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My Blog


Reasons why you should not pressure wash a roof

Posted on 21 June, 2012 at 9:02
I have been asked recently about some of the posts and comments I have been making on things like Facebook and Twitter regarding why you should never pressure wash your roof. 
A power washing company recently sent me an e-mail asking for reasons why I think this. So here you go...
The three most commonly used roofing materials in Ireland are probably concrete tiles, fibre cement (man-made) slates and natural slates. I can give you a few examples for each as to why they shouldn't be pressure washed, and anyone experienced in fitting these products will tell you the same.

Concrete Tiles
I feel like I've beaten this one to death over the last few months but it is the most common cause of leaks that we come across. On a concrete tile you have a lip either side, one high one low, so side by side they interlock. If the lower lip of the tile breaks, and the break is higher than the headlap (the coverage you have with one tile over another) then you have hole in your roof. This is a weak point on most tiles, and some so much so that a tap of a knuckle will break it easily not to mention a pressure washer. 
Leak sourcing and repair.These holes may be already present on the roof before a pressure washer gets there and becuase they are covered by the higer lip, would not be seen without inspection of every individual tile. Then follows the increased flow of water into the hole, with only in some cases an underlay to keep it from some very expensive damage. 

Fibre Cement Slates
These man-made slates were definitely the favoured choice of most  building contactors during the now distant construction boom in Ireland. They offered a cheaper alternative with the same sleek finish as the more expensive natural slate, but unfortunately have no where near the life expectancy or colour durability of the natural stone alternative. 
Over a short period of time weathering takes its tole on this type of material, on installation these slates can withstand a certain amount of bending before they break, after a couple of years on the roof they become almost as brittle as a thin sheet of glass and again a tap of a knuckle is enough to break them.
Effects of moss growth. Damaging moss growth. With only a thin protective layer on these slates, moss and lichen can eat through them very easily and expose the the inner layers rendering the slate next to useless. Also a with moss's ability to hold water like a sponge, expansion between the joints and ends of the slates can cause the now brittle slate to snap. The slates are fixed at three points, two nails either side and a copper crampon at the bottom, because of this even broken slates often stay in place and to the eye all looks as it should do without close inspection.
Add a couple of thousand p.s.i. to these weaknesses and you will only shave years of the already short lifespan of fibre cement slates.

Natural Slate
If well maintained and cared for correctly, natural slates will last a number of lifetimes and in most cases will only give problems when they've outlived the man made fixing they were fitted with.
Like all natural products no two are the same, gradient, texture and shape differ with every slate. Because of this each slate has its own level of durability. When laid out on a roof it is next to impossible to tell which slates are strong, which are weak, or if they may be cupped slightly. For this reason, not matter how careful, foot traffic should be kept off natural slates completely so not to run the risk of any damage. 
The very same has to be said for pressure washers. 
Before pic natural slate roof kildare village. Dark algae and lichen.after image of natural slate roof cleaned by HD Roofing Services Ireland 
With each of these materials, moss and algae can grow in and under joints and completely out of reach of the spray of any pressure washer.
The reason the person is having the roof cleaned in the first place is so that these growths are removed. Leaving bits behind to continue the organisms reproductive cycle is pointless and at best will only keep the roof looking clean for a few months, before things start heading back to square one.

These are only an example of reasons not pressure wash a roof, and i'd be here all day if I was to go on.
We, at the same time, do highly recommend keeping a roof cleaned and well maintained for reasons I will touch on more next time. Just know your options, you really do get what you pay for.

Categories: Effects of moss growth.