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Sealants for movement cracks in wood


markocosic

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I think I'm going to need to fill some checking / cracks around knots on painted board on board cladding that we're doing all wrong.

 

 

Short version: 

 

1-2 mm cracks. Maybe 0.5-3 mm cracks. Can I use a low modulus polyurethane building sealant in wood?

 

https://www.everbuild.co.uk/product/puraflex-25/

https://www.everbuild.co.uk/wp-content/uploads/2017/11/TECNIC-PURAFLEX25-V2.1.pdf

 

 

 

Long version:

 

We're using the wrong type of timber (darn yanks buying all the good stuff) wrong time of year, (coulda shoulda woulda started in april), wrong application (roof) etc. This is mitigation.

 

 

They are (or might be) in the range 0.5 to 3 mm. Mostly 1-2 mm. 

 

The filler will be overpainted with an acrylic facade paint. This is flexible but won't bridge large cracks. It'll be black and reach 70C in the sun.

 

The purpose of this filler is to try keep standing water out of cracks that aren't full depth and are on the roof and reduce the amount of drying through to the ventilation gap behind that needs to happen.

 

Ideally the wood wouldn't have these but we've got what we've got and can't afford to bin 5 metre pieces for the sake of a knot or two with checking around them.

 

The boards will be mechanically fixed this summer. They'll be warm (I'll take from a covered pile) when screwed on then get hotter (anthracite in July sun) and colder (winter...) It's debate-able how dry they are and which direction they'll move in and whether there'll be a tendency to open or close the cracks. I'm going with opening cracks as that's worst case!

 

 

Anybody know about bodging fillers?

 

 

I'm thinking the stuff used in expansion joints that can do +50% expansion, rather than acrylic (7.5%) or butyl rubber (5%) BUT you're only supposed to use it in joints that are 10-40 mm in width (presumbly to give it half a chance of the expansion not being -100% to +200%)

 

https://www.everbuild.co.uk/product/puraflex-25/

https://www.everbuild.co.uk/wp-content/uploads/2017/11/TECNIC-PURAFLEX25-V2.1.pdf

 

https://www.everbuild.co.uk/product/175-universal-acrylic-sealant/

https://www.everbuild.co.uk/wp-content/uploads/2012/05/EVERFLEX-175-Universal-Acrylic-Sealant-V1.1.pdf

 

https://www.everbuild.co.uk/product/145-butyl-rubber-sealant/

https://www.everbuild.co.uk/wp-content/uploads/2012/05/EVERFLEX-145-Butyl-Rubber-Sealant-V1.1.pdf

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Thanks gents!

 

18C and raining in August... perfect time to be fitting them, heh!

 

We found that priming then one topcoat, stack and wait, then fit to roof and wait for some sun baking and raining, then topcoat and get a good dollop of the acrylate topcoat in each check mark/screwhead has hidden a multitude of sins. Much better than doing all painting then fitting. Looks like no need for "fine" filler but time will tell.

 

The more visible wall boards will be getting done in October at current rate of progress. Roof needs to go on now (whilst I have hols) though!

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14 minutes ago, markocosic said:

We found that priming then one topcoat, stack and wait, then fit to roof and wait for some sun baking and raining, then topcoat and get a good dollop of the acrylate topcoat in each check mark/screwhead has hidden a multitude of sins. Much better than doing all painting then fitting. Looks like no need for "fine" filler but time will tell.

Is this timber a rainscreen, IE there is a surface below that actually does the work of stopping water penetration?  

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It's a (board on board) roof. The timber does the same job as tiles.

 

Back in the day there'd be nothing underneath. We've added a membrane just as you'd with tiles. Partly a quick way to get watertight. Partly to catch any wind driven rain or seepage through damage. The membrane isn't the primary waterproofing though. You've got to keep the timber dry (using pine tar in ye olde days, or acrylic paint these days) if you'd like it to last, so the coating on the timber is the primary waterproofing that you seek to maintain.

 

 

PXL_20210808_183718845.jpg

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