Jacks, Plumbs and Bird’s Mouths

Spent yesterday working on the complicated bits of the dormer wood work. It’s all a bit too slow, especially working alone when moving wood up and down for measuring and cutting. Although having other people around at this stage would probably not help as I need to concentrate.

I’m sure that there’s a method the makes all this easy, but I can’t work it out. In theory you just cut the end of a jack rafter with a compound angle: plumb (20/9.5 on the framing square) and 45 degs (10/10 on the square). That’s all fine but the valleys are not a perfect 45 and their faces are not perfectly vertical. So the cut is easy enough, but is not a great fit. A bit of tuning and I can get the jacks to mate up to the valley rafter with only a 3 or may be 4mm gap in places. It shouldn’t matter as there is a good point of contact and I’m using steel framing brackets designed for jack rafters. This is where the strength comes from.

My general impression of working the 5 inch sections of rough cut timber is that the mathematics becomes fuzzy. So is you want 30mm here and you start with 120, you measure 90mm, mark it, cut it and you end up with something in the region of 30mm. Put that up on your frame where you measured 30 and the a 5mm gap? My mindset is all wrong. I’m used to working in fractions of a millimeter, not fractions of a mile. As Andy kept saying, in this game “similar” is the same.

I’ve now got most of the dormers rafter cut. Just 4 easy ones to do today. The weather is going to be great. The sun is just coming up now, cutting between the mountains. As soon as it hits the house, I’ll be on it. Must finish today.

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One Response to Jacks, Plumbs and Bird’s Mouths

  1. ian says:

    I found out why it doesn’t work. It’s not 45 degs! You must adjust according to the roof slope. I think the framing square is marked out with graduations to work it out, but I can’t see how. Anyway, I measured it and now the joints are tighter.