Another building type question for you

[baj25]

Hi All, I am having a porch built (215mm no-cavity walls, about 2700x1200 floor area, double pitch roof) which is tied into existing structure with firfix wall starters. Builder says OK to fix the 2 rafters against existing wall with hiltibolts or chemical anchors and do away with horizontal ceiling joist (it will still have birdsmouth onto wall plate). Other rafters will have the joists, this one would just look better without joist immediately over door into the house. Googled it to death, can’t see if this is acceptable, but it sounds OK, loads taken into existing structure etc, but I still have doubts. Please can anyone advise? TIA, Bri

[Mr Bean]

Hi All, I am having a porch built (215mm no-cavity walls, about 2700x1200 floor area, double pitch roof) which is tied into existing structure with firfix wall starters. Builder says OK to fix the 2 rafters against existing wall with hiltibolts or chemical anchors and do away with horizontal ceiling joist (it will still have birdsmouth onto wall plate). Other rafters will have the joists, this one would just look better without joist immediately over door into the house. Googled it to death, can’t see if this is acceptable, but it sounds OK, loads taken into existing structure etc, but I still have doubts. Please can anyone advise? TIA, Bri

Speaking as an engineer rather than an architect/builder:
Fixing the first rafters to the building is fine providing the remaining rafters are triangulated by what would be the ceiling joists were it to have a ceiling. In the absence of the horizontal components the roof could attempt to do the splits at its other end by forcing the walls apart at the top. So providing I have not misunderstood your description the structure should in theory be fine. Viewed from one end If you visualize the whole structure with flexible joints it should still remain standing.
No doubt others will add comments.
CS

[baj25]

Thanks CS, I too am an Engineer, of the electrical variety, my thought processes seem to be on the same lines as yours. I think I'll let him crack on as proposed. Cheers, Bri

[Mr Bean]

To be quite honest a bit of common sense reasoning could save a whole lot of grief across a whole range of disciplines. You were quite right to question the design. See this:https://en.wikipedia.org/wiki/Hyatt_Reg ... y_collapse to see how a well meaning subtle change to an otherwise sound design can come back and bite the earnest efforts of honest engineers.
Cheers
CS
PS
I also recall the Ronan point Collapse in 1968 where a gas explosion on the ground floor brought down every corner floor of a 22 story High rise. Have a breeze through https://en.wikipedia.org/wiki/List_of_s ... 80.931950s" onclick="window.open(this.href);return false;

[dapple]

No problem with it ,you can have no ceiling joists ,but you would have to have steel ridge beam or two large timbers bolted together to make solid ridge one end chopped in to existing build other end sitting on cable end but you would need engineer's calcs for this

[lodgey62]

A load bearing ridge.

[Smosh]

From what you describe, I think that removing the one collar tie shouldn't be problematic. However, if the wall starters are debonded, this may allow a degree of movement from the building, however fixing the rafters to the existing wall and therefore the roof, you may end up with some differential settlement and cracking.

[what2do]

To be quite honest a bit of common sense reasoning could save a whole lot of grief across a whole range of disciplines. You were quite right to question the design. See this:https://en.wikipedia.org/wiki/Hyatt_Reg ... y_collapse to see how a well meaning subtle change to an otherwise sound design can come back and bite the earnest efforts of honest engineers.
Cheers
CS
PS
I also recall the Ronan point Collapse in 1968 where a gas explosion on the ground floor brought down every corner floor of a 22 story High rise. Have a breeze through https://en.wikipedia.org/wiki/List_of_s ... 80.931950s" onclick="window.open(this.href);return false;

The gas cooker survived and the old dear took it with her to a her new residence - fabulous, every cloud has a silver lining.