Pitched Roofs (Houses) - Part 2 – Moss Growth and Flashings

It is worth spending the time to look closely at a roof, from a maintenance point of view, for those living or occupying a property and from an investment point of view for those thinking of acquiring, disposing of, or leasing property

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In my previous article (Link) I discussed what an important component a roof is to a building and that to be effective it needs to be weather and watertight, and if deterioration starts to occur it can affect many other parts of a building and ultimately, a roof is often one of the most expensive components to repair or replace. It is therefore worth spending the time to look closely at a roof, from a maintenance point of view, for those living or occupying a property and from an investment point of view for those thinking of acquiring, disposing of or leasing property. In last week’s article I discussed sagging roofs and provided some examples of likely causes and what to look for and for the purposes of the remainder of this article I plan to discuss moss growth and flashings and identify some of the things that I would look for when carrying out a survey to a traditional timber pitched roof to a low rise domestic building (for the layperson I am referring to a typical house).

Moss Growth – ‘vegetation’ or ‘deposits’ can appear on the surface of a roof in many forms and the type of vegetation or deposit will be determined by a number of different factors.  Typical terms used to describe these are lichen, algae, fungus or moss growth.  Limited presence or absence of sunlight on a roof surface is often an influencing factor in the amount of moss or lichen growth. Usually there will be less growth if there is regular sunlight, so consequently when there are shady and cool/moist conditions, the presence of moss/lichen growth is more likely.

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Apart from looking a little unsightly the impact of small deposits of moss/lichen on a roof surface are likely to be insignificant.  However, valley gutters, rainwater gutters, downpipes, hoppers and other outlets should be regularly checked to ensure that deposits, which may detach from a roof surface, are not allowed to block the rainwater system. If on the other hand, deposits are allowed to develop and cover larger areas of the roof surface then this can have a more detrimental effect to the building. Moss/lichen growth on roofs in large deposits have the capability of holding surface water and moisture and as such will keep the roof surface moist and cool in the areas where it is present.  This can also result in the internal surface temperature being cooler than it would normally be, creating an environment with an increased possibility of condensation occurring in the internal roof space.  Consequently, this can introduce moisture into a roof space which can be the catalyst for quite a number of other building problems.  You can view my previous article on condensation by following this (Link).  During colder periods of the year and moisture held within moss/lichen deposits may freeze causing expansion (when water freezes it expands and therefore increases in volume).  This can introduce stresses to the roof surface underneath and can damage or even displace roof tiles, particularly where water is trapped in the moss/lichen at the junction or under the ‘lap’ of roof tiles. 

Therefore moss/lichen growth on as roof surface may look innocent enough however it can be seen from the information above that it can be quite problematic if it is not dealt with.  Caution should be taken when removing moss/lichen deposits from a roof from a personal safety perspective (particularly safe access), as well as the method adopted to remove any deposits. After all it would be pointless clearing the deposits and then damaging the roof surface in the process.  The use of a high pressure water jet for example will damage certain types of roof tiles so always carry out some research before deciding on which method to use.

Source: http://metalrooftodays.blogspot.co.uk/

Flashings - When any part of a roof meets a vertical surface or abutment such as an extension, dormer window or a chimney stack etc. the joint between the vertical surface and the roof covering needs to be completely water tight to prevent moisture ingress. Nowadays, lead is commonly used however in older buildings zinc, aluminium, slate, tile and even cement mortar were used in these locations.  Whilst undertaking surveys, flashing are a vulnerable point within a building and it is common to find evidence of damage and deterioration to flashings as well as signs of moisture ingress internally.  It is always worth looking closely inside a building at the underside of where flashings are located externally, looking for historical signs of moisture ingress such as staining in addition to areas that may be visibly damp, peeling/blistering of paint and finishes and other signs that there may be a problem with the flashings.

Things to look for include evidence of temporary repairs around flashings including the use of ‘flashband’, which is basically a bitumous tape.  The use of flashband should never be considered as a permanent solution and in fact it will not be long before moisture ingress starts to re-occur unless a more substantial repair is not carried out quickly.  To a Surveyor, flashband screams out problem!

Source: http://www.1stassociated.co.uk/

The use of cement based mortar as a flashing is also inadequate and should be replaced with something more substantial.  The whole point of a flashing is that it should be watertight.  The problem with using mortar is that it is porous and therefore has the ability to absorb moisture, which can easily find its way through to the surfaces below. As discussed with moss growth above, any water or moisture that is trapped within the mortar will expand if it freezes and this can cause the mortar to crack, move and even fall away. This will expose the junction it is trying to protect to further moisture ingress.  

A Surveyor will also commonly see problems associated with roof flashings which are a result of poor workmanship. The reasons for poor workmanship are wide but will include cutting corners to save money, use of inappropriate materials, lack of knowledge and understanding, ‘getting the job done quickly’, if it’s out of sight, it’s out of mind’ etc.  I used to think I’d seen it all however there was always something new just around the corner that I had not seen before!  Whatever the reason, poor workmanship creates a lot of problems in buildings and something that I am sure will continue to do so in the future.  The photographs below provide some examples of poor workmanship of roof flashings.

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