Roof for new extension - 15deg pitch - Recommendations

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We're looking to build a rear extension to our house later this year. My preference initially was to have a tiled pitched roof as this will match closely to the existing look of the house and should last the longest compared to flat roofs. The roof will also be installed with velux/skylight style windows. The architect has come back and said he can make a pitch roof with 15deg slope which I believe on research now is on the limit for most low pitched tiles (i.e. Sandtof 20/20 or Marley Wessex). Also from my searches there seems to be a number of people experiencing leaking roofs especially with skylights in which I assume is also on the 15 deg limit. I’m now wondering if this is the best way to go or just go with a flat roof. Would a 15deg tile pitch roof be better without any skylights, thus minimising discontinuities in the roof? Would it be better/possible installing Velux windows designed for flatroofs on a pitched roof? We don’t need these windows to be opened, only for light.


On looking at flat roofs, the common types seems to be EPDM or GRP. EPDM seems cheaper with a slightly shorter lifespan (still good at 30years+) – whereas GRP is should last longer, though more expensive and very dependant on good installation.


Any advice/recommendation as to which system is better in the long run?
 
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Have you looked at a flat roof with a glazed lantern?

regarding flat roof membrane -GRP or EPDM have similar lifespans really -long enough not to consider that as a reason for one over the other.

GRP -if not installed correctly may crack at abutments and it may suffer banging noises.

EPDM -has seams which need doing correctly and wrinkling is an issue if not fitted correctly.

So the longevity of the systems is more dependant on installation than choice of material.....

Alternatives are single ply membrane (sarnafil for eg), liquid rubber roofing.

for a modern look on a shallow pitch, zinc looks good -dear I expect though!

Or you could choose a low pitch roof tile with onduline below (or whatever is needed). Personally Im not a fan of very low pitch roofs with tiles -it can look a bit naff.....
 
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I used Sandtoft 20/20 and Velux windows at 16 degrees and had no leaks in over 2 years in all sorts of weather.

The added extras Velux sell sometimes get overlooked in installations but at such a low pitch are critical to a leak free roof. The insulation collar and more so under felt collar and transverse gutter must be fitted not just the flashing kit.

If done correctly there's no reason why you can't have a tiled roof with Velux windows at 15 degrees.
 
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Have two roofs with 17 degree pitch, doubled up on the membrane and increased the head lap slightly and both have been fine ( one has a velux in it).
The tiles used were suitable down to a 20 degree pitch .
So if care is taken you can get away with tiling a low pitch .
 
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Crap designer.

Redland Regent tiles go down to 12°.

All rooflight manufactures can supply a low pitch adapter for their products.

Any tiled roof can be designed with a boarded and sealed sarking which allows any roof tile to be used below it recommended minimum pitch
 
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2020 a nice little tile and will be fine at 15 degrees @ 100mm headlap, Velux are fine at 15 also..
 
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I am just going through a similar project and I am a little concerned about the Low pitch velux

How important is the the insulation collar and more so under felt collar and transverse gutter fitted because I was just going to buy the window and flashing kit.
 
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I don’t claim to have fitted loads of these, but I’d say at that pitch you definitely want the underfelt collar. Mine were all at 17ish degrees and I didn’t bother and have had no issues, but I reckon at 15 it’s a definite risk.
 
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That's the common mistake most builders make not bothering to fit the bdx option on low pitches.
 
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Leofric

Similar to what woody says ,low pitch roofs can have tiles on battens on counter battens on suitable membrane on boarding over rafters.
 
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low pitch roofs
Saying it and building it are very different. People assume that if there is a tile suitable for their low pitch, then all is well. There are however caveats and consequences that are not discussed often enough.
The lower the pitch, the greater the care must be taken during construction (similar with shallow fall drains) and a simplistic roof design to boot.

Architects and designers must be more vociferous in getting the message out there about low pitch roof construction methods and what should be avoided.
 
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Hi...do you have any update if you have gone with pitched or flat roof and what is your decision making factor? I am in same situation but unable to decide if 15 degree pitched roof is definitely the option in long run?
 
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I doubt the OP is still around or will reply as the post is from last year. There are experts on the forum who can respond to specific issues. Researching previous posts can give you an idea of the relevant issues.

Blup
 

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