Looking for some advice on structural calculations/loadings for a domestic roof. I've done some basic plan drawings for a friend who is extending the existing loft space in his house to include a small gable dormer to the front and a box dormer to the back. He's had planning permission granted for the front dormer and then submitted the same drawings to the council on a building notice for both the front & back dormer, and another one of our friends who works as a carpenter/joiner is doing the main building work for him.
The drawings that have been submitted to the council were admittedly a little vague structurally, in that they stated the new dormer/rafters would both be supported by the existing purlins subject to SE calculations and any additional strengthening or support that would be required as per their recommendations.
On the front of the roof which carries the new gable dormer the joiner has strengthened the existing 225x75 purlin with a 150x10 steel plate on one side, fixed with M10 bolts at 400 centres. At the back which carries the box dormer he’s done the same detail but with an additional 200x50 purlin fitted back to back on the existing purlin as well: almost like a flitch beam but with the steel on one side as opposed to in the middle. The new flat roof joists for the box dormer are then supported on hangers off the new purlin.
The BCO has been round to have a look and he seems happy in principle with it but still wants to see the calculations that were done to show what has been installed is ok regs wise…only thing is the joiner didn’t actually get any done and instead has just cracked on with installing the new purlins/plates as described. He has done quite a few of these kinds of jobs before including at least 2 houses that he owns himself so I’m fairly confident that he knows what he’s doing and what he’s done will work, but in this instance I don't think the inspector is going to accept previous experience as so we’re now trying to ‘reverse calculate’ what has been installed instead.
I’ve sent all the information we have to quite a few local & online SE firms to get a quote for producing some calcs but the vast majority of them are either too busy right now/haven’t got back to me at all, and of the couple that have given a price the cheapest is nearly £400. Considering all of the surveying/drawings and the work on site has already been done that is quite a bit more than we were expecting to have to pay so just wondering if there is an alternative where I can do some basic calcs myself that would satisfy building control?
The drawings that have been submitted to the council were admittedly a little vague structurally, in that they stated the new dormer/rafters would both be supported by the existing purlins subject to SE calculations and any additional strengthening or support that would be required as per their recommendations.
On the front of the roof which carries the new gable dormer the joiner has strengthened the existing 225x75 purlin with a 150x10 steel plate on one side, fixed with M10 bolts at 400 centres. At the back which carries the box dormer he’s done the same detail but with an additional 200x50 purlin fitted back to back on the existing purlin as well: almost like a flitch beam but with the steel on one side as opposed to in the middle. The new flat roof joists for the box dormer are then supported on hangers off the new purlin.
The BCO has been round to have a look and he seems happy in principle with it but still wants to see the calculations that were done to show what has been installed is ok regs wise…only thing is the joiner didn’t actually get any done and instead has just cracked on with installing the new purlins/plates as described. He has done quite a few of these kinds of jobs before including at least 2 houses that he owns himself so I’m fairly confident that he knows what he’s doing and what he’s done will work, but in this instance I don't think the inspector is going to accept previous experience as so we’re now trying to ‘reverse calculate’ what has been installed instead.
I’ve sent all the information we have to quite a few local & online SE firms to get a quote for producing some calcs but the vast majority of them are either too busy right now/haven’t got back to me at all, and of the couple that have given a price the cheapest is nearly £400. Considering all of the surveying/drawings and the work on site has already been done that is quite a bit more than we were expecting to have to pay so just wondering if there is an alternative where I can do some basic calcs myself that would satisfy building control?
