Flat roof with long slope

[Cal01]

 

 

Hi Everyone,

 

I’m looking to replace and raise the roof of my garage and kitchen. Before I bought the house the kitchen was built into the back of the garage, and the ceiling was fixed directly to the roof joists, so it has a low ceiling which isn’t insulated.  

 

The felt needs replacing so I was thinking about raising the garage roof before the roofing is replaced so I would have a warm roof and higher ceiling in the kitchen. This would also give me the option in the future to have an RSJ installed and an open plan kitchen diner. 

 

The garage roof adjoins the neighbours garage  which has an extension on top of it. The roof currently falls from front to back over approximately 6m and the joists run from side to side. How do I create the falls if the joists run from side to side using a warm flat roof method? 

 

If I put firrings on the joists running side to side, it will create the fall to the nieghbours side, but how would I get the fall to the back of the house where the gutter is? 

 

Hopefully that makes sense my apologies if not. 

 

I really appreciate any help and advice. 

Thanks! 

 

 

Edited April 8 by Cal01

[Temp]

OK so joists running left to right and you want a fall running front to back. You add firings on top of each joist that aren't tapered. Instead each is a different thickness to create the fall. Bit of maths is all you need to work out the height of each firing. But add say 1" to each so the firings at the low end aren't super thin. Cut them carefully and do a dry run to check for no hollows.

 

Edit: Perhaps fit noggins flush with the top of the firing for extra support.

Edited April 8 by Temp

[Cal01]

Thank you Temp!

[ETC]

...................Or.................put plywood over the joists and then tapered firing pieces from front to back.

[SBMS]

  On 09/04/2025 at 08:31, ETC said:

...................Or.................put plywood over the joists and then tapered firing pieces from front to back.

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@ETC can firrings be on top of plywood (sub deck)? Do they have to be below PIR?

[JohnMo]

Must be missing something. Your flat roof already has a fall, looking at the brickwork, so why do you need to make another fall?

 

 

 

 

[Mr Punter]

  On 18/04/2025 at 13:39, JohnMo said:

Your flat roof already has a fall, looking at the brickwork, so why do you need to make another fall?

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  On 08/04/2025 at 07:13, Cal01 said:

I was thinking about raising the garage roof before the roofing is replaced so I would have a warm roof and higher ceiling in the kitchen.

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[Spinny]

How much do you need to, or want to, raise the ceiling and therefore roof ? Perhaps it is feasible to build the new roof structure above the existing roof and then remove the original garage roof and ceiling structure from below ? Perhaps you could put a nice rooflight in the new roof ?

 

Our warm roof has the ceiling joists - set level for a level ceiling - then firring strips on the joists to create the slight runs needed, then the plywood deck, then a vapour barrier/underlayer of bitumen type roof felt on the deck, then 150mm PIR, then a single ply PVC membrane adhered. Photos and piece of a drawing attached. As you can see internal gutters can be created in the insulation layer if you wish, and they don't necessarily need to run the same way as the roof slope i.e. they can be made to carry water around the roof to wherever you want a downpipe by sloping the bottom of the gutter that way.

[Spinny]

You can see the firring strips here. They can go parallel on top of joists, or transverse to joists depending on the slope(s) you want. The carpenter worked out the taper required and then got them precut (i.e. angled machine cuts down the length of timbers, so you get two firring strips out of one length of timber.

 

(PS If you do add a rooflight, don't forget to work out how to get the back edge of it to drain properly - a bit of additional firring on top perhaps ? This was a glitch for us and we get some ponding behind the rooflight).

 

Also make sure the roof is good enough to crawl/walk over when finished to facilitate cleaning.

 

Also now I think of it there are also suppliers that can provide PIR boards cut to an angle so they fit together like a jigsaw to create the roof slope when laid onto  a horizontal roof deck, here is one...   https://ukflatroofing.com/tapered-insulation-boards?srsltid=AfmBOooDUJXxa7Ewu-xdJ5OhOos63D0eRS9mlahXWSUBBy1vSc5mBICy

 

 

[Spinny]

To drain properly around rooflights and things you need crickets - double tapered triangle pieces as explained here (They can be made of cut insulation board before the membrane/covering is put on)...

https://www.premierguarantee.com/insite/ponding-on-flat-roofs/

 

Something our architect omitted to specify, and the builder and roofer just scratched their heads over, and it ended up too late to for me to source any in time.

 

oh, and you might want to get some sound insulation in under the roof, which we are doing now because it can sound like being in a shed when it rains.

Edited April 18 by Spinny

[goodbyegti]

  On 18/04/2025 at 16:12, Spinny said:

To drain properly around rooflights and things you need crickets - double tapered triangle pieces as explained here (They can be made of cut insulation board before the membrane/covering is put on)...

https://www.premierguarantee.com/insite/ponding-on-flat-roofs/

 

Something our architect omitted to specify, and the builder and roofer just scratched their heads over, and it ended up too late to for me to source any in time.

 

oh, and you might want to get some sound insulation in under the roof, which we are doing now because it can sound like being in a shed when it rains.

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That's a great link Spinny. A must read for anyone building a flat roof.

[Spinny]

Looking at that photo of your garage roof, is that the corner of an upstairs window in the side of your neighbour's extension ?

 

oh wait, I guess that is a window in your house which is on the right in the photo.

Edited April 18 by Spinny

[Spinny]

I think you might want to strongly consider getting that lintel put in as part of putting the new roof on. You could actually have the joists for the new roof cut into the web of the steel to support them at that end - you can see how that is done in my ceiling joist photos above. Then you may achieve the steel being hidden in the ceiling structure - might depend which way the joists run in the ceiling in the main house.

[Spinny]

@goodbyegti

You might even do something like this picture with a rooflight against your neighbour's outside wall.

(Might need some party wall agreement, which you might need to replace the roof anyway.)

Edited April 18 by Spinny

[ETC]

  On 18/04/2025 at 13:22, SBMS said:

@ETC can firrings be on top of plywood (sub deck)? Do they have to be below PIR?

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Speak to Kingspan or Trocal for a detail although I would assume that you would want to place your insulation above the deck and firing pieces.

[ETC]

I would always advocate full insulation below any gutter. Your detail shows a potential thermal bridge.

[Spinny]

  On 18/04/2025 at 20:18, ETC said:

I would always advocate full insulation below any gutter. Your detail shows a potential thermal bridge.

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@ETC Thanks. You have got me wondering, do you think I should be adding some insulation on the inside as sketched in red in the attached image ?