Answers to some of the most common and topical roofing queries

Every month our technical team receives thousands of calls.  From questions about products to legislative changes, sustainability advice, fixing specifications and estimates, our expert team is there to help contractors, specifiers, builders, merchants and homeowners.

Whilst we receive many product related calls, we also receive more general technical roofing enquiries.   We’ve identified and answered twenty five frequently asked roofing questions, which we are making available on our website. However, I wanted to take this opportunity to answer a few of the most common queries.

I have a roof pitch that is below the recommended pitch of your product.  Is there anything that can be done?

The proposed relaxation of planning regulations for ground floor extensions means we are seeing a growing number of technical queries about low pitch roofing solutions.  The first piece of advice we give in this situation is to consider a different type of roof tile. Some tiles and slates incorporate design features allowing them to be laid at a lower minimum pitch.  It is uncommon to find a pitched roof below 15° and there are several products within the Marley Eternit range that can be laid at this pitch (for example our Birkdale and Thurtone fibre cement slates and our Wessex and Mendip large format interlocking tiles).

In the very rare instance that your roof is below 15° you do have options available to you, for example there are several manufacturers that provide flexible bituminous sheeting systems laid underneath the tiles, but these will always raise the overall height of the roof, which can be problematic if you have obstructions such as windows.   You could also consider using a marine plywood deck which can be installed using counter battens to provide adequate means of ventilation.

What is a breathable underlay and can I use it?

 Breathable underlays have often been the subject of misunderstanding. Many people tend to group these products together as one and do not distinguish the different types, including their differing capabilities. Breathable felts come in two types, vapour permeable and fully breathable (vapour and air permeable).

Vapour permeable underlays are often the cheaper of the two types. The fibrous structure of vapour permeable underlays is sufficiently dense to prevent liquid water from penetrating; while allowing water vapour to diffuse.  Although water vapour can diffuse, there is still an argument for having additional ventilation to carry this vapour out of the roof space.  In fact, the NHBC has recently implemented guidelines that state when using a vapour permeable underlay, there should also be high level ventilation to provide sufficient air flow to draw this vapour out of the building.

Fully breathable underlays are generally the most expensive form of underlay and (according to their manufacturers) have the lowest vapour resistance, negating  the requirement for any other roof ventilation.  Whilst this is a claim that is supported by these manufacturers, there are still some questions over the long term performance and their suitability when specified with a close fitting roof covering.  There is also often a requirement for additional components such as sealant tapes.

In all cases, the use of an eaves vent system in conjunction with a ventilated dry ridge is not only an economical way to ventilate a roof, but also the most effective and assured in supplying over and above the minimum free air flow required to meet building regulations.

Should I be using dry fix or mortar to bed hips, ridges and verges?

Although mortar has historically been the popular choice to bed hips, ridges and verges, there are a number of factors that can cause mortar to fail. Dry fix systems provide a mechanically secured alternative to mortar which is not only maintenance free, but in the case of ridge and hip systems, also provides ventilation to meet current Building Regulations, British Standards and National House Building Council (NHBC) guidelines.

The question of whether to use mortar or dry fix really comes down to the issue of future maintenance and cost.  Mortar has a limited life span and is also affected by building movement and environmental conditions, so can lead to long term maintenance costs.

If carrying out NHBC work, then ridges and hips must be mechanically fixed – either through a complete dry fix system or using mortar bedding with additional mechanical fixings. We have just launched a mechanical fixing kit for mortar bedded ridges and hips to help meet these guidelines but provisions still need to be made to provide adequate means of ventilation at both high and low level.

Can I clean or jet wash my roof?

This is a particularly topical question in light of a recent episode of Watchdog, which showed an unscrupulous company offering a roof cleaning and repair service, including a full high pressured jet wash, leaving several homeowners with damaged roofs.  Since that episode, we have received many queries about whether it is safe to jet wash a roof as there are many companies on the internet offering similar services to homeowners.

Our advice is that the use of high powered jet washes is not recommended as it can damage the surface of the slate or tile, thus reducing its expected life considerably.  If there is moss or lichen that needs removing, this can be done professionally by spraying with a toxic wash, using copper wire or scraping.  For further information on moss or lichen and its effects, please contact our technical team.

Can roof tiles be walked on?

This is related to the last query, as on the Watchdog programme the contractors in question were seen walking all over the roof.  Marley Eternit does not recommend foot traffic directly on the roof tiles or covering.  Cracks or micro-cracks can occur on the roof covering if roofs are trafficked without appropriate access equipment. British Standard BS 8000: Part 6: 1990 'Workmanship on building sites' states that any roof or vertical work in slating or tiling should be 'treated as fragile'. The standard recommends that an adequate number of crawling boards and ladders should be used when accessing completed areas of roof, using packing between boards and tiles to avoid damage.

If you have a technical query that hasn’t been answered here, you can see all 25 frequently asked questions and our advice on our website, or you can call the Marley Eternit technical team on 01283 722588. 

Here at Marley Eternit, we are always on hand to offer practical and experienced support. Select an option below or call us on 01283 722588.