Roof coatings and surface paints advice

There are companies who specialise in cleaning and re-coating or painting existing tiled roofs.

The claims made by these companies include statements that the application of a coating or surface paint will increase the thermal efficiency of a roof and will prolong its life. They also claim that mosses and lichens ‘eat into the strength of the tiles’ and ‘roof tiles are not waterproof’. Unfortunately, none of these claims are true, but before you read about why, we should also talk about price. It is not uncommon for home owners to be quoted prices for roof coatings that are equivalent to a completely new roof. If you do want to consider a roof coating it pays to ask for a quote for a new roof from a reputable roofing contractor, as a comparison.

 

Thermal Efficiency
Because roof tiles and slates are ‘discontinuously laid’; ie they have gaps around them, a tile or slate roof covering does not provide any real resistance to the transmission of heat. When architects and designers are calculating how well insulated a building is they will only apply a negligible value (U Value) for the tile/batten cavity/underlay and structure. The application of a coating onto the tiles will not make any appreciable difference because the coating will be relatively thin.

It is also important to note that these gaps between tiles are important in allowing air movement, which reduces the presence of moisture laden air from the batten cavity (the space between the tiles and the underlay). If this is not ventilated then condensation problems can occur. If the tiles are sealed, the moisture laden air that escapes from the dwelling into the batten cavity has nowhere to go, and instead forms as condensation on the underside of the tiles.

Moss and lichen growth
There is no evidence that mosses and lichens cause damage to or affect the strength of roof tiles. Mosses and lichens tend to grow on the surface of roof tiles because their rough surface filters dirt out of rainwater. Spores and seeds can be blown onto the roof or are carried there on the feet of birds and then take root in the dirt on the tiles. Sanded or granulated tiles tend to promote growth the soonest. Growth is also most likely to happen where there are trees nearby and in shady, damp conditions, particularly on North-facing roof slopes.

The principal effect of moss and lichen growth is that it can hold water for longer. So if heavy growth in tile interlocks (water channels) or in valleys and abutment gutters etc is likely to impede water flow then it is advisable to remove it. But it should be stressed that these growths are not necessarily deleterious to tiles and slates and can actually impart a mellow and pleasing appearance. In fact, older roofs are more often likely to have moss or lichen growth on simply because age has given the spores time to grow on the surface of the tile. The fact that such roofs have lasted so long would refute any spurious claims about tiles losing strength. Indeed tile manufacturers receive as many enquiries asking how to promote growth on the roof as they do asking how to remove it.

Porosity
Claims by some of the companies who spray roofs that roof tiles are not waterproof are very misleading. As is the case with most building materials, roof tiles absorb a very small amount of water. This is completely normal as the function of a roof tile is to shed water onto the tile below without allowing it into the structure below. The industry works to a standardised and well-established impermeability test that ensures that water does not pass through roof tiles, despite the fact that some might get absorbed.

Maintenance
Mortared junctions such as ridges, hips, valleys and verges all require periodical inspection and maintenance. Modern roof components such as underlay, battens and fixings are fully specified in today’s Standards and are generally long lasting and reliable. But in the past less attention was sometimes paid to some of these components and so their quality and durability could be questionable. Therefore it is worth checking them out during a roof survey.

In the majority of cases where there is water ingress into the building or where water damage is evident within the structure this is often due to problems associated with poor workmanship, incorrect detailing or lack of adequate maintenance of mortared junctions, rather than through any fault with the roof tiles.

If you have concerns about the state of your roof you should seek the services of a reputable roofing contractor who will be able to carry out a roof survey and then offer recommendations. A list of roofing contractors who are members of the National Federation of Roofing Contractors can be found on their web site by clicking here

 

In summary: -

 

  • Coating existing roof tiles does not increase the life of the tiles and gives no advantage other than a temporary aesthetic improvement which will require further maintenance in only a few years.

 

  • Coating does not give the opportunity to renew other components and elements which may have reached the end of their useful life.

 

  • Coating can seal the joints and gaps around the tiles, which prevents efficient drainage of water to the gutter and limits ventilation of the batten void which will mean a build up of condensation.

 

  • Replacing the existing roof tiles with new designs gives the freedom to replace other elements and take advantage of the latest advances and technology in roofing such as dry fix systems, high performance underlays and insulation membranes

 

 
 
Contact The NFRC
The National Federation of Roofing Contractors Ltd,
Roofing House,
31 Worship Street,
London,
EC2A 2DY
Tel: 020 7638 7663 
Fax: 020 7256 2125
 
 
Maintenance Plan

Missing tile? Cracked Mortar?

Download our Maintenance Plan here.
 
 
 
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