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White mastic for sealing roof vent?


michaelmorris

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Just doing a bit of fettling on our new-to-us 12 year old motorhome. We've just discovered what appears as though it might be evidence of a past damp problem. The ceiling of the cupboard above the gas heater has some wrinkling in the surface and has a slightly sticky feel to it. Looking at the roof vent for the heater I can see that white 'gunk' around where the vent joins with the roof has a 2mm wide split in it on the 'downhill' side of the roof slope.

 

What would be the best (white) material for sealing up this split?

 

Thanks

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Guest frankie640

The best thing is to remove it and start again. Make sure you clean off all the old sealant and reseal it. I use EvoStick Seal and Bond which I get from B&Q. Completely water proof and seals better than Silicone and bonds like the proverbial to a blanket.

Whatever you end up using, do not use Silicone, it's totally unsuitable for this purpose.

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Guest Peter James
Agree you need to dismantle it, clean off the old sealant and start again. I use Plumbers Mait. People will say its not the recommended stuff, but you have that and it hasn't worked. Its like sticky plasticine that never sets. So its not a strong bond. But because it never sets it won't shrivel and crack like the conventional stuff does. And its easier to dismantle. Its what plumbers rely on to seal the waste pipe into the back of toilets and stop the contents coming down through the ceiling into the room below when you flush them.

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Guest Peter James
frankie640 - 2015-06-15 4:00 PM

 

What a mess.

Not now, I cleaned the surplus Plumbers Mait off much easier than the usual sealants. But to get the best seal you have to fill all the void with sealant. The only way to do that is to put plenty on, then clamp it together so the surplus squeezes out. Then clean it off if its going to be visible.

People who are more concerned with speed and neatness than making a good seal can just use less sealant.

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The advice to remove cover and reseal is right. Rather than use gallons of sealant from a tube you can get butyl sealant on a roll. It sticks like the proverbial to the blanket but does not set so it can ( with difficulty as it behaves like the stringiest cooked cheese) be pulled apart. You get an even layer and after tightening down the excess that squeezes out can be cut away with a sharp knife or chisel.
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Guest Peter James
George Collings - 2015-06-15 10:21 PM

... on a roll. You get an even layer .

with my van I didn't want an even layer because it was going on to a corrugated roof.

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In that case you can get the butyl in a tube and my guess is it can deal with vibration and wind buffeting. I am not sure if plumbers mate would put up with that in the long term but if it worked for you fine. There is a lot of difference in dealing in the frequency of 'movement' between heavy landings on the throne and wind buffet at 70 mph.
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