Caulking/sealing between timber windows and bluestone

[Ikimashou]

G'day all,

I'm working on arched windows in a bluestone building. The sill was rotting away and opted for cutting out and replacing the worst of it with hardwood, rather than the whole thing.

The lime around the frame is mostly gone and gaps between 2-5mm are letting water in, which runs down and gathers under the sill. The stone is angled inside, which explains why the sill was rotting in and behind more than on the front and surface. The rebate in the stone is cavernous and was very wet. The wooden sill is flush with the stone, no drip channel, so I think a lot of water gets in under the sill gap and runs backwards too.

After repairing as much as I could, I've undercoated and reglazed the glass with linseed oil putty and blended it in with the old glaze, which is all intact. Waiting to paint the frame.

I live in Australia and oakum is very difficult and expensive to get, but I understand it's the ideal thing to use between timber and stone. Then you follow it up over the top with your lime pointing. Hopefully any deep rot that remains will stabilize if I can keep the water out.

Two questions: Because most of the gaps are so narrow, is it okay to not use oakum and just generously lime the cavity between the timber and the stone? It's almost stone on timer in some places. If not, what about the linseed oil putty for glazing? It's suitable for plumbing work too, so surely it would be a good, breathable seal? Has anyone tried it? Clearly it works well between glass and timber, what about stone and timber? Can you lime over linseed oil putty? Let me know if you have experience with any other sealers.

Cheers;

 

[Cubist]

Hi There and Welcome,

For your home I would not recommend using oakum. The stone may not be very porous but.. the oil in oakum would nevertheless bleed into the stonework and produced unsightly stains.

I would suggest you try Caulking Cotton, twisted and braided where necessary to get it to the required diameterofr a snug fit, and pressed firmly into the cavities before finishing with a lime mortar.

Let us know how you get on.

Cheers,
Cubist

 

[Ikimashou]

Hey Cubist!

Thanks for the welcome. Could you tell me more about caulking cotton? I've had a look online and a few boating suppliers in Australia do sell it, but don't say much/anything about what it is.

I found a Youtube video of a woman caulking the deck of a boat with what looks like fluffy, raw cotton and then putting oakum in over the top. But the caulking cotton these suppliers sell just looks like cotton sash cord, which I can get cheap locally. For the very tight gaps there is natural jute cord at the hardware store too.

Would cotton not get sodden and rot quickly on it's own if it's not tarred like the oakum is? Same for jute? I wouldn't use plastic rope but there is a 10% polyester cotton cord. The lime would have to keep the water out in that case to protect the cotton, if it's not water repellent in any way.

Also how deeply would the oils in the oakum penetrate the stone? The gaps aren't that big so I wouldn't be using much. I'm surprised in that case the timber doesn't soak up the oil and get stained itself.

Online boat supplier - https://shop.classic-boat-supplies.com.au/rope-cordage/other/caulking-cotton-1-2kg/
Online boat supplier -

What Is A Caulking?

Australia's leading Boat Accessories & Marine Electronics Supplier. CH Smith Marine Supplies Fish Finders, Boating Parts, Boat Propellers, Boat Trailer Parts, Fishing Equipment, GPS Chartplotters and Satellite Phones together with Excellent Customer Service, Advice and leading Boat Equipment...

chsmith.com.au

Natural cotton sash cord (hardware store) - https://www.bunnings.com.au/grunt-5mm-x-5m-cotton-sash-cord_p4310236
90% cotton, 10% polyester cord (hardware store) - https://www.bunnings.com.au/grunt-4mm-x-50m-macrame-cotton-cord_p0133146

Thanks for your help.

 

[Cubist]

Hello Again,

In addition to its oil content oakum is not suitable for your purposes as it is not a 'woven' material produced in regular dimensions. It is produced from short strands of jute or hemp that hold themselves together in a loose matrix. That matrix is pulled into long strands that can be anything up to 4-5 centimetres thick and this is coiled into bales of usually 5 or more kilos for sale. It most definitely is not suitable to fill a 2-3 mm wide gap be.

Caulking cotton is sold in rope form and is produced by a weaving process to acheive regular width/thickness. The 'loose' weave and the natural properties of the cotton allow it to be pulled, twisted and braided to allow a tight fit when it is pressed into a narrow cavity - for a boat or timber frame house it would literally be 'hammered' into place, this is not advisable for your stone walls. Sash cord and other ropes are likely to be too tightly/densely woven to permit such use and should not be considered.

The caulking cotton will absorb water as it, like the lime plaster/mortar used as the final filler of your gaps, is a breathable material.

 

[TudorCottage]

I’ve been using a lot of Oakum in between the window frames and Timber frame on our place and have found it really effective. Probably too early to see if the oil content stains anything.

I don’t know the exact size in one of the gaps in an upstairs window frame but I could see into the bedroom
I just used more oakum, tightly bound and tapped home. I tested for any gaps with a small bright light and all was good. I then covered the recess on both sides with a lime filler.

