Doing a loft conversion - am taking every opportunity to strengthen the roof as I come across it; so the ensuite wall is going to act as an extra roof support wall / knee wall type thing running about 1000mmm parallel alongside the purlin. Whats the best way to attach the timber for this support wall to the rafters at the top? I'm using c24 2x4's. I think I have 2 options, screw the 2x4 straight to the rafters then chamfer the upright struts at the top to meet this on the angle of the pitch, or fit the 2x4 top timber straight into the straight edge of the struts and somehow fit this to the rafters? I'd birdsmouth them but can't figure out how I'd get accuracy across all the existing timbers. I'm sure I'm not being clear here and the answer is obvious. I've attached 2 images of the al;ternatives - the photo is one where the timber is made straight but I can't see how it attaches to the rafters to offer any structural support - and the alternative I've drawn makes me think a chamfered top edge would be weaker as the weight would be coming down at an angle?
Purlin support wall
- Thread starter liamruairi
- Start date
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If you do it either of those ways the thing to bear in mind is that for the rafters to sag, all they have to do is tilt your wall into the room. The most important thing is doing a good job of creating the connections so that there is rotational stiffness in the wall.
Either method is equally good really, but have you considered running individual verticals up to the side of the rafter and sticking a pair of coach bolts through? That is going to be stiffer in rotation. Similarly, rather than bearing onto a plate over the floor joists, if you have a double joist under the wall, your verticals sit against the face of that and are bolted through, again that's much more stiff in rotation. Then you can add a binder across all the verticals at the top on the outside to tie them all together. Probably beyond what is necessary, and either of your methods will be fine, but it's basically the same amount of wood for a lot more strength.
My loft top-tip (I'm doing mine at the moment) is to pay the extra for C24 as C16 is generally much worse quality and the cost difference isn't huge. I have loads of C16 from earlier in the project that's going to end up on the fire it's so bent.
Either method is equally good really, but have you considered running individual verticals up to the side of the rafter and sticking a pair of coach bolts through? That is going to be stiffer in rotation. Similarly, rather than bearing onto a plate over the floor joists, if you have a double joist under the wall, your verticals sit against the face of that and are bolted through, again that's much more stiff in rotation. Then you can add a binder across all the verticals at the top on the outside to tie them all together. Probably beyond what is necessary, and either of your methods will be fine, but it's basically the same amount of wood for a lot more strength.
My loft top-tip (I'm doing mine at the moment) is to pay the extra for C24 as C16 is generally much worse quality and the cost difference isn't huge. I have loads of C16 from earlier in the project that's going to end up on the fire it's so bent.
Thanks for your advice, it all makes sense. The current rafters all need strengthening (some are split) and increasing in size anyway for the insulation so I'm adding extra 100x50 timbers to the face of each rafter for added strength and depth which should also help. For all the structural timbers I'm using c24 all the way
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