Roofline and Rainwater Systems; Crucial elements in housing construction

// the building envelope

David Osborne, Technical Services Manager for Swish Building Products takes a look at roofline products - crucial elements in the fabric of a building and ones which, if not designed, selected and installed correctly, can seriously affect their long term structural viability.

On average, this country receives almost 900mm [36 inches] of rain each year with some areas of Cumbria and North Wales experiencing as much as four times this amount.

How we capture, manage and dispose of surface water from a building’s roof and, at the same time stop it getting into those vulnerable areas around the eaves today, tomorrow and in 30 years’ time, is possibly one of the most important and neglected considerations of house design in the UK.
Every millimetre of rain falling on a roof has to be channelled and disposed of to prevent moisture ingress to the supporting structure. So the system of guttering, down pipes and the supporting fascia assembly needs careful planning to be effective.
So here are the main considerations in our self-help guide to specifying roofline and guttering products.

First things first

Roofline – the awful truth – timber is a lovely material but it doesn’t cut it in the long term as a roofline material. I like wood, especially in the form of the high quality, tactile material of my dining table - but I wouldn’t trust it to survive more than a decade as a soft wood fascia board. Especially if I didn’t get around to painting it at least once every three years.
Cellular PVC is probably the most cost effective material for the roofline because it costs roughly the same as timber to install, requires no maintenance and so suits the exposed and inaccessible location at the eaves. Most importantly, especially from any landlord’s point of view, when it is installed correctly it will probably last the lifetime of the dwelling.

What are the technical considerations?

The roofline is a tricky area where two expanses of quite different materials meet; the roof tiles and the brick wall. Each does its job well but neither is satisfactory for covering the point of union.
Importantly that point has to be defended against water and wind ingress whilst allowing proper ventilation of the roof void to take place. So why ventilate the roof void at all – why not seal it all up and be done? The answer is rot.
Every day you and I generate a lot of hot air which is full of water vapour. Baths, kettles, saucepans, boilers, washing machines and even breathing sets off clouds of the stuff. And, as we all know, hot air rises. In addition moisture occurs naturally in the air outside the home and a lot of this wet air finds its way into the roof void.
Now consider that the average attic is a cold uninviting place that encourages moisture to condense on its vulnerable timber surfaces and you have the perfect micro-world in which mould and rot can take hold if the moisture is not kept moving.
PVC roofline systems, the broad, industry term to describe fascias, soffits, bargeboards and all the associated ventilation components, are rather good at the double trick of protecting the eaves while fully venting them.
The preferred method these days is to use a continuous venting unit that sits on top of the fascia board and supports the roofing felt and the lower row of tiles. This method does away with slots or ugly vent discs set into the soffit board.
PVC roofline systems are also good at providing both visual and physical support for the other essential components in rainwater management – guttering and downpipe systems.

Design and installation

To work out what our gutter system has to cope with we’ve got to calculate the quantity of rainwater that might run off the roof. We do this by deciding [a] The rainfall intensity that the system must cope with [litres per hour per m2]. [b] Then calculating the roof area to be drained. [effective roof area m2]. Once that’s done we must choose a gutter system with sufficient flow capacity (litres per second) and finally decide on the system layout including the fall of the gutter and the number and position of outlets required to maximise the flow.
BS 12056:2000 sets out the method for calculating roof drainage. It gives likely rainfall intensities across the UK, which may be experienced as unusual events once every 1, 5, 50 and 500 years, for a duration of two minutes or more. The standard says that domestic gutter systems should be designed for a storm event that is likely to occur once a year with an intensity of 75mm per hour per m2 or a flow rate of 0.021 litres per second. However anecdotal evidence tells us that parts of the country are now experiencing much more intense “50 year plus” events every few years.
The area of roof that drains into any one gutter (effective roof area : m2) can be calculated in two ways. Looking at the roof diagram and pitch factor table, we can either use (H/2) + WxL or LxWxPitch Factor, whichever takes your fancy.  The area of roof to be drained should now be compared with the maximum area that the manufacturer claims a given gutter type is able to drain.
If the figure for the chosen gutter is too low the designer has a number of options: firstly to increase the fall of the gutter. With a longer gutter, this has its limitations, as water coming off the tiles is more likely to overshoot a tilted gutter at the lower end.
This may not even be an option where a shallow fascia is fitted. In addition once the length of a gutter section exceeds 50 times the height of water that it can realistically hold, the flow efficiency starts to decline.
Alternatively the outlet point can be moved to a central position. Generally speaking, down pipes have significantly more capacity than gutter units and a centrally placed outlet in most gutter runs will increase the gutter’s drainage capacity.
The down side is that underground drain points are unlikely to be available directly below gutter outlet positions that are chosen late in the design process, eg after the footings have gone in; and running a drain pipe diagonally across a facade is not a pleasing option.

A case for over engineering?

Finally the designer can select a system with a higher capacity. As I’ve suggested earlier, maybe we should think seriously about “over specing” guttering in the future as previously infrequent events increasingly become the norm.
I have heard it said that once the heavens really open no one cares if the gutter copes or not because we will all be inside. True enough for us, but at the eaves there may be damage occurring because of a poorly specified gutter that we can’t see. What is for certain is the effort we put into roofline and guttering specification shouldn’t be wasted forlack of an appreciation of the challenges posed at the eaves.

David Osborne is Technical Services Manager for Swish Building Products.

Swish Building Products: 01827 317 200
www.swishbp.co.uk

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