The NHBC has taken the unusual step of issuing new guidance ahead of a published change to its standards. What are the implications?
Last year the NHBC revealed that in 2010 it paid out a whopping £11 million on claims against its Buildmark Warranty that were down to problems with workmanship. By far the largest issue related to mortar failure on pitched roofs. Clearly the organisation needed to take action and it did late last year.
So serious is the issue that the NHBC states that from 1st January 2012 all NHBC registered homes must include the new working practice, with a strongly worded recommendation that contractors should work to the new guidelines immediately. The changes concern two specific issues: the need to use mechanical fixings on all ridge and hip tiles and the composition of any mortar used on a roof.
Previous guidance issued by the NHBC has recommended mechanical fixings (preferably in a dry fix system). This latest incarnation makes the advice more specific and much more emphatic.
The traditional fixing method for hip and ridge tiles involves the use of deep-bed mortar, and it is the failure of this mortar that causes so many problems. Cracking of mortar can result simply from the slight movement that occurs as new buildings settle. Mortar is not tolerant of movement and can crack or de-bond, making the tiles insecure – particularly in high wind.
This situation is worsened if the wrong mortar mix is used – a practice that the NHBC has highlighted before. In the new recommendation the organisation is very precise in its instruction on how any mortar used on a roof should be made up.
For fixing, the current NHBC guidance asks for ridge and hip tiles to be mechanically fixed for the first 900mm. The new document states that the entire ridge and hip should be mechanically fixed. For this there are two options:
- Continue to use mortar for bedding, but, in addition, use mechanical fixings for all ridge and hip tiles
- Use a complete dry fix system
If using the first option, then it is considered imperative that the mortar mix used is correct. This is described as: 1:3 cement:sand mix with added plasticiser. This mix should be based on sharp sand with soft sand added to achieve good workability. The sand content should not exceed two parts soft sand to one part sharp sand.
NHBC recognises that appropriate bagged mixes can be acceptable, but also states that builders should no longer use adapted mixes or factory-produced retarded mortars.
The simple reason for this level of detail is that wrongly constituted mortar is considered to be responsible for a large number of roofing failures.
There is no doubt that there is a general shift towards the use of dry fix. In Scotland, where wind and rain conditions can be particularly severe, dry fix is the first choice installation method. It is also widely used for social housing projects in the UK where the main concern is for the roof to perform over decades.
Although adopting an entirely dry fix system will involve a change of working practice for contractors, it is a change that gives benefits as well. Dry fix is a fast installation method – saving site time. It also requires lower levels of craft skill than effective mortar bedding and can be used in weather conditions in which mortar could not be applied.
Given that mechanical fixings will now be required – with the attendant need for power tools on site – it is a relatively small step to move towards a dry fix solution. Statements such as this from the NHBC merely support our view that this is the future for standard roofing practice.
NB: This article first appeared in RCI magazine, December 2011
