External rendering

[stevedresden]

I have an 1800s terrace property where the rear wall has been badly weathered and affected by damp in the rear chimney stack.

The previous owner has made some attempt to repoint, bodged it and then just painted over it – hence it looks a mess. I’ve had a quote to render the rear wall (based on the fact that it’s weathered beyond the stage where I could remove the paint and repoint)

The builder is suggesting a couple of coats of SBR bonding agent, two coats of 3/1/1 sand cement lime mortar with nylon fibres and then one coat of limestone chippings.

He seems to know what he’s on about and claims that this will hold and not crack.

Reading about rendering issues on old damp period properties, it seems pretty clear that whatever he does has to be breathable – will this do the right job or should he just be using lime render rather than a cement lime mix?

 

[TheForge]

Whether it will do the job isn't easy to say but it certainly won't be very breathable with cement on the mix. A true lime render has no need for cement.

 

[plasticpigeon]

Hello, welcome to the forum!!! Are you sure it can't be cleaned and or rebuilt, I don't think rendering is often a good option, especially over a crumbling chimney stack, it seems like papering over cracks to me.

 

[jackpoint]

I can tell you how well cement render worked on our hovel if you like....but it's not a pretty story :lol:

 

[malcolm]

A cement lime mix is cement with a lime plasticiser just to make it more workable. It's still cement.

My take on rendering is old buildings were lime mortared and not built on much in the way of foundations so move around a bit. The render must be capable of moving around a bit too so is better in the same materials - lime. Secondly the solid wall behind will have no means of drying out if the render is not very permeable, so lime is better than inappropriate waterproof coatings like cement again. Otherwise you'll get the same sodden walls and failed coatings that you presumably already have under the inappropriate paint.

My place has been painted too. I'm getting quotes to have the paint chemically stripped (not sandblasted as that would ruin the bricks) and planning to leave it as brick. Looking at somewhere between £40 and £90 per square metre for the stripping. I guess at or below the low end if there is just one coat on there. Pointing would be in the same ballpark again, though I can do that myself. I reckon it will be worth it in the long run.

Post some pictures and someone will have better ideas.

 

[Zebra]

A 3/1/1 mix sand cement lime render won't be breathable, and if the house moves around a bit, having only minimal foundations, it could well crack, exacerbating your damp problems as water will then be able to penetrate but not escape. It will also stick very firmly to your bricks, and therefore be irreversible without severe damage to the house. Many a period property owner has cursed the previous owner who put a cement render on, and gone to a lot of expense and trouble to remove it in order to let the house breathe. I can't help but think that there must be a better solution.
A breathable render would have no cement in it, just lime, but most "normal" builders will look at you funny if you suggest a render with no cement in it. You'd be looking quite possibly at specialised lime renderers, which might not be as cheap as "normal" cement rendering.
Is that really the best solution? Can the paint not be removed, and the pointing repaired? When you say the rear wall has been badly weathered and affected by damp in the rear chimney stack, have you been able to identify the cause of this damp? Has it been addressed and a solution found? Is it rising, or penetrating damp? Or condensation? Or faulty rainwater goods, or raised external ground levels - the usual culprits?

 

[stevedresden]

Thanks for all the advice, I'll get some pictures posted later.

The damp has almost certainly come through the chimney. The previous owner claims that it was hit by lightning and that's why the brickwork was damaged - but to be honest I no longer believe anything he told me about this house!

I had the roof completely replaced a couple of years ago and they re-leaded the chimney and told me that would stop any further damp (previously it didn't have lead flashing, it had a layer of cement of some kind).

The other thing to bear in mind about the rear chimney is that the lowest section of it in the kitcen was removed a log time ago. There is a couple of foot left in the kitchen . which has a vent in it. We hadn't even realised this until we ripped out the old kitchen and found that the chimney stopped behind the cupboards. The bottom of the stack was full of damp brick dust, which we cleared out as best we could. I guess as the chimney doesn't reach the ground then it can't be rising damp

It's a shared stack with next door and they have external air bricks on their side, whereas we don't.

 

[stevedresden]

A couple of photos attached.

 

[plasticpigeon]

That's nice to see, your house looks similar to mine!! I too had, and to a small extent, still have some damp problems with the chimney. I had mine rebuilt above the ridge and vented cowls put on top of the pots to stop rain ingress ( i have changed the cowls for different ones since this picture was taken). I now just have a small problem with the flashing at the ridge. Are you certain that water ingress isn't at the top of the chimney? Also if you have the top capped off and no ventilation, I imagine that could trap damp. Having seen the pics I personally think that taking the paint off and repointing would be better than rendering.

