building an extension with warm roof tieing into a pitched cold roof

Ill be building an extension on the back of my house with a flat roof that will tie into a 40d pitched roof. The flat roof will be a warm roof however what im wanting to know is what sort of problems could arise when running a warm flat roof into an existing cold pitched roof. I have worked on warm flat roofs before but never one that would tie into pitched roof as mentioned.

Has anyone any detail of this spec or pictures of jobs theyve did of this type of work.

Thank you in advance.

 

Are you planning on resting your new flat roof joists on your existing wall plate on the house?

 

TO be honest, im not to sure about this. What would you recommend?

 

No problem doing it like that if it works height wise, you can then screw the new joists to your existing rafter feet.

 

I am not stuck for height. I can go as high as 10ft ceiling height which should run into the rafters. If this is how ill do it, the insulation should but up to the wall plate which is ontop of the cavity closer block.

 

I've done numerous orangeries with this arrangement.

In terms of weathering the flat roof surface has to run up the pitch, then the tiles are dressed down over.

The abutment of the fascia on the flat roof to the pitch can be awkward and not very pretty.

Running a flat roof into a pitched roof with hips can be a right headache.


Internally you may need to return the insulation downwards as you don't want airflow from the cold roof reaching the warm side of the warm roof.

And then you need to be careful not to compromise the cold roof ventilation.

 

You could also drop joist hangars of the existing house wall plate if that helps.

 

HAve you any pics or diagrams of the weathering of the flatroof surface running up the pitch. Do you mean the finished surface running under the cold roof felt over the fillet?

The cold roof ventilation is what was worrying me. This is where im finding it difficult to visualise!

The fascia detail, im thinking if i have an open cavity, i can run the cavity insulation up and itll but up to the insulation in the warm roof creating the closer of the insulation needed for the warm roof where no hot air can escape.

 

Would there be any need for themif the joists will be sitting on the cavity closer blocks and bolted/screwed to the rafters of would this not be strong enough hold.

 

Not if you are happy with your heights. Your going to need to block the ends of your joists off with insulation.

 

Put the flat roof joists on the plate, fix to rafters, and be sure to run the deck boards and flat roof cover at least 300mm up the slope.

The roof membrane should then lap over the flat roof cover, with a tilting fillet to prevent blown water going under the tiles.

Notch the underside of any battens that are fixed over the felt/membrane to allow any water that gets under the roof tiles to run out

 

A simpler way than notching the batons Woody is to fold a few bits of lead under the baton to allow drainage.

 

As long as it don't kick the tiles up.

 

Im finding it difficult to visulise where id need to cut the battens. This will be tied into a slate pitched roof.

 

"Cut" the battens?

From NHBC, showing the principle.

That first tile batten does not need to be notched if its not fitted on the extended plyboard