timber roof options when attaching only to the wallplate

[gwam]

I would appreciate some help! I am planning on building a 1 3/4 cottage style house (due to planning restrictions.) The inner block work will rise approx 1 metre above the ceiling joists so the rafters will need to rest on the wallplate with no lateral tie. The property will have dormer windows and is 10metres by 7.5 metres with 45 degree roof pitch. The gable ends will be masonry.

Will I need to use an strengthed extended truss method or can I still use ridge boards and purlins?

Are there other possibities which will give me the open roof space and ridgidity??

Many thanks!

 

[gangman]

Extending both the joists and rafters will create your A frame and give you a lateral tie but it would mean an overhang of over a metre.
On my own cottage it's about 500mm and we did build a garage with a flat over it and that has an overhang of around 750mm and that looks ok.

 

[Bowness72]

Personally, I would use the ridge "beam" and purlin method and make sure the rafters are well connected to the beam/purlins, possibly bolted through the ridge beam with a pressed metal bracket. I would then install a ceiling tie at the lowest level that you are satisfied with if possible. Dont use a ridge "board" with the rafter spanning all the way down to the wallplate as you will risk pushing the wall out with the horizontal thrust from the rafter. It would also help stiffen the roof if you use a deeper rafter and also reduce the spacing. There is still a chance you might get some horizontal cracking along the eaves due to the outward thrust but that is always a risk without the tie at the base of the rafter.

One other thing that will play a part is the thickness and therefore strength of the cantilevering wall. Obviously the thicker the better.

Building control and your structural engineer (if involved) should be able to advise further.

 

[noseall]

similar to above. use a steel ridge support.