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neb



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Topic # 239903 9-Aug-2018 23:17
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I'm in an older house with fibrolite cladding and alu window frames fitted into the fribolite, with wooden surrounds to seal the edges, for which prolonged water exposure in one has partially rotted the bottom strip of wood, see the photos below. The alu window is installed over the top of the fibrolite, so the whole lot seems reasonably waterproof (the flashing at the top and the wood surround down the sides looks fine), it's only that lower piece of wood that's rotted, presumably from water running off the alu frame and then sitting on the wood for awhile. It's nailed onto the fibrolite, not onto wood, so it looks like only this one strip of wood has rotted, not any framing timber behind the fibrolite:

 

 

 

 

 

 

The obvious solution here is to replace it with another strip of wood (the existing one is nearly forty years old so it's held up pretty well), however I'm wondering if I should do anything extra to deal with water ingress at the bottom of the window frame. I was going to spray some penetrating fungicide up underneath the alu frame to deal with anything that may have got in there and then replace the wood strip, but since I've got the thing opened up I could also do other things. I'm not certain whether (say) sealing it up tight will mean moisture is kept out or kept in.

 

 

At the moment I'm considering using a thinner piece of wood so I can run it up under the alu frame rather than having it flush, which is where it looks like the water is getting in now, or running a bead of sealant along the top edge of the wood before I fit it to form a watertight seal between the window and the top of the wood.

 


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  Reply # 2070984 10-Aug-2018 00:11
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If you go to the Branz website, they have details of that with a sill junction. That timber sill beading shouldn't stick out like that, as the aluminum sill should lap over the top of it. Something like this. https://www.weathertight.org.nz/assets/Uploads/_resampled/ScaleWidthWyI2OTYiXQ/B130Fig4.jpg 


neb



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  Reply # 2070985 10-Aug-2018 00:18
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mattwnz:

If you go to the Branz website, they have details of that with a sill junction. That timber sill beading shouldn't stick out like that,

 

 

It didn't, until I pulled it back to see what was underneath :-).

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  Reply # 2070986 10-Aug-2018 00:31
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neb:
mattwnz:

 

If you go to the Branz website, they have details of that with a sill junction. That timber sill beading shouldn't stick out like that,

 

It didn't, until I pulled it back to see what was underneath :-).

 

 

 

Yeah, but the aluminum sill should overlap over the top of the timber. If it overlapped, you wouldn't be able to pull it out. It appears from your photo that the top of the timber just sits back at the same level as the bottom of the sill. So water basicly then tracks backwards from the aluminum window sills bottom edge, back to the top edge of the timber beading, and possibly gets drawn in by capillary action too. So the water gets trapped  or sits on top of the timber, and just rots it over time. This sort of thing is probably quite common on some older builds.  The diagram I linked to shows Branz recommends it is done. E2/AS1 also should show something similar.


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  Reply # 2070987 10-Aug-2018 00:36
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Incidentally, which page on their site was that from? In terms of what to replace it with, I was looking at something like this, which isn't meant to be a sill but, referencing another thread, is HMR MDF so should last longer than the usual 3.1 treated stuff, might be the best option. Since the depth window frame only protrudes around 10mm it'd also be about the only thing thin enough to fit in there.

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neb



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  Reply # 2070990 10-Aug-2018 00:40
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mattwnz:

It appears from your photo that the top of the timber just sits back at the same level as the bottom of the sill. So water basicly then tracks backwards from the aluminum window sills bottom edge, back to the top edge of the timber beading, and possibly gets drawn in by capillary action too. So the water gets trapped  or sits on top of the timber, and just rots it over time.

 

 

Yup, that's exactly what's happened.

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  Reply # 2070991 10-Aug-2018 00:46
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mattwnz:

 

 

It's on this page. https://www.weathertight.org.nz/new-buildings/detail-solutions/wall-cladding-installation/cladding-penetrations/

 

 

 

 

Perfect, thanks! I can't really add flashing without taking the whole window out (which would also mean putting up scaffolding, getting extra people in to help wrestle it around, and who knows what else), might be able to fold some up and poke it into the gap as far as I can before I replace the wood strip... it's not meant as a long-term thing, at some point the whole house will need a reformat and reinstall anyway.

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  Reply # 2071026 10-Aug-2018 09:06
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It's important to seal cut any ends on the new timber surround.

 

Apologies if I'm stating the obvious, but it's often overlooked - including by builders.





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  Reply # 2071027 10-Aug-2018 09:09
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neb: Incidentally, which page on their site was that from? In terms of what to replace it with, I was looking at something like this, which isn't meant to be a sill but, referencing another thread, is HMR MDF so should last longer than the usual 3.1 treated stuff, might be the best option. Since the depth window frame only protrudes around 10mm it'd also be about the only thing thin enough to fit in there.

 

I had thought from the pictures this was external? I wouldn't use even HMR MDF externally. If it does get wet for any reason it will swell and there goes any kind of seal. H3.1 is rated for external so long as it is painted. If you want something more durable than that, you could try a hardwood moulding, probably cedar (cedar needs an oil primer, not acrylic, if you end up painting it though).


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  Reply # 2071291 10-Aug-2018 13:44
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mdf:

I had thought from the pictures this was external? I wouldn't use even HMR MDF externally. If it does get wet for any reason it will swell and there goes any kind of seal. H3.1 is rated for external so long as it is painted.

 

 

The reason why I was nervous about 3.1 is that, due to the window/house construction it ends up soaking in water for long periods of time [*], so H3.2 or 4 seemed safer. I'll soak it in timber sealant/fungicide and then paint it (BRANZ says a 3-coat paint finish), in any case though it only has to last until the whole house gets redone so I'm not desperate to get a 15-year lifetime from it.

 

 

[*] This place was built during a period when some cretin decided it was fashionable to not have eaves. Water runs down the sides of the house and into any gap or seam it can find, where it sits for indefinite periods of time.

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