Tuesday, 26 March 2013

A good roof overhead ~ Part 3 ~ flashing for water deflection

This article about flashing precedes the one about tiling as these components are such an integral part of all roofs.  Flashing takes a number of forms, some of which are put in place underneath the main roofing material, and some of which go on top of it!

Flashing is any material which is used to deflect water from the main roofing or cladding material and includes the guttering and spouting by which that water is carried away.

The flashing materials used on this project were roofing paper; butynol, a synethetic rubber membrane; (both of which come in rolls), and stainless steel.  

For the technically minded the stainless steel is of two grades: 304 and 316:
  • The 304 will be visible and have a slightly dull finish
  • The 316 flashing will be a long strip under the bottom row of slates to deflect any wind-blown water into the guttering.  316 is very durable.  As it will seldom get rained on it will be a higher grade of Stainless. It is a seeming paradox that corrosion occurs mostly where a material is less rather than more exposed to water and weather; this is because water tends to sit there in one place rather than draining or getting washed away. 
The visible ridging, which in the old roof was a combination of galvanised iron with a lead edging, will be replaced with the same basic shape but in stainless steel.

Here you can see a roll of roofing paper by Thermakraft.  The labelling describes it as "Bituminous self-supporting roofing underlay".

Here John is rolling up a length of butynol. 

And here is a length of stainless steel flashing which will be fixed underneath the lower edge of the tiled roof area where it will deflect any water which might be wind-blown from the guttering:

That metal strip will also prevent birds from getting inside the roof and building their nests there.  As the old roof was removed a number of birds nests of at least several seasons habitation were revealed.  The scale of the one immediately below can be measured by the (sadly) deceased sparrow lying next to it which you can see at the left.  The nest was in fact a starlings nest.  Those birds had put an extensive collection of pine needles to good use:

I don't know what bird would have build this very different nest from inside another corner of the roof:

Here is a length of that steel sitting the right way up and ready to go into position:

And here you can see it tucked under the edge of the tiles with the clip which fastens the guttering in place fitting snugly underneath it:

Here is what the old ridging looked like - definitely in need of replacement.  That strip of lead along each of its edges is usual.  Lead is relatively malleable, which enables such ridges to be shaped to fit the neighbouring corrugations of galvanised iron. 

While the roofing was being replaced section by section the old ridging came in handy in providing protection of the ridges from rain.  Each evening it was re-secured until the new ridging could be screwed into place.

The ridge area of any roof could be likened to a gusset: these require all the components to be carefully fitted to the correct size and shape.  Below you can see the ridging batten fitted between two flanks of the roof and covered with roofing paper.  The pale-coloured shape underneath the gap in the tiling is a supporting batten, which has roofing paper directly underneath it. 

Here the same ridge can be seen in profile at the left.  That ridging batten has to be exactly the right height for the stainless steel ridging to sit properly in place over it...

...But before the steel ridging is screwed into place the ridge is covered with butynol to be extra sure of waterproofing it:

So how do you get a nice curve for the spine of ridging in stainless steel?  The answer is that you don't - well not quite: the apparent curve is made up of numerous small folds.  In the image below you can see these in an off-cut as viewed from the underside:

So the flashing and tiles are pieced together part by part and layer by layer.  But what about the two chimneys?  Here is the one that got done first:

And yes, it's flashed with butynol, carefully fitted to pull down over the main chimney and the little vent next to it in a single piece with no joins.  It has enough stretch to fit around the base of the chimney like a stocking.

And there is it all glued into place - so neatly that it is hardly visible:

This handsome brick chimney required a different treatment:

The roofers' attempt to fit a steel collar on the diagonal was such a disaster of aesthetics that John has vetoed my inclusion of any such photograph.  You can see the diagonal grove cut into the brickwork though, which presented another problem in finding the right treatment:

But perseverance paid off: here it is neatly clad in - you guessed it - butynol!  John spells it 'beautinol' which I consider apt!  Ideally the coverage would have been in one-brick height steps, but the two-brick height is unobtrusive in the black and covers that undesirable diagonal groove:

By this time the roofers and John had parted company and John ended up finishing the top roof himself.  Here he is working away on the topmost ridging, which has to be the final piece to be fixed in place:

Here he is working out the junction of the final piece of downward ridging with the top one:

What could be neater!  In the image below you can see how neatly the solar panels fit in with the other tiles.  I'll write more about these in a separate article.  The observant reader will notice that the depth of the tiles that go with the solar panels is larger than those on the neighbouring flank:

The lower level of the roof is much more complex with multiple angles and joining areas to be taken into account.  You can see a sample of them here:

The corner of the weatherboarding shown above is covered with weatherboard casing.  Again, the observant reader will notice a discrepancy: the one shown below does not match the corner shown above...

The seam along which the weatherboards from upstairs meet the lower roof also has to be flashed.  All those angles take a lot of careful working out as steel is not easy to cut to shape, and angles have to be formed in the factory.  In the image below you can see that the base of the chimney has been tastefully flashed with steel.  That design took a great deal of thought and consideration!  Seen from this angle the chimney's black butynol flashing on the upper level of the roof is visible but hardly noticeable.  One wants no visual distraction from the clean upward sweep of the brickwork. 

The valleys between neighbouring roof angles had to be flashed quite differently from any other angle.  Below you can see the roofing paper with some battens already in place where they butt against the side of the 'V' shaped flashing:

And here is the same area all tiled in.  It had been another long day!

One form of flashing that I haven't mentioned here is the guttering.  In the context of this roof stainless steel guttering had been installed previously so didn't need to be replaced.  It adds a crisply handsome outer edge:

The departure of the roofers originally taken on to do the job was disappointing but inevitable as they did not seem to have the right skills to do the job or to be willing to learn from John, who turned out to be more skilled than they were.  I'll be generous and say that the complexity of replacing an older style of roof with a newer and different one is more demanding than putting a roof on a building for the first time.  While they expressed an interest in gaining further experience they soon tired of having basic errors pointed out which ended up being paid for by the client - not what you would expect from skilled tradespeople or indeed service providers of any description.  

The good thing about all that not working out was that John's young friend, Andrew McCurdy, an experienced builder, was able to pick up where the others had left off, and he and John settled into the remaining work with gusto, amiably working through design and construction issues to which each contributed from their own knowledge and skill.  That's the sort of working relationship we want!

But Andrew couldn't begin work immediately, so there was a hiatus in which that flashing of the upper level of the brick chimney was worked out and we all wondered what would come next.

It was during this time that I took an interest in the choices other people had made about their roofs and chimneys and took a collection of photographs which showed a range of them.  An article about these follows this one:

Readers can click through to other articles about this project via these links:

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