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New roof rafters

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johnfarris

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Hello Folks
I need to sister new common rafters along side existing ones and i am a little unsure on how to
configure it. The new rafters are 47 x 150 and the existing are 50 x 100m. If i cut a birdsmouth to suit the plate i would be taken more than a third out the rafter which obviously is a big no no. So do i move the plate forward to get over this? The pictures show some offcuts i used to figure out how much the plate would have to move forward which is exactly 80mm

Any suggestions, comments welcome




New rafters.jpeg
 

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MikeG.

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Who has told you to do the job, and who has sized the new timbers?

A birdsmouth only exists so that the rafter can extend past the plate and form the eaves overhang. If you don't need to form an overhang, you don't need a birdsmouth, and can just extend the seat cut for the whole width of the rafter. However, there may be something else going on here, which is why I ask about the design/ designer of this work. It is curious that the plate in the photos is sitting on top of joists, rather than a wall, and that makes the "truss" incomplete. In other words, I can't see anything tying this roof together. Are there principle trusses and purlins, then? And if there are, how would sistered rafters work?
 

johnfarris

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Hello Mike drawing and calcs have been drawn up by a design professional that i have lost all faith in.
Thinking about it I do not need to form an over hang as the existing rafters are staying and will continue to support the facia and soffit.

The new rafters will be supported by a dwarf wall sitting on a steel beam at the lower end of roof and also tied with collars. Apparently purlins that are in at the moment will not be needed. Their are no principle trusses. The pole plate the rafters are sitting on at the moment are sitting on 50 x 100mm ceiling joists. These ceiling joists are being supported by under slung joist hangers off steel beam.
 

TheTiddles

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I’ve nothing to say that Mike already hadn’t put better than me (I was going to say the whole thing looks odd)... but on people who do drawings one of my colleagues says “anyone can draw a picture, I need a engineer to make it work” he mainly says that in an exasperated way, on “special” occasions

Are you in the midst of a complex restoration? The whole setup sounds complex

Aidan
 

johnfarris

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TheTiddles":toy0gf5c said:
I’ve nothing to say that Mike already hadn’t put better than me (I was going to say the whole thing looks odd)... but on people who do drawings one of my colleagues says “anyone can draw a picture, I need a engineer to make it work” he mainly says that in an exasperated way, on “special” occasions

Are you in the midst of a complex restoration? The whole setup sounds complex

Aidan
What makes it complex is the lack of information in the drawings. I specifically requested at the very beginning to the Architect that i didn't just want drawings that would pass building control but i wanted working drawings that would give me all the details i needed and i was willing to pay for that extra detail.

Here is an example of the drawings and specs. The opening for the staircase was to small I did not find out until i had put half the floor joists in. In hindsight I should have checked the drawings at the start, you live and learn. As you can see on the drawing nothing on their detailing how to deal with the pole plate, hence my original question. I will go back to the Architect and ask him what to do as well as doing my own research.


Spec.jpg
Section aa.jpg
Section a.jpg
 

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MikeG.

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A 1:50 section is not a lot of use to anyone. Ask for a labeled 1:20 section. And careful of that stair. Without the min 50mm going at the turn, it doesn't work.
 

Lons

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I'm not suggesting this in your case but unfortunately as in all walks of life there architects who are excellent and those who are hopeless.
I've had dealings with a lot of the former but also several who really shouldn't have been practising. One of those after a fraught experience on a job he was involved with where I had to rectify his mistakes I was later offered another sizeable lucrative project but when I inspected the plans closely saw discrepancies and realised it was him again I turned down the work, a difficult decision as the customer was a friend. The decision however turned out to be a good one because the job turned out to be a pigs ear with building control issues and the builder had a major fall out with the architect.

The major issue with another one was on a grade 2 listed building I was about to convert and huge errors on the plans cost a delay of 2 months in starting as the LA wouldn't accept them due to the errors and would it have been much more if I hadn't re drawn sections of the plans myself. That was 2 months of reasonable weather wasted and we ended up working under tarpaulins. My customer was a regular who I did a lot of major work for and it was so bad he ended up refusing to pay his invoice and sending him a counter invoice for extra costs incurred.
 
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