Casting window cills on-site.

[epsilonGreedy]

Just musing the practicalities of this...

 

On a long walk around the village I noticed that most properties built over a 100 year time span have rough cast concrete window cills and this includes new heritage houses that were build around 10 years ago under the regime of a very strict conservation officer. The rough cast cills are typically 1 brick high with a shallow gradient for water run off and are painted in the same colour as the window frame.

 

Would this type of window cill be cast in-situ in the window aperture or on the ground and then fitted like a normal art stone window cill?

 

Part of the motivation for doing this is to avoid the modern estate look with 2 brick high bathstone coloured cills and I also assume it would be cheaper.

Edited May 9, 2021 by epsilonGreedy

[tonyshouse]

I have done the several times, need to mitigate thermal bridging, concrete is a bit naff so we buttered the shuttering box with a clever mix of yellow pit sand and white cement , add a few bits of reinforcement then careful concrete, finally top off with the sand mix ensuring no concrete shows 

 

when set almost indistinguishable from real Bath stone. 
 

easier on the floor for upstairs, quoins etc -  

[epsilonGreedy]

9 minutes ago, tonyshouse said:

I have done the several times, need to mitigate thermal bridging, concrete is a bit naff so we buttered the shuttering box with a clever mix of yellow pit sand and white cement , add a few bits of reinforcement then careful concrete, finally top off with the sand mix ensuring no concrete shows 

 

when set almost indistinguishable from real Bath stone. 
 

easier on the floor for upstairs, quoins etc -  

 

 

This is encouraging to hear. If I read correctly, casting insitu is viable for downstairs window cills?

 

Very interested about your sand colouring idea, I assume a fine grade sand is preferred for a decent stone-like finish. My default local sand is mid way between soft sand and sharp sand.

 

When you say "a few bits of reinforcement" I assume this means strengthening the shuttering or did you place stainless rebar inside the cast cill?

 

The more I think about this the main challenge would be preventing concrete leakage down the face of the facing bricks at the junction between the shuttering that forms the external cill overhang. 

[tonyshouse]

We use steel reinforcing to act as anti cracking, my soft sand works perfect, it is very soft and yellow, Thames Valley, 

 

the bottom board of the shutter can sit 15mm onto the brickwork sides tucked in too, silicone to stop the slip oozing out. Sand mix can be fairly thick as can any concrete you use 

 

we use angled ends to help form the fall and squared up the horn bit that sits under the reveal brick. 

[PeterW]

I cast a load of coping stones to match a Yorkstone using a mould made from  kitchen cabinet offcuts. 
 

Mix is 4 parts washed plastering sand, 1 part white cement and 1/2 lime. Half filled the mould, laid in some cut up rendering mesh (glass fibre coarse mesh) and then topped up. Vibrate it with the SDS drill and left to set. 
 

all look good and tbh 1/10th of the price of York stone...!!

[Onoff]

36 minutes ago, PeterW said:

I cast a load of coping stones to match a Yorkstone using a mould made from  kitchen cabinet offcuts. 
 

Mix is 4 parts washed plastering sand, 1 part white cement and 1/2 lime. Half filled the mould, laid in some cut up rendering mesh (glass fibre coarse mesh) and then topped up. Vibrate it with the SDS drill and left to set. 
 

all look good and tbh 1/10th of the price of York stone...!!

 

Would that mix work for gate pillar caps? I need to have a drip bead on all 4 sides and that looks to be a nice "fine" mix that'd take up the drip bead detail.

 

Is washed plastering sand commonly available?

[epsilonGreedy]

1 hour ago, tonyshouse said:

concrete is a bit naff so we buttered the shuttering box with a clever mix of yellow pit sand and white cement...

 

 

Rookie Question: You don't mean you used butter as a releasing agent?

 

I assume you troweled a fairly stiff mix of fine yellow sand/white cement on the external facing sides at say 10mm to 15mm thick. Buttering the front vertical face of the cill could be interesting, if I used a laminate faced mould because I imagine the facing yellow mix slumping off before it has the weight of the concrete core to brace it.

