Flat Roof Falls

You are already here, so outlining the need for suitable falls on flat roofs may not be required. By definition a flat roof should not be perfectly flat, a shallow fall should either be incorporated into the structure or by other means. Inadequate falls can lead to accelerated ageing, safety when accessed and developing structural issues amongst other issues. So, what is the right level of fall to specify?

Set your Expectation of the Outcome

The design fall of tapered insulation will be achieved when applied to a level surface. It is common practice for a tapered insulation scheme to state ‘assumes a level deck’, and in the absence of additional project specific information, we believe this is a fair statement.

The tapered insulation industry has historically recognised that a level of unqualified deflection to a structural deck is a reality, which has resulted in a 1:60 design fall being the most common tapered insulation fall utilised. The principle being that ‘some’ minor deflection will be taken up and a satisfactory fall in the region of 1:80 achieved. Will it?

Structural roof decks can be typically categorised as concrete, timber or metal. Each takes on its own set of likely tolerances, and of course as with any element of the building industry can be drastically affected by the quality of workmanship. It would be a little unfair to set an expectation for tapered insulation to put right the wrongs of a poorly cast concrete slab, without qualification. On numerous occasions we’ve been asked if a tapered scheme can be designed to counter the cambers and steps in a poorly installed beam and block deck! The challenges in this scenario possibly don’t need explanation.

Breaking the structural deck types down, the likely deviation from level can vary, see below some indications/examples of deck tolerances:

Concrete Slab Constructions

Standard construction tolerances for concrete surfaces are generally accepted as having no deviation of more than 9mm across a 3m distance, with no abrupt irregularities of more than 3mm as identified in the National Structural Concrete Specification for Building Construction. Correction of any slab outside of these tolerances resulting in a negative fall would be recommended. This can be rectified by using levelling compound in the affected areas.

Timber Deck Constructions

The NHBC Standards 2021 states that when installing timber decks “tops of boards should be laid flush with no deviation greater than 2mm”. It is unlikely that a deviation of 2mm across any distance will create a negative fall in the roof deck that would adversely affect the design fall of tapered insulation.

With the rigid nature of timber decks and the close centres required for the structural joist supports, in our experience roof decks constructed from timber tend to be installed very close to level.

Steel Deck Constructions

Deviations and deflections within Steel deck roof constructions are often more difficult to qualify. The extent of any deviations or deflections in a steel deck will be subject to the span of the steel sheets, with the deflection in the deck being centre to the span. It is typical for rainwater downpipes to be located close to structural columns which can then result in a back fall having to be dealt with.

On a recent project we were informed of a maximum possible deflection of 25mm in the steel sheets, as a result of the span between structural columns. This potential deflection would equate to a negative fall on the roof of 1:80 across the longer spans. To ensure a positive finished fall of 1:80, the design fall of the tapered insulation would need to be 1:40. This design resulted in an insulation thickness that could not be accommodated by the upstands.

Upon inspection the installed sheets showed maximum deflections of 5mm. This resulted in a design fall of 1:60 being utilised to achieve suitable finished fall

How does this affect the design falls provided by Tapered Insulation?

Using the National Structural Concrete Specification for Building Construction guidance for acceptable tolerance:

A Tapered insulation scheme with a design fall of 1:60 is a rise of 50mm across a 3m distance. The maximum accepted tolerance of 9mm across a 3m distance is a fall of 1:333.

Installing a 1:60 design fall to a 9mm negative fall will result in a finished fall of approx. 1:73 or 41mm across the 3m distance. The finished fall of 1:73 is still greater than the minimum recommended finished fall of 1:80 as stated in BS6229:2018 guidance.

Insulation types supplied in tapered format fall into two manufactured types. Widely used PIR insulation (Polyisocyanurate), is manufactured to a fall on a production line, whereas other products such as EPS (Expanded Polystyrene) and Stonewool are fabricated to a fall from a donor board or block. This is where the term ‘Cut to Falls’ originates.

Currently the most widely produced fall across all U.K. insulation manufacturers is certainly 1:60, with a lower percentage being produced to 1:80 and more recently 1:40. The tolerance variation for tapered insulation, regardless of the insulation type, is low. It can be assumed that if the structure is constructed flat then the design fall of the insulation will be achieved.

The distance of fall will have a bearing on the likely maximum design fall that can be adopted. Careful scheme design can help. For example a mitred tapered system will have a shorter distance of fall compared to a monopitch fall into a tapered gutter (see design considerations). But this may not be as practical for the intended use of the roof; laying decking/slabs on a heavily mitred tapered system can be viewed as ‘not ideal’.

Industry Guidance

There are two main sources of guidance on roof falls in the U.K industry:

  • BS6229:2018
  • NHBC standards 2021, section 7.1.5

Below is an extract from 6229:2018, specific to warm roof construction
and falls.

“Ponding water adds a dead load to the roof structure and in exposed warm roofs increased stresses in the waterproofing layer, therefore, a minimum 1:80 finished fall is recommended to both the general area of the roof and to any formed internal gutters.

BS 6229 recommends that all flat roofs should be designed with a fall of 1:40 to ensure a 1:80 fall is achieved once the roof is completed. This design criteria includes internal gutters and should take account of construction tolerances, settlement, and for deflection under load, both during and post construction.”

We understand how this statement can cause confusion, the second paragraph suggests ‘taking into account construction tolerances’, but then seems to indicate that only a 1:40 design fall will achieve the end result. Pre-qualification of the structural deck is paramount. The assumption that a 1:40 design fall will always achieve a positive outcome may also be unrealistic if the level of workmanship for the structural deck is particularly bad, or in the situation of a roof refurbishment and in dealing with a deck that may have suffered from difficult to qualify structural deflection or settlement.

The most recent Design Guide by SPRA (single Ply Roofing Association), section 3.3 provides some good advice in relation to best practice when considering design falls and references BS6229:2018

https://spra.co.uk/wp-content/uploads/2020/08/S01-2020-Design-Guide-1.pdf

NHBC guidance is a little more specific with a specific reference to ‘Tapered Insulation’

stating, ‘Where tapered insulation is used: drainage should be designed by the insulation manufacturer, with falls of no less than 1:60’. They also make reference to consideration being made for the structural deck

Flat roofs and balconies should:

  • be designed with a fall no shallower than 1:40 to ensure a finished
    fall of no less than 1:80, unless a detailed analysis which includes
    overall, and local deflection is used as justification
  • have a minimum finished fall of 1:80 (green roofs 1:60), unless it has a metal sheet covering
  • account for deflection in the structural design where falls are achieved using screeds (particularly on large roofs)

You can read the full guidance here:

https://nhbc-standards.co.uk/7-roofs/7-1-flat-roofs-and-balconies/7-1-5-
drainage/

It is important to state that BS6229 is ‘guidance’ and not a standard that has to be adopted, that’s not to say that we would suggest taking a flippant approach to your chosen fall.

It is also important to add that some third party warranty schemes will enforce BS6229 with minimal deviation from the guidelines.

Early consideration is key

Our simple advice would be to adopt the steepest design fall feasible, up to 1:40 for all field areas and to incorporate a fall into gutter zones. Being realistic about the likely deflections to the structural deck, or by confirming its status will allow good decisions to be made regarding the proposed design fall. Undoubtably early consideration of a tapered insulation scheme will minimise issues later in the building process.

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