything suspicious.
When we asked some further questions it turned out that indeed his room was only 4 meters wide, but the joists run parallel the long wall - a massive 21 meter long wall (spread over 3 connecting "rooms") Meaning that over 21 meter "long" the new Solid Oak floorboards run row next to row next to row (perpendicular to the direction of the joists), creating in fact a 21 meter wide area of flooring.
In this case the length of the room had effectively turned into the width of the room and another set of "Rules of Thumb" should have been followed:
never install Solid Oak floorboards in a room wider than 6 meter without adding extra expansion gaps (by ways of installing thresholds or flat dividers in the most logical places, for instances where two rooms have been knocked into one and still have "pillars" or small parts of the old wall)
So, keep your wits about widths and realise that with installing wooden floors the actual width of the room sometimes has to be measured along the length of the room, it all depends on how you are installing your floorboards: lengthways or widthways!