Warm Flat Roof Construction

[morg]

Hi,
Looking for some guidance on how to build a flat warm roof.

I'm currently building a small extension (approx 3m x 2m) nearly up to wall plate height and now trying to design the warm roof construction.

Many videos / guides they put the joists on top of the wall plate and then the deck, but what about putting the joists level with wall plate? (to save height mainly).

Can anyone see any problems doing it this way? Thanks

 

[Mr Chibs]

A friend's family member recently had a new flat roof on their extension... it was a cold roof, but they didn't even have a wall plate, joists were just resting on inner skin brickwork, had some noggins here an there.

120mm joists seem a bit small for a 3m span would think you might want to upsize these... a guess at least 150mm maybe more.

Trada span tables (if you search) will give you the size.

I saw a pergola last week that was made from 120mm x 45mm and it was already bowed/sagged. It had only been up for 6 months, with twin wall polycarbonate as the roof, so very light.

 

[morg]

Hi, thanks for the help

Good point on the joists being too small, its could be possible to run these in the other direction (2m) or perhaps double them up, but I'll check the trada tables for reference

Thanks

 

[blup]

How are you going to ventilate it?

 

[morg]

Hi,
Generally warm roofs don't require ventilation with a VCL.

But I'm no expert

Cheers

 

[datarebal]

You don't vent a warm roof

 

[Notch7]

Many videos / guides they put the joists on top of the wall plate and then the deck, but what about putting the joists level with wall plate? (to save height mainly).

Can anyone see any problems doing it this way? Thanks

It doesnt save any height, the overall roof thickness is exactly the same

the only difference it makes is the design of the eaves detail

if you want a wall plate but want the external brickwork or masonry to be flush to underside of joist, then you can simply finish your internal blockwork shorter by the wall plate thickness -thats if you are plasterboarding internally.

Generally the strongest and easiest construction is to run the joists past outside face to form your overhang then make a ladder or whatever for the other 2 sides

Be very careful with warm roof detailing to make sure you avoid condensation issues or thermal bridging around the perimeter -avoid hybrid roof designs with insulation over and in between if you can -its very difficult to form a complete vapour barrier

 

[morg]

Hi,
Sorry late reply! Thanks for the help Notch,

Good point on the overall roof thickness, it is the same

Sounds good to do what you say with the insulation, perhaps use 150mm timber/insulation instead of 170mm (filling in the ladders all the way to soffit), then go over complete deck with 25mm PIR insulation (25mm pir and OSB screwed down with thermally broken fixings)

Cheers for the help