Potential major issue; water ingress at uncapped coping board during construction

[maxdavie]

Hello fellow builders,

 

I have what I think may be a major issue. 

 

For context - I'm an architect, so know about buildings in theory, not so much in practice; this is my first build; the purpose of this build - apart from providing us with a garden room - is to pilot a cnc-cut kit-of-parts construction system assembled from plywood. 

 

We assembled the entire building open to the elements during the summer months. It got wet occasionally, but dried out soon after. 

The roofing single ply membrane went down over a tapered insulation scheme. They lapped it up over a low parapet wall and stopped it half way over the top. Over the next few days, I adhered our airtight breather membrane to the walls, lapping this up and over the parapet wall and stopping 20mm or so from the roofing membrane (I now know I should have done these two activities in reverse order). 

I designed the system so that we'd slot plywood coping boards over the parapet wall top. The idea was that we'd then cap this ply coping support board with metal coping pieces. 

We have had a lot rain since the breather membrane went on. The coping support boards have been soaked and barely had time to dry before the rain came again and soaked them. 

I'm now waiting for a few dry days so I can install the metal coping.  

 

The major issue is this: water/moisture has moved straight through the double-plywood coping and reached the underside; water/moisture may have travelled beneath the single ply roofing membrane; water/moisture may have travelled down through the plies in the plywood and into the building. 

I know that trapping moisture is a major issue and as a rule in timber frame building, things should be totally weather tight before walls are sealed up with finishes. 

We've been cracking on in the interior with our VCL, plasterboard and skim. Floors have been raised and levelled with compounds etc, ie we are sealing things up. 

I want to know if other people see this as a major issue or think that it will just dry out in time before the timber begins to deteriorate.

Is it possible to use a dehumidifier to help it dry from the inside? If there is moisture trapped beneath the single ply roofing membrane, will it find its way through the insulation, the VCL over the roof deck and into the plywood roof deck itself. 

Thanks for sharing any thoughts. 

 

 

 

 

[HughF]

As it's technically an experiment (albeit an expensive one at current plywood prices), I'd stuff a dehumidifier in there and see what happens. And make sure to document this for future builds.

 

What grade of ply did you build it with? Looks like shuttering from where I'm sat. OSB would have been my choice, it doesn't delaminate as badly when it gets wet, and you could have looked at importing advantech or similar, with its built-in waterproof layer.

[JohnMo]

1 minute ago, HughF said:

OSB would have been my choice, it doesn't delaminate as badly

Strange I was looking looking at some left over bits and pieces from the build the other day.  Both have been exposed to the same weather and fully exposed outside for two years.

 

OSB has has gone grey, other than that looks fine.

 

External Plywood delaminated very badly and very twisted.

 

[HughF]

2 minutes ago, JohnMo said:

Strange I was looking looking at some left over bits and pieces from the build the other day.  Both have been exposed to the same weather and fully exposed outside for two years.

 

OSB has has gone grey, other than that looks fine.

 

External Plywood delaminated very badly and very twisted.

 

Exactly.... OSB puffs up and the flakes come off. Plywood suffers complete delamination. There's a metric ton of literature and arguments over in the US about what's better, and it's generally considered to be OSB.

Edited October 20, 2022 by HughF

[Carrerahill]

28 minutes ago, maxdavie said:

Hello fellow builders,

 

I have what I think may be a major issue. 

 

For context - I'm an architect, so know about buildings in theory, not so much in practice; this is my first build; the purpose of this build - apart from providing us with a garden room - is to pilot a cnc-cut kit-of-parts construction system assembled from plywood. 

 

We assembled the entire building open to the elements during the summer months. It got wet occasionally, but dried out soon after. 

The roofing single ply membrane went down over a tapered insulation scheme. They lapped it up over a low parapet wall and stopped it half way over the top. Over the next few days, I adhered our airtight breather membrane to the walls, lapping this up and over the parapet wall and stopping 20mm or so from the roofing membrane (I now know I should have done these two activities in reverse order). 

I designed the system so that we'd slot plywood coping boards over the parapet wall top. The idea was that we'd then cap this ply coping support board with metal coping pieces. 

We have had a lot rain since the breather membrane went on. The coping support boards have been soaked and barely had time to dry before the rain came again and soaked them. 

I'm now waiting for a few dry days so I can install the metal coping.  

 

The major issue is this: water/moisture has moved straight through the double-plywood coping and reached the underside; water/moisture may have travelled beneath the single ply roofing membrane; water/moisture may have travelled down through the plies in the plywood and into the building. 

I know that trapping moisture is a major issue and as a rule in timber frame building, things should be totally weather tight before walls are sealed up with finishes. 

We've been cracking on in the interior with our VCL, plasterboard and skim. Floors have been raised and levelled with compounds etc, ie we are sealing things up. 

I want to know if other people see this as a major issue or think that it will just dry out in time before the timber begins to deteriorate.

Is it possible to use a dehumidifier to help it dry from the inside? If there is moisture trapped beneath the single ply roofing membrane, will it find its way through the insulation, the VCL over the roof deck and into the plywood roof deck itself. 

Thanks for sharing any thoughts. 

 

I do not think it is a major problem, as it stands, think of the building that goes on in harsh weather and its fine in the end.

 

My only worry would be letting it dry. If you have wrapped it all up in airtight membranes then therein lies an issue, you have trapped it all.

 

I think I would try and open some of the roof structure/walls up and get heat and air moment in there to let things dry a bit.

 

Even if, as painful as it seems, you need to slice the membrane then tape it back up again later.

 

This airtight stuff is all good and well, but buildings must be able to breath and this indiscriminate use of airtight membranes and insulation and seals and all sorts can cause more issues than good. 

 

 

[maxdavie]

Thanks for thoughts. 
I’m using Wisa Spruce which has held up pretty well! The plies are spruce which has a durability class 4 ‘slightly durable’ (5-10yrs) and the glue layer is of bond class 3 (exterior grade) so it’s the next best thing really after marine ply. 

 
Re airtightness and breathability - these are not mutually exclusive. Airtightness just prevents bulk air loss and with it all the valuable heat it carries. The building can still breath. The VCL on the warm side however does slow the process considerably to prevent condensation accumulating faster than it can escape as it moves to the cold side. 
 

 

[Iceverge]

Any chance of a quick sketch? 

 

A thousand words and all that.....