I was lucky enough to win a set of vintage chaulking irons on eBay for peanuts, a set of 4 that range from tiny to large. I’ve used all 4 many times, they have slightly rounded edges so tap home the oakum rather than chop it up. They have proved invaluable.

 

[Cubist]

I tend only to use oakum when the gap is wide and deep enough to accommodate it. Very narrow gaps I feel are better and more easily handled with CC which, like oakum, compresses nicely when firmly pressed or hammered. Must admit that around window frames in a timber frame I would expect oakum and CC to get lost in the expansion/shrinkage gaps and for that reason I tend to turn to burnt sand mastic for these.

I resisted buying a set irons as I'd found that an old and very blunt stubby screw-driver and equally blunt bolster and masonry chisels were as effective.

 

[DianeEliz]

I have Portland or Bath stone around my windows. In the last years I had very expensive wood sash windows out in. They have sealed between the stone and the wood with stone filler. But ofcourse the wood has shrunk abit and there are cracks in the filler. It’s a mess for a novice, and not a professional in sight.

Should I fill these cracks with burnt filler mastic please? In some places the filler is just falling away leaving cracks for water to penetrate.

They put clear silicone over the top which bees ate…

Upstairs they’ve just gone around with transparent silicone filler. It’s a mess as you can see the gap…
Why is it that these so called quality suppliers can’t offer proper installation to go with their fancy windows. One is useless without the other.

I’m so sick of there not being proper professionals in building. Just quick cheap and easy rubbish.

 

[TudorCottage]

I would imagine getting a colour match with the existing stonework would be the hardest part with burnt sand mastic. I got some from Cornish Lime in it’s natural colour (haven’t used it yet). They do sell it in quite a few different shades with a longer lead time than natural, so that might be worth a look.

 

[Cubist]

The photos suggest that it's the render coat that has cracked in places. Burnt sand mastic is not appropriate for the repair of these. You should use a a material compatible with the render coat; either a lime or cement mortar.

 

[a twig]

I used a bath stone coloured lime based filler when I had a damaged fireplace, I’ll try and find the link

 

[a twig]

Found it - this stuff:

Stone Repair Mortar, Bath Stone 1 (10kg)

Breathable lime based repair mortar to match paler shades of Bath Stone, famed for its honey colour as found in the city of Bath.

www.lime-mortars.co.uk

Carefully applied into the cracks with an old butter knife or similar should tidy it all up nicely

 

[Stevers]

I've made 'plastic stone' (NHL, sand and Doulting stone dust) repairs to ashlar quoins round doors, leaving a gap and (then not knowing any better) filling it with a variety of mastics other from silicon. On the front door the acrylic mastic has shrunk and pulled the plastic repair away from the stone as per the OP example. The gap between frame and quoins to be filled after repair is between 1/8" and 1/4", and by far the most successful filler on the other doors (for such a narrow gap) has been simple linseed oil putty - cheap as chips and good for the hands. Also in my experience, short of a lot of undercutting of the original stone to create a proper key, plastic stone repairs of this ilk do benefit from corrosion resistant metal reinforcement (copper, brass or stainless), due to poor bonding of the repair to the original stone.

When I bought the house the wider gaps round all my sash windows had been filled with the very hardest cement mortar; although I would never have done that myself, it's given no trouble, and I just paint over it each time, as did the professional painters at least three times before me - in maroon, green then blue.

 

[DianeEliz]

Found it - this stuff:

Stone Repair Mortar, Bath Stone 1 (10kg)

Breathable lime based repair mortar to match paler shades of Bath Stone, famed for its honey colour as found in the city of Bath.

www.lime-mortars.co.uk

Carefully applied into the cracks with an old butter knife or similar should tidy it all up nicely

But it will crack again won’t as the wood will shrink more? I wondered if there should be lime putty between the stonework and the wood?

 

[DianeEliz]

The photos suggest that it's the render coat that has cracked in places. Burnt sand mastic is not appropriate for the repair of these. You should use a a material compatible with the render coat; either a lime or cement mortar.

It’s not render though, it’s bath stone… see my ident photo

 

[Cubist]

Thanks for the clarification and I think I now have a through understanding of what's going on.

Your problem is not the caulking material. None of these would be strong enough to pull the face of the stone away to form the observed cracks no matter how much the timber window frame may shrink. I think it more likely that the window installer used an unsuitable filler around the edge of the stone where it meets the new window-frame to provide a clean and tidy finish. This, I now think, has shrunk back away from the stone and has separated to form the cracks you are seeing. Sadly, all of this will need to be removed as it now poses a water ingress risk to the stone-work, and the wooden window-frame, that could be very damaging over time. Once all of it has been removed you should then make good with the Stone Repair Mortar recommended by Twig.

 

[how999]

The traditiional cauling between timber and stone or brick is burnt sand mastic