Waterproof Detailing and Achieving Compliance


In recent years, changes to various sections of the Building Regulations seem to have conspired to make it very difficult to design and install flat roof waterproofing to upstands that achieves full compliance.  Bauder Ltd’s Technical Director, Doug Ross explains the issues.

When constructing flat roof waterproofing systems, good practice dictates that the minimum required height for upstands is 150mm. For conventional flat roofs, this height is taken from the surface of the waterproofing, but for landscaped roofs, it should be taken from the surface of the finished landscaping.  This provides sufficient height to deal with rain splash-back off the roof surface. 

The two most common design conflicts are:

Meeting the requirements of Part M and designing flush thresholds to doorways providing wheel chair access for the disabled, but still achieving the minimum finished upstand height of 150 mm.

Meet the requirements of Part L and insulating the upstands of the waterproofing system,  the consequence being that the extra thickness imposed brings the finished waterproof upstand forward of the doorway opening and the abutment wall. 

The constructional elements that most affect achieving compliance are the increased thickness of roof insulation required to meet Part L, making provision for adequate drainage falls, and the depth of any landscaping, because these all limit the available upstand height, but particularly at access doorways with flush thresholds that open out on to balconies, terraces or green roofs.

To meet these combined design requirements, floor and ceiling levels would have to increase. Usually designers try to make the roof slab marry up with corresponding floor level to simplify and reduce the overall cost of construction. With Design and Build contracts, value engineering can have a negative effect as design compromise usually impacts on the waterproofing system, as detailing pre-tender is usually generic and then resolved once a system has been chosen.

This is often the case with flush threshold detailing at access doorways to roofs, because meeting the Part M regulations and providing the necessary wheel chair access, is seen as more important than compliance of the waterproofing and this element is more visible. Awareness of minimum height requirements to meet the waterproofing standards is understood far too late and usually only once the building is well under construction, when it is too late for any major changes.

The most common misconception seems to be taking the 150 mm height from the surface of the waterproofing, rather than the landscape surface when designing a doorway access with wheel chair access on to a hard or soft landscaped flat roof.

NHBC has recently revised its minimum upstands height requirement for balcony access from 150 mm to 75 mm, taken from the surface of the waterproofing to the underside of the threshold. They have set out a series of design principles to accompany this change and minimise the potential risks. These can be found in their Standards Extra technical news letter dated February 2010 and is downloadable from their web site. This acceptance of a lower finished upstand height only applies to balcony access on domestic buildings and does not include green roofs.   

It is important to carry the waterproofing through beneath the threshold, rather than just terminating the waterproofing at the top of the vertical upstand. Also, to ensure good mastic seals between the waterproofing and the threshold plate, particularly where fixings are likely to penetrate the waterproof membrane.

The waterproofing to the abutment upstand of the wall will usually be set 150 mm higher than the material under the threshold. This will need to return into the door opening to provide good integrity. Modern bituminous built-up waterproofing can be quite thick (up to 9 mm plus laps) and the correct work sequence means that the detailing to the door opening must be completed before the frame is fitted. Unless this thickness is taken into account and accommodated within the door opening, the frame will not fit.

The best solution is to recess the wall returns by 10 mm (and to a height) of 150 mm at the base of the opening, to accommodate the waterproof detailing to prevent reducing the opening or impede the fitting of the frame. Otherwise it is likely that the frame fitter will re-modelling our carefully crafted waterproof detailing with a Stanley knife and any hope of obtaining a waterproof detail and guarantee will be lost.

An important design element of a flush threshold detail is to include provision for drainage using a linear drainage channel or open grille plate where the upstand meets the landscaping. This prevents water sitting against the vulnerable interface immediately below threshold, allowing water to drain way.

In conclusion, it is certainly possible to design and construct waterproof upstand detailing that meets with current Building Regulations. Designers need to be fully aware of all the various requirements, how they impact with each other. Waterproofing system suppliers can assist in this respect, but the important thing is to establish these requirements and incorporate them into their design at an early stage.