Flat Roof Query

[murphyabu]

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.

 

[Bonni]

Sit the joists on a 4x2 wall plate

How to detail a wall plate: a sift through the guidance — Practical Architecture

A common detail that can be confusing

www.practicalarchitecture.com

 

[murphyabu]

Sit the joists on a 4x2 wall plate

How to detail a wall plate: a sift through the guidance — Practical Architecture

A common detail that can be confusing

www.practicalarchitecture.com

Thanks @Bonni but that link concerns pitched roofs,

This is for a flat roof.

 

[Notch7]

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

 

[murphyabu]

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

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......

 

[^woody^]

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.

 

[murphyabu]

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.

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

 

[^woody^]

Yes, it's good design to notch the joists or otherwise to remove a deep fascia. But a 400mm projection seems excessive.

 

[murphyabu]

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

 

[Bonni]

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.

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".

 

[^woody^]

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".

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.

 

[catlad]

No need not to have a wall plate.

 

[murphyabu]

As an update, finally found a break in the weather to tackle the flat roof with a notched/stepped detail to break up the depth of the fascia.
In hindsight an additional course of bricks (or two) on the outer leaf may have achieved the same but I think what I have done looks acceptable and creates a little more detail to otherwise boring fascia profile. Does add to the cost though as instead of needing to clad 12 linear meters I now need to clad 24m!

Now for a couple of questions from you knowledgeable folk,

The roof build up has caused a slight issue at the side of the roof where the water check will be. The roof build up now means I need to clad approx 490mm of timber (joist, firring, osb deck, insulation, osb deck). Would you use utility board to clad the sides?

Using PIR I intend to tuck the roof in (using 140mm PIR on top of the internal leaf wall between the joists and foil taping the edges) but would you also be inclined to fill the soffits with rockwool to reduce cold bridging between the joists/steel?

Thanks again

 

[freddiemercurystwin]

How have you ended up with seemingly a double layer of joists? There's no point any insulation going beyond the outer face of the outer skin of brickwork, whatever you choose to hide the soffit/fascias you want to ventilate the void. I would go for a some powder-coated aluminium if funds permit but no doubt nothing off the shelf will fit because you've not thought that far ahead.

BTW why are some of the joists cut short?

 

[murphyabu]

Not double joists, just the 220mm joist run through and notched over the outer leaf and then notched again to create a stepped detail.

Would you typically vent the void of a warm roof soffit?

Standard fascia will fit as the stepped detail was measured to accommodate standard sizes and hollow board soffit.

As for the joists cut short, the one day I had some assistance he squared the end of the joist not realising I was leaving them at a full 5.4m

I'll reinstate the length with a piece of 4x2 off the side of the joist.