Advice needed on repairing gable end parapet

[michael]

I need to do some rendering repairs to the parapet of a gable end wall on my nineteenth century property. The parapet stands proud above the slated roof by about a foot. It’s also about a foot wide and flat on top and pitched to follow the line of the pitched slated roof. The front visible face of the parapet has simple straight line mouldings which step back towards the face of the brick gable wall. The rear face of the parapet and top face are just flat rendered.

To all intents and purposes, it looks like old cement, very hard and grey buff in colour with a bit of discolouration from years of grime no doubt from the days of coal fires. Could it be a kind of Roman cement perhaps, albeit, I don’t really know what Roman cement is.

Both the flat top and the mouldings have a few old cracks which someone in the past has tried to repair with a kind of bitumen filler. One of these cracks is now letting in water. Dose anyone have any ideas on what I should use to repair these cracks correctly. Because the flat top is in direct 90 degree contact with the rain it has to be extremely waterproof. There is no evidence of the parapet ever having coping stones so I would like to avoid this if possible and keep things simple and original.

I think lime (hydrated or hydraulic) in this situation in direct contact with rain fall would probably just act like a sponge, eventually becoming saturated and allowing the downward moving water to seep through.

 

[MdB]

Pictures would be useful if possible.

 

[chuckey]

"Grey buff coloured", is this yellowish, grey-> white? How wide are the cracks? Does water pool on the top surface? Sorry about the questions but I have all sorts of pictures in my head and I need some more facts to clear my mind.
I think in general the repair will have to be done in two parts, one to water proof the inner core of the wall and two, to colour match the finished repair.
The first part is really vandalism, open up the cracks so they are at least 1/4" wide and 1 " deep. The wack in a wodge of your favourite water proofer, say silicon based caulking from a tube, cement with a water proof additive, "prompt " cement (water proof proof cement for drains). Lime mortar with lots of linseed oil in it. Leave a minimum depth of 1/2" for finishing with your colour matched mortar. This is a fiddly job, just make small batches of various mixes with different sands or use a cement dye. About a cupful of each, tip'em out on a board, label each one and keep a note of what you put in them. After a week decide which one is the best or repeat experiment. It will be a real pain cos' you will need different sands in minute quantities. In the end you will have to spend at least £2 on sand for the whole job, but which one??
Have done it.
Frank

 

[Jon Maine]

Is there a free stone water tabling on top of the coping?

Anyhow, What I usually do is bring up lead soakers from the tiles or slates by 75 to 100mm, fix stainless steel expanded mesh underside of the tabling down over the lead and render in hydraulic lime, use stainless screws and washers. Job done.

If removing the cement is going to damage the brickwork to much you could just mesh straight over the top of it. Although you could use a diamond grinder to remove the cement with no impact damage to the bricks but a dusty job.