Roofing

Temporary removal of truss webs to install a coffin water tank behind

Anonymous user 23/02/2024 - 3.54 PM

I previously posted about this and had some helpful replies which prompted further questions so I'm going to try and add more detail. Apologies in advance for any lack of technical knowledge. I am a DIYer and I have a coffin water tank which I want to install behind the roof trusses in the attic. I have had my plans approved by a plumber. I just need to redirect the pipework and remove the old circular tank which is blocking the entrance to my attic and reducing the usage space. The new coffin tank would sit longways behind the upright back trusses which I believe are called tension webs. I have a standard fink truss design. My issue is, I can't fit the tank through a gap in the webs as they are too close together for me to twist the tank around and slide it into position. My plan was to temporarily remove a tension web and possibly even one of the tall compression webs and brace them while I push the tank into place, then put them back after. I would brace them by screwing a piece of horizontal timber to the adjacent webs either side of the one I'm removing, high enough up to leave me space underneath to push the tank through the gap, and then screw a short upright onto the brace and screw that into the purlin, so that the weight is temporarily distributed down the two adjacent webs. The webs themselves are difficult to remove however because they are secured with nail plates that are hard to lever off. I wondered if it was ok to cut them out near the top and bottom and then put them back after with metal plates either side of the cut join. Replies to my previous post from roofers said that this should work but in addition, I should add another block of wood at the side and bolt it to the web for added strength. They mentioned the added weight of the water tank. I presume they mean the downward force of the tank on the rafters which in turn pulls down on the webs. But I'm no expert so I was unclear on how the weight of the tank would affect the webs. They also suggested strengthening underneath the tank to distribute the weight which I plan to do. The floor is boarded out but I was going to build a raised platform using joists that would go at 90 degrees to the direction of the ceiling joists, placed in a row and a flat base on top of them for the tank to sit on. So I just wondered if it's safe to temporarily remove a web as long as I brace it, and if it's ok from a building regs perspective to cut them out, as long as I secure them properly afterwards? I hope this makes sense. It would probably be clearer in a diagram. I would get someone in who's qualified but I don't have any reliable contacts locally and I also enjoy the challenge of doing it myself if I can. Cheers.