Wet vs dry fix

Dr Kevin Ley, Redland Technical Manager, takes a look at how traditional ‘wet’ roof fixing methods stack up compared to modern ‘dry’ fix alternatives.

Modern methods of construction have superseded many traditional construction methods. It’s no different in the roofing industry. For many years accepted roofing practice has involved use of wet sand-cement mortar to fix tiles and slates onto pitched roofs...............

With correct workmanship, fixing roof tiles with mortar still offers a long lasting, secure solution. Having said that, best working practice often falls down on-site, leading to problems in the future. This has been recognised by the NHBC in the form of changes to its technical guidance. That and changes to regulations are driving a shift in working practice towards dry fix methods and Redland has a number of new and existing products that fit the bill.

Failure

The NHBC has paid out millions over the last few years under its Buildmark Warranty, the mainculprit being failure of mortar on newly built residential homes. Subsequently, it has updated its technical guidance to require mechanical fixing of all ridge and hip tiles on new houses covered under its Buildmark Warranty. Even if working on roof refurbishment rather than new build, mechanical fixing offers benefits over the traditional way.

Usually ridge and hip tiles are fixed using deep-bed mortar. 

However, poor workmanship and the variable nature inherent with mortar make inconsistencies inevitable. Getting the correct ratio of sand to cement is critical to achieving a good result and this is easy to get wrong. In fact, some roofers use sand meant for bricklaying in their mortar mixes. This is totally unsuitable.

Whether working on new build or refurb a mix consistent with the mortar strength requirements laid out in BS 5534, the code of practice for slating and tiling is essential. Traditionally, mortar used for roofing should be 1:3 cement: sand. The sand grading and type specified in BS 1199 and 1200: 1976, Specification for building sands from natural sources, should be used. More recently the NHBC has changed its technical standards to accept a variation on the traditional 1: 3 mix with increased workability albeit still meeting the mortar strength requirements set out in BS 5534.

However, even when correctly mixed and applied, mortar on roofs can crack or de-bond due to movement within the building. The result is insecure tiles. In high wind conditions tiles can then work loose and blow off. Using an incorrect mortar mix on roofs just accelerates  these problems.

In new build, according to the NHBC, all ridge and hip tiles should now be mechanically fixed either in combination with mortar, or, alternatively as part of a complete dry-fix system. If a specifier or contractor fails to abide by this guidance, finished new homes will not qualify for protection by a Buildmark Warranty.

Dry fix alternatives

Even if not building a home to qualify for a Buildmark Warranty, using dry fix still represents best practice as it removes the requirement for any mortar at all, which can still crack and drop out, causing weather ingress issues, even if all ridge and hip tiles are mechanically fixed. Dry fix systems also provide the easiest and most economic way of introducing high level roofspace ventilation, often a requirement in new houses covered under the Buildmark Warranty.

Not only is dry-fix much more secure in harsher weathers, it involves far less technical expertise than the traditional mortar bedding fixing of tiles. Dry fix is incredibly easy to install, requiring no specialist skills only the use of simple power tools and screws.

Despite historical reservations that it is a costly method, dry fix is actually a faster installation method, therefore cutting down on labour costs, and unlike mortar, whose workability can vary with the weather, dry fix can be installed in most weather conditions.

Looking at the roofing industry as a whole, Redland is not the only company recommending dry fix solutions above traditional methods. Dry fix methods make sense in so many ways it seems likely they’ll be the norm for roofing practice in the not too distant future, as they already are on the continent of Europe.

Kro-clip-  New Universal Fixing   

For roofing contractors, the new Kro-clip represents a speedy solution to the long-standing problem of fixing cut tiles at hips, valleys and around roof window details. It’s quick and easy to install, removing the need to use unpredictable glue solutions which rely heavily on workmanship.

Much like using mortar on roofs, glue has many variables that can affect the quality of the finished roof. Using a mechanical fixing, such as the Kro-clip, removes these variables as each one fixes securely to the cut tile and around a nail secured to rafters or battens. Along with Redland’s existing C-clips and tail clips it forms part of Redland’srange of mechanical fixings for cut tiles at various roofing details such aships, valleys, roof windows etc.

With the NHBC pushing for roofs to be mechanically fixed in most areas, the Kro-clip launch is particularly timely. Whilst glue is currently accepted by the NHBC as an on-site fixing solution but site conditions can make glue very difficult to apply properly, making its effectiveness entirely dependent on workmanship. This is in stark contrast to the Kro-clip which has no such variable.

For this reason,  glue fixings in most cases cannot be backed up by guarantees, leaving it to the contractor to take the liability instead. 

Having said that, the Kro-clip is also a universal product named after its design which looks like crocodile jaws. It will work with most tiles and slates on the market, including Plain tiles andDuoPlain, comes in convenient packs of 50 and is available now from merchants.