Low Pitch Roof Tiling

[woodshavings]

I am planning a very small extension to our Kitchen - just 80cm out. I don't really want a flat roof and prefer to use a tiled roof to match the existing one.
Unfortunately, the existing roof is 22.5 degree slope, the minimum for the particular tile (Marley Ludlow) used. I need the extension roof to be at 15 degree.
Is there any kind of additional tray or lining that could be fitted under new tiles such that the existing tile type could be used at 15 degree pitch?
Any help appreciated.
John

 

[Lons]

You could sheath the roof with 11mm OSB (I always do anyway) and then double thickness the synthetic underfelt to waterproof against wind blown rain but if you reduce the pitch too much the tiles won't sit correctly.

I suggest you ring the tech dept at Marley as I've found them very helpful and they should offer a solution.
cheers
bob

 

[woodshavings]

Hi Bob,
I will check with Marley but fear they will stick to their published data - minimum of 22 1/2 deg for the type of tile I have. They do have alternative tiles that are specified down to 12 deg but of course, they would not match.
I will give them a call anyway and see what they say - I like the idea of an under board and membrane as you suggest - I wonder if building control will OK this.
Many thanks, John

 

[Lons]

Absolutely no problem with building control as it's a recognised method of construction. I use it always and have recently finished a decent sized kitchen / utility / WC extension on my sons house doing exactly that. Only difference is that pitch was greater and used man made slates.

The OSB makes a stronger roof and doesn't need the usual windbracing diagonally across trusses.
It will increase the height slightly though as the sequence is nail OSB across the trusses / rafters then you need a counterbatten nailed down the length of each truss (I cut a 25mm lath in half for that which is adequate as the purpose is to raise the felt so any water would run down the centre between trusses and not towards any nail holes). Underlay felt goes on next and shouldn't be stretched tight, followed by slaters laths as normal and tiles.

This is a very rough diagram and I might have missed something (had a few beers - ccasion5: but hope it helps
cheers
Bob

 

[midnightlunchbox]

This has been the standard method of construction in Scotland for decades, unless it's a slated roof. However, and not to tread on any toes, you DO require both diagonal and longitudinal bracing. The OSB sarking will bind the roof together to a degree but it is not an alternative to sarking. I have seen roofs collapse over the years due to lack of bracing so don't take any chances. Oh, and lap your felt/roof therm by a good 6-8 inches.

 

[Lons]

This has been the standard method of construction in Scotland for decades, unless it's a slated roof. However, and not to tread on any toes, you DO require both diagonal and longitudinal bracing. The OSB sarking will bind the roof together to a degree but it is not an alternative to sarking. I have seen roofs collapse over the years due to lack of bracing so don't take any chances. Oh, and lap your felt/roof therm by a good 6-8 inches.

Unless changed very recently, that doesn't apply to England for pitched roof extensions under 40 deg and certainly for the OPs tiny low pitch extension is completely unecessary IMO and I've never had a problem with building control on any of the roofs Ive constructed.

If I remember correctly many Scottish properties are higher pitch and OSB is used to give additional protection against the more severe weather encountered there although in England more notice is now being taken of wind loadingsin specific regions and actioned accordingly.

As an aside, I've worked on many turn of the century houses with slated roofs, no felt and no bracing and have yet to see one that's failed through lack of bracing. It's only moders thin section engineered trussed roofs that are vulnerable. I doubt that the OP with his 800mm roof will be using that method!.

As far as the felt is concerned, on a pitch as low as that proposed, I would have a much larger overlap and probably double thickness anyway as windblown rain is likely to breach the tiles.

cheers
Bob

Great part of the world you live in btw.

 

[Richard863]

Hi John
I had a similar problem many years ago with a tiling system insisted by my council. Then on a visit to Canada I saw a system that was fantastic, called DECRA on my return home they had a outlet in Crawley. After fitting it all my weather problems went. This has been on some 20 years now.
If you need any more gen on my pad PM me

Good luck
Richard

 

[woodshavings]

Many thanks Bob and Richard,
You have been a great help
John

 

[midnightlunchbox]

If I remember correctly many Scottish properties are higher pitch and OSB is used to give additional protection against the more severe weather encountered there although in England more notice is now being taken of wind loadingsin specific regions and actioned accordingly.

As an aside, I've worked on many turn of the century houses with slated roofs, no felt and no bracing and have yet to see one that's failed through lack of bracing. It's only moders thin section engineered trussed roofs that are vulnerable. I doubt that the OP with his 800mm roof will be using that method!.[/quote]

The main accepted reason for sarking under tiles in this area is the sarking supports the felt/roof therm and prevents it sagging. Should the felt sag there is a greater risk of leaks. It does bind the roof but not to the degree less bracing is required. It is the bracing that counteracts the wind or snow loading. It creates triangulation and forms a very rigid structure. There are southern firms building trussed roofs without sarking but it's kind of frowned upon. Slated roofs are a different kettle of fish. By the time the roof is covered in 22mm boards, the bracing is more or less redundant.

 

[Lons]

The main accepted reason for sarking under tiles in this area is the sarking supports the felt/roof therm and prevents it sagging. Should the felt sag there is a greater risk of leaks. It does bind the roof but not to the degree less bracing is required. It is the bracing that counteracts the wind or snow loading. It creates triangulation and forms a very rigid structure. There are southern firms building trussed roofs without sarking but it's kind of frowned upon. Slated roofs are a different kettle of fish. By the time the roof is covered in 22mm boards, the bracing is more or less redundant.

Yebbut... we're not all in wild and windy, snow covered Aberdeenshire :lol: ( At least it was on every visit I made which used to be once a month :wink: )

Most of the old slated roofs in my part of the world have no felt and little or no bracing and very few would have 22mm boards. My son's "new" house built in 1898 is so and still sound as a bell, no probems. Building inspector was perfectly happy for new single storey extension (not trussed) to have OSB under Cembrit slates with no additional bracing and I'm happy that it won't fail.
One of the reasons I always fit OSB is to support the felt. A much better job.

I repeat: OP proposes an extension with a projection of only 800mm. Nothing to stop him adding bracing but huge overkill in that particular case.

We'll just have to agree to disagree in this instance

Bob