Flat Roof Query

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murphyabu

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Evening all,
Thank you to those who have given me some pointers to date, much appreciated.

Just started framing the flat roof starting with a roof light, triple trimmers etc.

Unfortunately I was away while the blockwork was run in and didn't get a chance to agree a wall plate so I am going to rest the joists on the inner leaf,

The joists are 220x47, I'd like to triple check:

- is there anything preventing me from laying the joists on the inner leaf and using twist straps to tie them down to the blocks?

- is it acceptable to notch the underside of the joist (past the inner leaf blockwork) so it then clears the outer leaf of brickwork but will allow the soffit to run back against the brickwork.

(Yes - I've had a touch of the Clevetts and routered the hanger into the trimmers to make it easier putting in the plasterboard)

Thanks in advance.
 

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I’m not sure I would put a point load onto a thermalite block even if there is a steel just below, maybe swap the block below for a high 7 or a pad stone or steel spreader plate
 
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Notch7 said:
I’m not sure I would put a point load onto a thermalite block even if there is a steel just below, maybe swap the block below for a high 7 or a pad stone or steel spreader plate
Click to expand...
Understood @Notch7, but then what about first floor joists which are regularly built into thermalite blocks? Surely similar if not more point loading in those instances?
Could be a call into BC in the morning for clarification......
 
You don't have wallplates with flat roofs. The roof joists rest on the inner leaf.

Notching is OK as long as the joists are still deep enough for the span.

There are no point loads with evenly spaced joists, it's all uniformity distributed load.
 
^woody^ said:
You don't have wallplates with flat roofs. The roof joists rest on the inner leaf.

Notching is OK as long as the joists are still deep enough for the span.

There are no point loads with evenly spaced joists, it's all uniformity distributed load.
Click to expand...
Cheers @^woody^

The span (from timber trimmer in the web of the steel with heavy duty joist hangers to the inner leaf of blocks) is 4.7m the joist then runs out another 100mm (cavity) and is then notched to clear the outer leaf of brickwork.

The joist then projects past the outer leaf by a further 400mm to allow a stepped detail in the facia/soffit to save it from being a huge 350mm face
 
Yes, it's good design to notch the joists or otherwise to remove a deep fascia. But a 400mm projection seems excessive.
 
Noted, I've run them in at full lengths (5400) for the time being so I can figure out the detail later with a string line and a skill saw
 
^woody^ said:
You don't have wallplates with flat roofs. The roof joists rest on the inner leaf.

Notching is OK as long as the joists are still deep enough for the span.

There are no point loads with evenly spaced joists, it's all uniformity distributed load.
Click to expand...
Technically you're right and wrong. Although you don't need a wall plate on a flat roof, you can fit a wall plate. The advantage of doing so is that it makes securing the roof timbers down easier and much more resiting to uplift. I only know because of my structural engineer and what was specified on plan.

Fit a wall plate, strap it down in the usual manner and it's easier to draw on where your timbers are needed. Then they're easier and better fitted to the wall plate. A common misconception is that, "You don't have", as opposed to, "You don't need".
 
Bonni said:
Technically you're right and wrong. Although you don't need a wall plate on a flat roof, you can fit a wall plate. The advantage of doing so is that it makes securing the roof timbers down easier and much more resiting to uplift. I only know because of my structural engineer and what was specified on plan.

Fit a wall plate, strap it down in the usual manner and it's easier to draw on where your timbers are needed. Then they're easier and better fitted to the wall plate. A common misconception is that, "You don't have", as opposed to, "You don't need".
Click to expand...
The joists have to be strapped, not the plate. The plate does not hold the roof down.

Also, the joists should be built in, and that can't be done on top of a timber plate.

Yes you can fit a plate, and yes you can nog every joist, and yes you can build in lots of different ways, but the traditional proven way is that you don't need and therefore don't have wallplates with flat roofs.
 
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