Orangery Roof Construction
This article will expand further on the design considerations of our preferred method of roof construction, a “flat roof” with falls hidden behind the fascia.
We start with some basic assumptions, all of which can be challenged, but if they are not they lead us to the final outcome.
Ceiling height to match existing – in most cases this is about 8ft or 2.4m depending on the age of the house. If the ceiling is lower, many orangery companies use lower ceilings, then we have more height to play with on the roof. This factor is only relevant if the overall height of the building is critical, either due to planning approval or overall aesthetics.
We use a “Warm Roof” construction this means the insulation is on top of the roof and the whole roof space is on the warm side of the insulation. This does add the the thickness of the roof but provides for better insulation than a “cold roof”. In a cold roof, the insulation is on the inside of the roof, which not only would cause issues with fitting recessed spotlights, but would require the roof to be ventilated, which would create aesthetic problems.
The minimum thickness of insulation for a warm roof is 120mm of PIR.
Fascia height This differs slightly between timber and aluminium, but is typically around 550mm. We could, in theory, use a larger fascia, but this does affect the aesthetic balance of the building. A fascia of more than 550mm would look too heavy on most orangeries.
Upstand to Fascia Fundamental to the design is the fact that the fascia is higher than the flat roof and hides some of the underside of the lantern and flat roof itself. Building Regulations stipulate that the upstand to the lantern should be 150mm to prevent water ingress, however these is no requirement for the external upstand, and in any case it’s better that the external upstand is lower so that any water would overflow outside in the event of a blocked drain.
However, being a durable product, the Sarnafil membrane is not very pliable and cannot be folded sharply with folds close together. The Sarnafil goes on top of the fascia, and really needs a minimum upstand of 70mm for it to be able to adhere effectively.
Overall roof thickness
The aim is to keep this to a minimum. Even on large span roofs of up to 10 metres, we have clever methods (arguably trade secrets!) of using glulams, steels and joists with a maximum thickness of 225mm.
The make up of the roof, from bottom to top, excluding any falls is:
Plasterboard and skim 15mm
With a fascia height of 550mm and a roof thickness of 378mm, this leaves us with 172mm to achieve the desired falls on the roof and to have an upstand from the highest point of the roof to the fascia. If the minimum upstand to the top of the fascia is 70mm as stated above, this gives us 102mm to achieve the falls.
If we work on a bare minimum fall of 1 in 80, this equates to a fall of 12.5mm every metre, or 102mm of fall would give us a run of just over 8metres. A fall of 1:40 would only give a run of approximately 4metres.
In the example above, the distance from outlets to the furthest point away is 6.7m. To achieve a fall of 1 in 80, we would need a height of 84mm, just below the available height of 102mm. A steeper gradient, which would alleviate standing water, would not be possible.
This example is an average sized orangery, for larger orangeries achieving the desired falls can be more problematic. Depending on the situation, we may be able to reduce the thickness of the joists to gain a bit more height, increase the height of the fascia or fit additional outlets to reduce the run.
If those options are not available or desirable, then the least worst option is to have a shallower fall and accept there may be flat areas. As stated on the Sarnafil page, standing water has no adverse affect on the roofing membrane and does not affect the guarantee.