How to fit flat roof joists into/across big catnic lintel

[oly]

I have a sort of detail form my SE (who I can't now get hold of). But I don't really understand exactly how the flat roof joists will fit into/on these CX90/100 insulated lintels.

Catnic actually has told me these types of lintel are not suitable for this type of flat roof because their strength comes from having at least three rows of blocks above? Anyway, I'm assuming for now that they will do the job as the flat roof does not weigh a huge amount.

The detail shows the joists fitted about 130mm up the lintel. Do I cut a ros of blocks 130mm high and bed them down on mortar, then fit all the joists on top and then fill in the spaces between the joists with 350 wide bits of block (400mm spacing minus the 50mm thickness of a joist)? Or do I elaborately cut out slots in all the blocks before fitting them?

Should I incorporate any type of timber wall plate?

Help much appreciated!

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• oly
• 12 Feb 2015
• 1

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[theprinceofdarkness]

How you cut the blocks seems to be unimportant. If you use a wall plate then there will be no blocks to cut.
How are you going to fix the wall end of the soffit board? I think I would take the joists all the way through to the facia board, put a notch in it to clear the lintel (which should be foam filled). Put a string along it then saw notchs in it to set stringers in it to support the facia board and dangle a stringer down on stalks to fix the back end of the soffit.
You also need restraint on every other or third joist to stop the roof lifting off in a high wind.
There is also an almighty great thermal bridge (hole?). The ceiling void above the plasterboard will get as hot as the room, you need insulation between this and the outside world. say 4" foam on the vertical face on the inside of the block work or stuff that end of the ceiling cavity, say 12 " back with glass fibre. Else you will get condensation on the inside of the wall which will dribble down and spoil the end of the plasterboard.
Frank

 

[oly]

How you cut the blocks seems to be unimportant. If you use a wall plate then there will be no blocks to cut.
How are you going to fix the wall end of the soffit board? I think I would take the joists all the way through to the facia board, put a notch in it to clear the lintel (which should be foam filled). Put a string along it then saw notchs in it to set stringers in it to support the facia board and dangle a stringer down on stalks to fix the back end of the soffit.
You also need restraint on every other or third joist to stop the roof lifting off in a high wind.
There is also an almighty great thermal bridge (hole?). The ceiling void above the plasterboard will get as hot as the room, you need insulation between this and the outside world. say 4" foam on the vertical face on the inside of the block work or stuff that end of the ceiling cavity, say 12 " back with glass fibre. Else you will get condensation on the inside of the wall which will dribble down and spoil the end of the plasterboard.
Frank

Thanks Frank and good point about the insulation. I've just had another word with Catnic and they say these lintels def shouldn't be used in this way. The strength of the lintel can't be relied upon if it isn't installed correctly, so, that means I'll need to use a much heavier RSJ I think.