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Minera have just quoted for our POSIs. I'm wondering why there is an option for untreated metal web joists. Why would anyone want their floor structure to be more likely to rot? I have got soooo much to learn.

This week has been difficult and pleasing in equal measure. Never installed a joist before, but now 8 are up, sitting there waiting to be nailed in place. No nails yet - just push fit. Fussing them into level-ness was interesting. Mostly I managed to get them level enough to of split the laser beam - half on, half over the top. But doing that job made me look at the joists in great detail. The top surfaces have some very small bumps and dips above and below the laser line. Nothing substantial. Since I don't know what I don't know, should I get the belt sander out and make sure that the bumps are levelled out? Planning to nail them in and then put temporary boarding on them tomorrow: quite exciting because it's a bedroom - somewhere to put up guests?

Reading this thread and all the dire warnings written there sent me scurrying off into Tinternet to look and see what I can find. Well, the only method of fixing them I haven't found was by using bubblegum. There's oodles of guidance and I have read a good deal of it. MiTek say that the fixing method rests with the designer. So I had a little phone chat with two or three companies and they all said that 'we' could discuss how I wanted them to be fixed. And anyone who knows me, knows that that is going to be a one-sided conversation. So, come on BH, help me out here. help make me sound as if I know just a leeeetle bit about metal web joists rather than sound more of a complete plonker than I already am. I know what to avoid: floor bounce - because of @readiescards 's thread (thanks) not installing them back to front (PeterW here ) [What the Hell is back to front on a POSI fgs?] make sure the trimmer is correctly seated (Jamie same thread) make sure the chords of the joists (flange, web ?) are wide enough (Peter Stark same thread) use 400 centers (already knew that - by accident) make sure the bottom chord is supported correctly (PetereW same thread) AlexPHD (same thread ) tightly pack between the bottom chord and the wall (what with Alex?) Peter Starks are hung from the top chord on a ledger plate (Whassat fgs ?) and he uses a strong back (should I consider one?) PeterW's helpfully annotated image of where the problem lies (same thread) @JSHarris seems to argue against hangers (same thread) Try pre-cambering a little bit ( @JSHarris same thread) @CC45 has strengthened his POSIs (same thread) That's a good start.... Do people use hangers because they are easy, and becuase they are easy to use, they don't pay attention to detail (present company excepted)? My spec is for the POSIs to be hung on a wall plate (rim joist) Knowing all of the above, how would you make sure the problem didn't occur in the first place?

Roof on (ish) need to think about the flat roof and first floor now. Each is supported by metal web joists, POSIs in our case : MiTek. In measuring their length to assemble an order, how accurate do the measurements have to be? To within 5 mm? Its just that our wall is pretty wavy if you know what I mean (Don't ask, it just is ?)

Bit of help needed here please. Part of our roof is flat. I am determined to avoid at least some of the problems associated with flat roofs if I can. One strategy is to create a significant fall on the roof. The question is how to create that fall easily and hopefully cheaply. The plan: The elevation: The architect has noted that we should use soft wood (SW = soft wood I assume ?) tilting fillets. I note from this post that we should make the fall 1 in 40 or steeper. (Thanks @tonyshouse) The Fall: Is there any reason why we should not build that fall into the POSI joists in the first place? (instead of using tilting fillets?) Attaching the wall plate (rim joist) As planned, the wall plate (rim joist) attachment system (threaded bar resined into the concrete behind, I should imagine) will butt up to the Durisol block face: and that is the face which is backed by 185mm of graphite polystyrene - and then 120mm of concrete. On the other side of the Utility and boots room, the wall plate abuts the 'internal' face - the concrete 'side' of the Durisol block, so there is no potential problem with that face. In short, does the 185mm of insulation matter when attaching the wall plate (rim joist)? To help you visualize my concern, here's a schematic image of a Durisol block.

As I suspected, this learning curve really is pretty steep. So in an attempt to make sure that I reduce the number of errors in establishing our first floor at the correct level, I thought it might be an idea to start a thread - hopefully I'll be able to summarise it as a check list later. First: the end in mind Accurately mark out the position of the top of the wall plate (rim joist) such that our 202 POSIs sit at the correct height. Follow this bit of maths through with me please: Assuming the top of the POSIs is the same level as the top of the wall plate , it (the rim joist) needs to be 20mm (the depth of the decking) below FFH; (2350+255); that is (2605-20) = 2585mm above FFL In order to mark it out, since our floor is currently 25mm below FFL (currently we are at sub-floor) we need to add another 25mm = 2610mm above our current floor Naturally, it isn't that simple is it? Our timber spec is C24, treated, 200 by 50. Well wadya know? Timber of that spec isn't. It's less than 200 and less than 50 (195 by 45 ish). Which means the bottom of our POSIs are going to be hanging low (to coin a phrase). Our hanger spec is 200 by 50 Does it matter that the bottom of the hangers are going to be below the bottom of the wall plate?