Thanks JobAndKnock for your responses. My apologies if you think I am criticising your advice, that's not my intention. You clearly have a lot of experience in this area and your posts are very knowledgable.
At no point have I suggested that wood doesn't expand or contract, or questioned the degree to which is does under different circumstances. In fact I thought I had made that clear by giving the example of my outside door which expands and contracts every year. I'm also not having this discussion in the context of what you or other professionals should do, or what should be done in other houses, I am talking about what I do in my own home, and like I always do, I am trying to understand a little deeper why I am doing things, rather than blindly following standard guidelines that are intended for worst possible scenarios. I may not be a joiner but I am an engineer, so I understand a fair bit about materials and structures in general so your suggestion that I am ignorant of the issues is way off.
When you do a job you have to consider the fact that you don't know whether the customer will have an heat recovery unit that keeps the house at a constant humidity and temp, or if they will paint all the windows shut and keep the doors open when having a shower. I get that. I'm not challenging that. I am trying to think through the issues as they apply specifically to my own home. There are a lot of things you can get away with when working on your own house, or make choices on, that would be risky for a professional. Likewise a manufacturer has to specify in ways that reduce their liability to comeback. I could make a choice to fit wood flooring in a bathroom because I know we don't tend to use the bath in that room and there is always low humidity, you probably couldn't recommend that because you don't know that the customer won't be splashing water all over the room.
In any case, the point of the original post was not really about the degrees to which wood expands or how much of an expansion gap to leave, it was about the fixing at the edges where the expansion is greatest. If the outside board moves to take up the whole of the expansion gap of 15mm then the fixing also needs to move or bend by 15mm. You are right that nails are probably more forgiving in this aspect than screws, but its actually because I was considering using tongue-tite screws on a wood floor I am going to fit sometime in the next few weeks that made me think of this. I've mostly fitted floating floors in the past and with a floating floor there is nothing to resist expansion at the edges. With fixings there is, and the two most obvious scenarios, are that either the fixing holds and the floor buckles anyway, or the fixing breaks. Third scenario is that the fixing flexes by 15mm and maybe with nail will just pull out by that much, but I can't see how screws can move by this much and although I don't really know about the type of adhesive normally used, it seems a lot for it to flex without shearing. This is the nub of the question, can typical fixings really move by this much or is it more of a case that if the floor was to expand by this much the fixings would be sacrificial. If this is the case, maybe screws are a bad idea, as they are less likely to give or pull out.
At no point have I suggested that wood doesn't expand or contract, or questioned the degree to which is does under different circumstances. In fact I thought I had made that clear by giving the example of my outside door which expands and contracts every year. I'm also not having this discussion in the context of what you or other professionals should do, or what should be done in other houses, I am talking about what I do in my own home, and like I always do, I am trying to understand a little deeper why I am doing things, rather than blindly following standard guidelines that are intended for worst possible scenarios. I may not be a joiner but I am an engineer, so I understand a fair bit about materials and structures in general so your suggestion that I am ignorant of the issues is way off.
When you do a job you have to consider the fact that you don't know whether the customer will have an heat recovery unit that keeps the house at a constant humidity and temp, or if they will paint all the windows shut and keep the doors open when having a shower. I get that. I'm not challenging that. I am trying to think through the issues as they apply specifically to my own home. There are a lot of things you can get away with when working on your own house, or make choices on, that would be risky for a professional. Likewise a manufacturer has to specify in ways that reduce their liability to comeback. I could make a choice to fit wood flooring in a bathroom because I know we don't tend to use the bath in that room and there is always low humidity, you probably couldn't recommend that because you don't know that the customer won't be splashing water all over the room.
In any case, the point of the original post was not really about the degrees to which wood expands or how much of an expansion gap to leave, it was about the fixing at the edges where the expansion is greatest. If the outside board moves to take up the whole of the expansion gap of 15mm then the fixing also needs to move or bend by 15mm. You are right that nails are probably more forgiving in this aspect than screws, but its actually because I was considering using tongue-tite screws on a wood floor I am going to fit sometime in the next few weeks that made me think of this. I've mostly fitted floating floors in the past and with a floating floor there is nothing to resist expansion at the edges. With fixings there is, and the two most obvious scenarios, are that either the fixing holds and the floor buckles anyway, or the fixing breaks. Third scenario is that the fixing flexes by 15mm and maybe with nail will just pull out by that much, but I can't see how screws can move by this much and although I don't really know about the type of adhesive normally used, it seems a lot for it to flex without shearing. This is the nub of the question, can typical fixings really move by this much or is it more of a case that if the floor was to expand by this much the fixings would be sacrificial. If this is the case, maybe screws are a bad idea, as they are less likely to give or pull out.