Render over bitumen and paint?

[twitcher]

Alright lads, been lurking for some time. So here goes..

Been asked to s & c an external gable end wall on a detached terrace - it's the only one in the street that's detached. The chimney breast on the downstairs internal wall is showing signs of damp - high up in the corner of the wall near the chimney there's salts coming through, there's another patch on the chimney breast and I've noticed a patch by the bay window on another wall. They don't reckon the damp is rising so they're considering having it rendered to stop the damp. The external wall (see pics below) seems to have been painted with bitumen some time ago and then painted over. Question is, will the render stop the damp (scratch coat with water proofer) and can you render over the bitumen and the paint or do you need to hack it off? Could I just SBR it then throw a scratch coat on?

 

[twitcher]

trying to stick some pics up, but it says 'the uploader file is full' or something along those lines..

 

[beddy]

i think it wont let you upload them because youve been stealing our advice for ages without contributing and now you really need our help its giving you the finger mate

 

[Gibbo]

you could give it another coat of bitumen and when wet blind it with some sharp sand to form a key

 

[twitcher]

i think it wont let you upload them because youve been stealing our advice for ages without contributing and now you really need our help its giving you the finger mate

stealing?! I've just been lurking mate.

is it cos I didn't fill in that survey?

 

[essexandy]

Wow some question for a first post and all was going well until the end when you said "could I just SBR it and throw a scratch coat on". Ahrrrrrrr! surely anyone even thinking about doing some plastering/rendering should have enough knowledge to not ask that.
Sorry if that sounds a bit harsh but........................

 

[pwi]

You need to make sure the damp is what you say it is ie check room _ cavity wall ? _ windows etc . But if its the render then I would need taken off or hpx an mesh with mec fixings is it hollow at any point??

 

[twitcher]

there's no cavity in the wall, and the damp seems to be coming through the brickwork and not travelling up the wall.

if I follow wizard's advice and reseal it with bitumen plus the sand, how long do i need to wait till I can scratch coat it?

 

[essexandy]

there's no cavity in the wall, and the damp seems to be coming through the brickwork and not travelling up the wall.

if I follow wizard's advice and reseal it with bitumen plus the sand, how long do i need to wait till I can scratch coat it?

It depends on the make you use just make sure it's set because the bitumen must hold the sharp onto the wall well because it's got to support the weight of two coats of render. Personally I'd rather use building paper behind EML/riblathe which is covered with two coats of render and painted with a good quality masonary paint.

 

[McPlaster]

Stop the damp first then render.

 

[twitcher]

the damp appears to be coming straight through the external wall. there's some new bricks that have been put in recent, maybe the pointing's fcukd?

What about the damp on the actual chimney breast? it's about half way up the wall on teh ground floor.

 

[lucius]

Is the chimney still in use or is it bricked up or the flue sealed in anyway.

 

[Render Systems]

no cavity... use nhl lime mortar mate... sand and cement will make it worse... take all the c.rap off with angle grinder down to brickwork then render

 

[twitcher]

yeah been thinking that the damp coming through is where new bricks have been put in with cement mortar. Whereas where the old bricks are with lime mortar there aint any damp.

only used s&c render over the past 6 years.

What's nhl lime mortar like? Went on a mike wye course recently for tadelakt and venetian and i'm sure he was saying to these lime plasterers that they should use lime with pozzolans. what do you reckon?

 

[Render Systems]

hahha hes baffling you with science mate the pozzolan is sand or any other aggregate such as brick dust etc... buy your lime mix ready mixed

 

[merlin]

pozzolan is a volcanic ash that sets fat lime quicker than it can carbonate so you can use on external renders , it has no place with NHL !

 

[Render Systems]

pozzolan is a volcanic ash that sets fat lime quicker than it can carbonate so you can use on external renders , it has no place with NHL !

yeah your right mate

 

[martinemj]

i would think twice about rendering back over the bitumen

do find that render tends to crack or blow after a while (blinded sand never seems to stick properly either due to being poorly applied or just being s**t material)


would tend to agree with use of nhl render/finish if possible


www.emjenkins.co.uk

 

[The Apprentice]

I used to do loads of this type of work, bitumin was the main stay in the dark distant past to control damp. The problem was always that you only stopped the damp coming through in that specific area and it usually appeared in an area very near to it.

 

[twitcher]

pozzolan is a volcanic ash that sets fat lime quicker than it can carbonate so you can use on external renders , it has no place with NHL !

yeah your right mate

yep you're right it's a volcanic ash.

seem to remember him dissing nhl

 

[lucius]

The bituman needs to come of and rerender with lime morter be it putty or NHL for a proper job, there are some lime based monocouches available for renovation work, well there is in Spain anyway but they havent stood the test of time as they are quite new, You didnt answer my question about the chimney but anyway if it is redundant and bricked up it needs air bricks in it otherwise it may condensate and cause damp patches.