Product Product Ponding Board (Roofing) (Support underlay)Supporting sagging underlayEnquire...
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Underlay collapsed through weight of rainwater getting in through tile laps. Underlay is laid beneath the battens but is lifted over the last batten so water drains into gutter. That leaves the underlay close to horizontal and sagging and eventually becomes holed allowing rainwater into the walls and ceilings.
Item Ponding Board for roofs Folder
Document 01 ponding board detail
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Ponding Board

Supports roof underlay

The red is a retro fitted ponding board fitted under the roofing underlay (blue).

Underlay is a requirement to temporarily absorb condensation and to capture roof leakage and drain it out to the gutter. Underlay is conventionally a bitumen soaked paper required to absorb around 100grms of water per square meter. It is therefore expected to get wet, repeatedly.

Ponding boards can be found on older homes often in the form of sarking. The sarking keeps the underlay from sagging and splitting which if that happens water will be dropped onto the insulation in the ceilings and cause durability issues.

In this drawing the house has no soffits so if the underlay was not supported and sagged and split water would get into the wall and ceiling. If the timber is H1 or untreated and it got wet it would decay.

Not all roofs have underlay.

Document 04 underlay failed
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Underlay perished

Caused leaks and decayed framing

This roof is an expensive Italian style clay tile. The tile lap design is very poor and not rain proof. Basically the tile joints leak in every rain event. When the underlay is new water leakage will run down and into the gutter as designed.

But above this underlay failure lies a down pipe from the roof above. Now the tile joints have to deal with not just rain but the entire water load from about 100 tiles on the upper roof all converging and discharging at the down pipe. Spreaders have helped and hence the underlay failure is spread across several tile widths. 

Document 06 underlay failed concrete tile
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Underlay perished

No ponding board

This concrete tile roof is very popular in New Zealand. It comes with the 'safe as brick and tile'. Yes the cladding is brick as well.

But the underlay has failed, water is dropped onto the soffits, found its way into the walls and wet the framing. But some good news this home was built with wet frame boron and has resisted advanced decay.

So fitting ponding boards and on with life.

The leaves are blocking the Taylor Fascia which adds to the problem. But one fix at a time.

Document 08 underlay failed metal
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Underlay sagging under metal pressed tiles

No ponding board

This detail bless its heart is a stupid internal valley through the center of the house. The ceilings were staining and after rain scanned wet. But the butynol valley was in good condition. So why?

I removed the first row of tiles to see how well the butynol had been turned up under the tiles. Not well but - water was ponding in the underlay. Small tears had started so when the water level rose it spilled onto the ceiling. 

You're going to say ridiculous and I agree. What next? But its the little things that are often at the center of issues. This is an older home and had well treated framing throughout. even the internal walls. Damage was very minor. Water was also ponding on the butynol so the valley was reshaped to provide drainage.

A good candidate for ponding boards.
Document 30 decayed wall
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Underlay failure

Decayed framing and toxic moulds

This is directly under the Italian clay tile job. Untreated timber decayed, severe stachybotrys and other toxic moulds. Carpets were ruined, linings had to be destroyed, a sorry state.

This is a spare guest room and no one had really taken any notice of the smell. Visually it held together but underneath no.

What is wrong here is a multitude of failures.
  • External timber framing behind absorbent stucco is meant to be H3 (ref NZS 3602:1995 Table 1B) it was untreated so decayed
  • No cavity was provided to manage condensation (NZS 3602:1995 BRANZ reference Cunningham 1983 rp031) so water entering through the failed underlay accumulated and provided conditions for moulds to grow
  • Winstones GIB linings were changed on or about 1994 to remove boron so have no mouldicides to prevent mould growth
  • NZ Fibreglass started adding starches to dampen down carcinogenic glass fibres getting into installers lungs and blood.
  • No testing of any of the roofing systems to determine the need for underlays and if so how will the be supported and remain durable for the life of the building eg wire netting, ponding boards, glass reinforced - or whatever
  • No monitoring of moisture - note Prendos emails 1999 requested monitoring to find out if details were weathertight
And when I did invent monitoring in 2004 none of the regulators and only a handful of decent experts and builders supported them. I was the only manufacture to use them. They didn't want them - why - of course - allowing home owners tools to test moisture properly would discover their negligence. Claims they don't want.

Now its your problem. Its never too late to find out before your home gets into this sort of state.

Document 50 underlay detail
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Underlay theory

Paper laced with tar is defending your home

The blue line is in theory the intention of the underlay. It fits under the battens but must be lifted over the bottom one so water can run off into the gutter.

Not all roofs have underlay. Why? Because regulators don't know what underlay is for. Without underlay builders saved 3 rolls at $50/roll. With underlay roofers had safety issues on stepper pitches. On lower pitches the last batten meant the underlay went upwards so why bother as water does not drain uphill. Roofs without underlay leak everywhere. This wets the roof  and eventually the ceilings and mould grows. As the house ages these become more noticeable - but well outside the liability period.

Some of the HRV type fresh warm air systems are inefficient without underlay as roofs loose heat quickly.

Good performing underlay collects rain water from the occasional cracked tile, or heavy rain event forcing rain through tile joints, or when tiles don't lap properly. This keeps the roof insulation dry and prevents ceilings from getting mouldy. That's the theory anyway. 

Except when the underlay gives up.

Document 55 underlay defect
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Underlay failure

Leaky building

Following on in the end the underlay gives up, water gets in, wets the insulation, ceilings and gets into walls. Wet H1 or untreated timber framing decays. Linings go mouldy, insulation is lost and gets mouldy, pathogens and toxic moulds grow.

All hidden from view.
Document 65 metal ponding board
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Ponding board

Lifts sagging underlay and spills water into gutters

I have found both metal, Fiber Cement boards and plywood to be good candidates for ponding boards. This ponding board is colour steel with a stiffener at the top edge and fold down at the front to allow the concrete tile to sit down squarely.

It worked as the soffit dried out, the walls dried out and there has been no more mould on the soffits.
Document 66 metal ponding board supports underlay
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Ponding Board

Supports underlay

This is the now supported underlay. Sure it is not the only demon in the room. The Taylor Fascia now has overflows and we're working on tree removal at the neighbours. Interesting how a $5 sapling on special 20 years ago now causes thousands of dollars of damage. But for a chainsaw.
Document 80 new Acceptable Solution ponding board
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Ponding boards now Acceptable Solution

No backdated upgrades though

It never phases the regulators. Perhaps that's why they get the big bucks. If this was Toyota or Honda, reputable manufacturers, they would recall all these defects and straighten them out to preserve their good name.

But not the NZ building industry. Make the change and then hide.

Yes we have known about ponding boards since ceiling insulation under tiles came about. But we had the 'belts and braces' so sure ceilings would go mouldy, spray them and get heaters in and ventilate. Sensible no claims.

But the leaky building claims changed that. Damage was found. Oops needs a correction. Ponding boards used to work so lets go back to them. But we'll call them Anti-ponding boards. New.

This detail comes from the 2004 Acceptable Solution E2/AS1. Changes needed. Delivered. Done.

But no recalls. Sorry. It's up to you as owner. If you want dry walls this upgrade may be needed.