Hi - a quick question about throat width for an open fireplace, which according to regs (doc J) should be 10-12cm.
I'm using a 22" wide fireback, for which unfortunately nobody seems to sell a matching throat-forming lintel, so I'll be casting one using refractory screed. I'll also be making 2-3" of extra depth to the fireplace itself at the front.
So if I just did things the lazy way, the throat would be 16-18cm wide, which is far too wide to get a good draw, and would violate building regs.
My question is: in order to get the 10-12cm, should I narrow the throat at the front (quite easy, because I just need to make a deeper throat-forming lintel) or at the back (e.g. cobbling something together with firebricks poking out from on top of the infill, made good using mouldable firebrick or fire screed)?
My main concern is not to let smoke come into the room, so I'm think the answer is probably build up 5cm at the back. Am I right or wrong? :-)
Any advice would be much appreciated!
Thanks! Harry