Sunday, 31 March 2013

A good roof overhead ~ part 6 ~ roofing membrane goes on the flat part

The replacement of our neighbours' roof proved to be a surprisingly complex project.  Most of the existing corrugated iron was replaced by tiles made from synthetic resin to look like slates, but one area on the seaward side of the house had never been done properly and required a different solution.  Although previously been clad in iron this had never been satisfactory as the slope was slight and not sufficient to promote adequate drainage.   

After much deliberation and research the decision was made to cover it with Enviroclad, a roofing and waterproofing membrane.

The technical term for Enviroclad is thermoplastic polyolefin, commonly abbreviated as TPO.
A compelling point in its favour is that it comes in 3 meter widths and roof is 2.850 wide, meaning that the only laps (joins) would be in a small area where there is a 'dogleg'.  Also in its favour was the choice of colour: grey would not transfer heat into the house as black would.

TPO is said to be eco-friendly, but when I was looking for background reading I had difficulty finding independent reviews which substantiate this.  However, articles posted on the site linked to below do provide a good overview.  I gather that these articles are written by 'contributors' who may or may not be professionals in the field:

The company John contracted to provide the material and service was Viking.
They describe this product as "a scrim reinforced sheet membrane made from TPO (Thermoplastic Polyolefin)."

The specifications for preparing the basic plywood structure on which to lay the TPO are rigorous, to put it mildly.  Here is a sample:
  • Framing supports at maximum 400mm centres (if thicker plywood is to be used support spacing needs to meet NZBC and NZs3604, [etc.]
  • Minimum 17mm F8 Ecoply CCA H3.2 treated structural graded plywood (not LOSP).
  • For all roof decks over living spaces, all cavities must be ventilated and insulated in compliance with clause H1 NZBC.
  • [...and so on.]
The full checklist can be found here - look in the right hand column under the heading for Product Downloads.  The document title is 'Substrate Checklist  - Plywood'

But even before the plywood could be put in place a lot of more basic re-construction of the roof area was found to be necessary:
The construction of the roof framework had been done in a slip-shod manner, so had to be re-done.  As builder Andrew McCurdy, pointed out, if the basic construction of a building isn't done properly it affects all the work done subsequently, so it's important to establish things properly right from the start.  Hooray and Amen to that, and Do it once, do it right, I say. 

Because of the remedial work required the preparation of the area for the overlay took much longer than expected - about a week all told.  Here are Andrew and John hard at it getting that framework sorted out:

The whole area underneath where the plywood was to be fitted had to be ventilated.  Here the two of them are working on vents which came up under the soffit of the adjacent area of roof, which by then was tiled.

Once the basic framework of the area was complete a further framework of slotted ply was fixed over it.  The slots allow air circulation within the roof space.  The ply was H3 treated, 90mm thick and the slots 50mm wide.  Here is one of those lengths of ply ready to be put in place:

Below you can see part of it all framed in with the plywood strips shown above and with insulation in place.  The cavities on the left hand edge are over the eaves so do not require insulation.  (Before you read on take note of the one furthest away - you'll see why shortly.)  

Looked at in the context of all this detail the whole area is a considerable expanse:

The specifications for securing the plywood sheets required very close attention to detail: they state that:
  • Plywood to be screwed with 10g x 50mm SS counter sunk screws at 150mm centres on all sheet edges and at 200mm centres through the body of the sheet.
  • All screws counter sunk 1-2mm. 
The number of screws used was prodigious: they used 300 screws in total.  These were stainless steel - specified 316 stainless and 65mm long.

In addition, each of those boards of ply had to be exactly 3mm apart and no four-way joins were allowed:

That night the whole area was carefully covered to keep out all moisture.   Next morning John pulled the tarpaulin back and set to work, but who is this curled up in the corner?  It's an opossum!  This whole section of roof had long been a favourite bunkhouse for these little creatures which indeed had been something of a problem, so this little fellow was keeping up a long tradition. 

He or she proved surprisingly difficult to rouse: John had been crashing around up there for some time before he even noticed it.  After repeated prodding it sat up, blinked sleepily for a few moments, then whisked over the side of the roof and down a handy tree.  That would be the last night it would be able to get in there...
Those interested in reading more about opossums can find out more in my article:

Finally the arduous preparation was complete and everything was ready for the laying of the roofing membrane:  The day dawned fine and John was up bright and early.  Here he is cleaning an edge to ensure proper glue contact:

Every little bit of unevenness would show through the membrane so Andrew did some final evening out with a pink paste commonly referred to as 'builders bog':

Those 3mm grooves between the sheets of plywood had to be maintained: 

Just a little bit of final checking and adjusting was done:

All the careful detailing had been done.  The slightly raised outer edge will guide water run-off into the guttering:

All those carefully countersunk screws and neatly applied lengths of angled beading...  The beading will provide the support for the membrane to sit against it in a gentle curve:

A little after 9.30 on the big day the contractors arrived.  Readers of the earlier article about tiling will see that this all took place on the same day as John removed the section of roof you can see exposed here:

Once the men set to work it was all go!  Here they are on the big continuous strip of membrane getting the placement exactly right:

Once the exact positioning of the membrane had been established it was rolled back in portions and the reverse side coated with adhesive.  Make no mistake, this is skilled work:

After the contractors had left I was able to get a closer look.  The joins were masterfully neat...

...And the coving was perfectly smooth:

I held the camera over the edge of the roof to take this photo below so that you can see how evenly the top surface curves over to the vertical one above the guttering:

Now that it has been completed the whole expanse can take any kind of pedestrian traffic - not that there is likely to be much!  In just half a day the whole section of the roof was sealed off and completed, and beautifully so.  Those contractors were first rate! 

My next article will be about the solar panels:
Readers can click through to other articles about this project via these links:

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