Sparrowhawk Posted July 26 Share Posted July 26 I have a joist parallel and 6cm away from the wall I need to get my MVHR ducting through. The joist is approx 7 x 2 inches. Is it permissable to notch the bottom of a joist? In this case taking off a 45 degree bit where each duct goes, say across half the width of the joist. The guides online are all about notching the top. The ceilings this side are lower than the other side of the wall, so I need to go through at an angle and this would give more space to bend the ducting through 45 degress than cutting entirely through brick. Link to comment Share on other sites More sharing options...
MortarThePoint Posted July 26 Share Posted July 26 What is the distance between the joists? Link to comment Share on other sites More sharing options...
MortarThePoint Posted July 26 Share Posted July 26 (edited) https://www.labc.co.uk/news/how-get-it-right-notches-holes-solid-timber-joists Lots of US guidance has notches in bottom as well and there the rule is maximum depth D/6 and maximum width D/3 and avoid square edges (where D is the depth of the joist). On that basis a 1" deep notch wouldn't be a problem. Not in the middle third of the joist span though. I know US code is irrelevant to the UK, but it sometime provides me with some comfort. Your image makes it look like you'd only be shaving a small amount off. Perhaps a triangle that is 1" x 1". That means that you are leaving 75% of the material that would be cut out by a full width 1" deep notch. Should be fine, especially given it is so close to the wall and therefore not supporting as much floor above as an ordinary joist. Edited July 26 by MortarThePoint Link to comment Share on other sites More sharing options...
Sparrowhawk Posted July 26 Author Share Posted July 26 20 minutes ago, MortarThePoint said: What is the distance between the joists? 30-34cm from this joist to the next (working on screw holes), and then 40cm spacing for the rest. 14 minutes ago, MortarThePoint said: Not in the middle third of the joist span though. That is where I'll be - a 3m span, and I'm 1-1.5m out from one wall. Link to comment Share on other sites More sharing options...
MortarThePoint Posted July 26 Share Posted July 26 Just now, Sparrowhawk said: 30-34cm from this joist to the next (working on screw holes), and then 40cm spacing for the rest. That is where I'll be - a 3m span, and I'm 1-1.5m out from one wall. How deep would you notch be at its deepest? Your image suggests about 25mm Link to comment Share on other sites More sharing options...
MortarThePoint Posted July 26 Share Posted July 26 Assuming there is no wall or unusual load above, a 6x2 would likely be sufficient for a 3m span anyway https://nhbc-standards.co.uk/6-superstructure-excluding-roofs/6-4-timber-and-concrete-upper-floors/6-4-8-timber-joist-spans/ It's all looking promising. Also, this joist is carrying (34cm/2)+2.5cm+6cm = 26cm of floor where all the others are carrying 40cm. Link to comment Share on other sites More sharing options...
Sparrowhawk Posted July 26 Author Share Posted July 26 23 minutes ago, MortarThePoint said: How deep would you notch be at its deepest? Your image suggests about 25mm Yes that seems about right. If I could do more great, but there's 12.5mm plasterboard below the joist so doing the same 45 degree cut in that gains me another 12.5mm. 19 minutes ago, MortarThePoint said: Assuming there is no wall or unusual load above, a 6x2 would likely be sufficient for a 3m span anyway There is a doorway in the wall above, so is only footfall that comes in over this part. This is looking promising. Link to comment Share on other sites More sharing options...
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