 

[malcolm]

I have a damp chimney which had been sealed up in the past and was completely un-vented. For now I've wedged a tile under the capping to allow a bit of ventilation. The triffid growing from inside the chimney died this year which I take to be a sign it is drying out. I'll put a vented pot on at some point.

The damp in the walls behind the paint will be there because it can't evaporate. That causes the paint to fail and makes the problem worse. I removed the paint from one of my walls which was in a similar state to yours and it must have been damp as there was a lot of efflorescence from the wall drying out. The bricks were fine underneath - they looked like they had all failed under the paint, but that was just the paint bubbling.

If it were me I'd have the paint chemically removed. Cheaper than rendering and less risk of brick damage, damp, and the need for maintenance.

 

[Penners]

If it were me I'd have the paint chemically removed.

I agree. By the look of it you need to remove the paint chemically, replace any failed bricks and repoint the whole wall properly with lime mortar. Then wait and see whether the dampness resolves itself, which it probably will.

 

[stevedresden]

Are you certain that water ingress isn't at the top of the chimney? Also if you have the top capped off and no ventilation, I imagine that could trap damp. Having seen the pics I personally think that taking the paint off and repointing would be better than rendering.

Thanks for all the advice, I've taken it all on board and I'm getting some quotes for chemical paint stripping.

I'm not sure if the water ingress is at the top. It certainly was up until the point that the roof and flashing was replaced about 5 years ago. The roofer said the chimney pot was fine, but I realise that you couldn't see the top in the photos I posted, so here is the best I can get. Ours is obviously the small metal grilled cap

I don't have a tall enough ladder to get up there and look for myself up close (plus I'm not great with heights!) and I don't know if the fact that you can see some moss growing on the top of the stack is indicative of anything?

 

[Zebra]

I don't think you have enough overhang at the roof verges to render anyway. I agree with thoughts so far, strip paint and repoint would be better. Repoint in lime, preferably, ie with no cement in the mix! Many houses of that age do seem to get repointed in cement with no ill effects, all the time, but a mix with all lime and no cement will help breathability, particularly if your bricks are quite soft and porous.

 

[plasticpigeon]

I don't think you have enough overhang at the roof verges to render anyway. I agree with thoughts so far, strip paint and repoint would be better. Repoint in lime, preferably, ie with no cement in the mix! Many houses of that age do seem to get repointed in cement with no ill effects, all the time, but a mix with all lime and no cement will help breathability, particularly if your bricks are quite soft and porous.

Not sure about that Zebra. Many old places are pointed in cement, but also many old places suffer terribly from spalled bricks. The brickwork on my house is some of the best on the street because of the neglect it suffered over the years. The parts where the bricks were worst was where it had been repointed in the hardest cement.

 

[Penners]

I don't have a tall enough ladder to get up there and look for myself up close

It may be a trick of the photograph, but I think I can see a broken edge of the flaunching roughly midway between the pot and the vented cap. If this is indeed the case, then you'll need to remake the flaunching to ensure that the chimney is properly weather-proofed.

 

[Zebra]

Not sure about that Zebra. Many old places are pointed in cement, but also many old places suffer terribly from spalled bricks. The brickwork on my house is some of the best on the street because of the neglect it suffered over the years. The parts where the bricks were worst was where it had been repointed in the hardest cement.

I don't disagree about the damage that cement pointing can do, I just don't think it necessarily always means death to old brickwork, it depends on how hard the bricks are.

 

[DJH]

It looks to me as though the front apron doesn't extend far enough past the brickwork under the front side slate which leaves a weak area prone to water ingress.

The front and side flashing look to be made from one piece which will likely fail at some stage t the corner because of the forces exerted on this area by expansion pushing it out.

Ideally the side flashing would be better tucked into the joints of the bricks (step flashing) finishing in the joint one brick above the one the front flashing is in.

The only way to really stop ingress of damp through a chimney is to build in a lead dampcourse at the front flashing level with the lead turned up inside the liners and also where it projects in the roof space. This tray should be painted with bituminous paint to resist corrosion from lime and stop unsightly stain from running out down the flashings onto the slates.

Doug

 

[stevedresden]

Well my first quote for chemical removal of the paint came in at £3k - which doesn't include the scaffolding or any post paint removal brick/pointing restoration.

That's higher than I expected!

 

[plasticpigeon]

That's a lot! Has anyone had any experience of stone health products?? Their process is to paint on some goo to soften the paint and then wash it of with their own brand pressure washers. I'd have thought a normal hot water washer might be a cheap substitute.

http://www.stonehealth.com/stone-cleaning-products/no-7-7s-paint-softener/

 

[plasticpigeon]

I've ordered a few samples of this companies products. They have an approved contractor list and so may be able to recommend someone near you. They seemed very helpful on the phone.