Structural report, roof modifications

[nads1978]

I just purchased a property, the Structural Engineers report says:

there is no rigid connection between the rafters and the ceiling joists. We recommend that this area is strengthened with the adoption of ties between every other rafter and the adjacent ceiling joists to give continuous ties from front to rear. 75mm deep x 50mm wide C16 timber ties should be adopted at as shallow an angle as possible and rigidly fixed to both the rafters and the ceiling joists. A check should be made in all of the other roof voids to ensure that these are also tied in a similar manner.

From my picture, it shows in green the vertical 'tie' that is randomly placed around the roof structure (not many of them). As I understand it, the Engineer is asking for some bracing to be put in the position I have illustrated in red. What does he mean by 'shallow angle as possible'? Please reference the numbers/letters to help me understand. Would it be more 7g or 2f? just as a guide.

Or am I barking up the wrong tree?

Also (I'm not at the property right now), if the rafters and joists are not flush, is it okay to 'notch' the new timber as below:

 

[pinenot]

Without seeing the job, i would say your on the right track although he may be suggesting propriety (metal ties). For you I think that would be a lot simpler and easy. The easy way is normally the correct way, in my experience.
So count the number of ties required i.e. for every other one, as noted, cut straight galv. restraining straps to the required length, and screw fix these in (use three holes min. per end) size ten galv. screws would be best. You only require to do one side per rafter/joist joint, but if these are not in line perpendicularly, you will have to create a suitable bend each side to get a touching face joint so the screws are not under any pull out load...pinenot

 

[nads1978]

Very informative and useful, I'm now interested in using galv straps. Thanks.
However, can you/anyone elaborate on what sort of angle, or is it okay 'roughly' where I illustrated it?

 

[tony1851]

r is it okay 'roughly' where I illustrated it?

No;
What he is suggesting is to get the join beween the tie and the rafter as low down on the rafter as possible - say your position 2 or - preferably 1 if you can get that low.
Similarly, the tie should connect to the ceiling joist a reasonable distance away from the eaves.
The reasoning here is that the tie is more effective at restraining the rafters if it is as near to the horizontal as possible.
For connecting them, use 10mm coach screws as they give a firmer fixing than bolts.

 

[pinenot]

Exactly where you indicate it and don't worry about an angle, plumb will be better...pinenot

 

[tony1851]

Exactly where you indicate it and don't worry about an angle, plumb will be better...pinenot

Pinenot, what are you on about!!!

If by 'plumb' you are referring to the green ties, they are just hangers nailed to the ceiling joists to stop them deflecting too much. They do NOT offer any lateral restraint to the roof as a whole.

The red tie on the left is what he needs, though at a shallower angle.

 

[pinenot]

Just as an aside, here's a return of your sketch modified View media item 72513 ...pinenot

 

[tony1851]

That won't prevent roof-spread. The tie needs to be at as low an angle as possible to perform any restraining function.
It's basic structures, really!