Looking for advice on how big this step needs to be to accommodate a warm roof from the top of the steel. The block built in is 150mm with the thought I’d go with a cold roof system but didn’t like the ventilation options and been told to avoid cold roof so going with a warm roof and going to build this step bigger. The opening is going to be a patio door opening outwards (Juliet balcony). 
My current plan is to go,
50mm firring
12mm ply
150mm insulation 
25mm ply
75mm upstand sitting under door 
Totaling 312mm 

This makes the patio door pretty small but it’s a compromise we’ll have to make because the Juliet balcony is a late addition, was previously a window. Any tips to make opening bigger would be great.

 

Get rid of that block kicker wall and replace in Marmox?

 

That will give you height and insulation in one product, vs trying to insulate over a cold block.

 

https://www.tilefixdirect.com/marmox-thermoblock-load-bearing-thermal-insulation-block-100mm-140mm-sizes/?sku=MXTHERM100&gad_source=1&gad_campaignid=20339692481&gbraid=0AAAAAD4kYe83bkF1nlWN7wup5I8-8mnFJ&gclid=CjwKCAjwuePGBhBZEiwAIGCVS1_y7lij0zE5CX-P_-XA50nzCox6eHe0feq1-MVu5MwkQvw0s9k_8xoCJxkQAvD_BwE

That block is the outer leaf so insulation will be butted up to it. Planning to take it up to around 320mm. It’s the depth of the warm roof that I’d like to reduce so door can be as big as possible. 

Edited September 28, 20251 yr by James T

44 minutes ago, James T said:

That block is the outer leaf so insulation will be butted up to it. Planning to take it up to around 320mm. It’s the depth of the warm roof that I’d like to reduce so door can be as big as possible. 

I'm saying that the block can go, and the step can be reduced.

 

Is there insulation under that small section of what appears to be a flat roof? Lots of info helps get better replies, so sorry if I'm assuming it does. Hard to tell from the pic what's actually in front of the 'step'.

Insulated metal panel roof. 

 

https://www.kingspan.com/ie/en/products/insulated-panels/roof-panels/quadcore-roofliner/

My current thought is it needs to be 320mm high. My sketch below shows how I come to that total.

Tapered insulation would save most of the firring height for approx same u value (but costs more). You can probably lose the top ply too (but I'd use thicker ply at the bottom). Realistically you won't save much from a warm roof buildup, maybe get it down 40-50mm.

 

Presumably you're planning to install a cavity tray along the whole length of the wall, not just the opening?

I’ll look into tapered insulation cheers. Yeah cavity tray full length of the wall. 

You could do a hybrid roof too,

 

Or maybe SIPS depending on your span. 

 

Both work and reduce the thickness of the roof substantially.