Building in joists or joists hangers for flat roof with parapet.

[2020c]

I dont know if this should be in the building section or the roofing section.

As the title says. Im building an extension with a flat warm roof and will be building the block next week. What i want to know, do i build the joists on top of a wall plate and build the block in between them and on top or should i build joist hangers in.

If i build the joist hangers in, my concern would be if i only have one or two block high above the hangers, would this be sufficient strength to help hold the hangers in place without the weight of the roof pulling them with gravity force. I could be over thinking this but it has concerned me. Building them in does seem the better option.

 

[Notch7]

I hope you have the damp arrangement correct on the parapet....they will leak or get damp if it's not spot on.

I'd be thinking of parapet cavity closures

 

[Alastairreid]

Joists on wall plate oversailing brickwork to accommodate soffit and facing.

Cancel that, just noticed parapet being built.

 

[2020c]

I hope you have the damp arrangement correct on the parapet....they will leak or get damp if it's not spot on.

I'd be thinking of parapet cavity closures

The cavity will be getting pumped with beads and the top of cavity closed with a cavity closer. I will have 150mm insulation mechanically fixed above the Alutrix vapor barrier and 25mm upstanding insulation all around the top of the insulation where it meets the blocks. I will have EPDM all over the roof in one layer and up the parapet and over the cavity closer. If budget allows, i will cover the cavity/EPDM with aluminium coping. I did have in mind for an overhang but i have a 6m span(without the overhang) and this will be a 6inch fall going by my maths. Would this look silly? I would prefer the overhang TBH but it is a long span for firrings. Th longest firrings i can source is 4.8m and its a 95mm one end tapperd off to 0mm . would you have any suggestions for this to keep the overhang?

 

[Notch7]

I dont know if this should be in the building section or the roofing section.

As the title says. Im building an extension with a flat warm roof and will be building the block next week. What i want to know, do i build the joists on top of a wall plate and build the block in between them and on top or should i build joist hangers in.

If i build the joist hangers in, my concern would be if i only have one or two block high above the hangers, would this be sufficient strength to help hold the hangers in place without the weight of the roof pulling them with gravity force. I could be over thinking this but it has concerned me. Building them in does seem the better option.

you could resin anchor a wall plate (same section as joist) to the wall and put your hangers onto that

 

[2020c]

Joists on wall plate oversailing brickwork to accommodate soffit and facing.

Cancel that, just noticed parapet being built.

I would prefer the overhang, see above. I have one concern about the overhang with a warm roof detail i will post soon.

 

[2020c]

you could resin anchor a wall plate (same section as joist) to the wall and put your hangers onto that

Would you say a 4x3 wallplate or a 9x2 same as a joist?

 

[2020c]

One question i have about cold bridging in a warm roof. If i was to build a similar roof with the timber bridging the cavity like in this picture(taken from the internet) would this be classed as a cold bridge. The insulation will be inbetweek th etwo blocks up to the underside of the OSB boards with the 150mm insulation on top however if the timber that is bridging the cavity will continue out onto the exterior face for a fascia and soffit to be installed to. will this have any negative effect on having a warm roof.