[epsilonGreedy]

Another thought...

 

If the cill is moulded in-situ and the cill's concrete core flows into the facing brick frogs, that will result in a nice structural bonding & reinforcement of the base of the window aperture.

[epsilonGreedy]

14 minutes ago, Onoff said:

I need to have a drip bead on all 4 sides and that looks to be a nice "fine" mix that'd take up the drip bead detail.

 

 

I was thinking of using a rubber or plastic strip to form the drip gully and leaving it there for a week after the mould has been released, before pulling the strip out. 

[PeterW]

1 hour ago, Onoff said:

 

Would that mix work for gate pillar caps? I need to have a drip bead on all 4 sides and that looks to be a nice "fine" mix that'd take up the drip bead detail.

 

Is washed plastering sand commonly available?

Yeh it would and just a square of 10mm copper pipe pushed in, or even just scrape off the excess and when it starts to set you could carve in the drip detail with some sort of tool. 
 

Plastering sand in tonne bags is about £40 around here and available from the Jewson/TP/MKM merchants and also from Wickes I think. 

[Onoff]

6 minutes ago, PeterW said:

Yeh it would and just a square of 10mm copper pipe pushed in, or even just scrape off the excess and when it starts to set you could carve in the drip detail with some sort of tool. 
 

Plastering sand in tonne bags is about £40 around here and available from the Jewson/TP/MKM merchants and also from Wickes I think. 

 

Cheers. I only need to make 2 caps so these would do:

 

https://www.wickes.co.uk/Tarmac-Plastering-Sand---Major-Bag/p/131886

 

Plus some of my hydrated lime!

[epsilonGreedy]

2 hours ago, PeterW said:

I cast a load of coping stones to match a Yorkstone using a mould made from  kitchen cabinet offcuts. 
 

 

Is this because the material was cheap or does a smooth laminate mould release neatly?

 

2 hours ago, PeterW said:

Mix is 4 parts washed plastering sand, 1 part white cement and 1/2 lime. Half filled the mould, laid in some cut up rendering mesh (glass fibre coarse mesh) and then topped up. Vibrate it with the SDS drill and left to set. 
 

 

Does the mesh inlay stop the decorative outer layer mixing with the concrete core?

 

2 hours ago, PeterW said:

all look good and tbh 1/10th of the price of York stone...!!

 

 

And perhaps 1/4 the price of factory fabricated artstone cill? 

[PeterW]

2 minutes ago, epsilonGreedy said:

 

Is this because the material was cheap or does a smooth laminate mould release neatly?

 

 

Does the mesh inlay stop the decorative outer layer mixing with the concrete core?

 

 

And perhaps 1/4 the price of factory fabricated artstone cill? 

Yes laminate is smooth and it releases with ordinary veg oil. 
 

Mesh is midway through just for strength

 

And I recon they are £3 each or so - Yorkstone would be £35-40 for small bits 65mm thick. 

[epsilonGreedy]

3 minutes ago, PeterW said:

Mesh is midway through just for strength

 

 

I see. I assumed I would need to replicate the spec of the factory cills I was thinking of buying. The factory cills were 100mm high, 1400mm wide and had a 70mm rear cavity overhang, these had 3 stainless steep rebar rods embedded. I suppose the manufacturer has a dual concern of preventing a stress crack during transport as well as incorporating strength to handle movement in a masonry wall.

 

If I mould my own cills I would reduce the rear cavity overhang to 45mm, something similar worked for @joe90. I think the depth of a sash window frame is about 150mm so, given a full brick reveal, then a 40mm seat on the moulded cill rear overhang along the front of the sash frame and a 55mm rear seat on the inner wall blocks should be enough support.

[Onoff]

@Construction Channel made his:

 

 

[epsilonGreedy]

1 hour ago, Onoff said:

 

 

 

@Construction Channel's photo of the cill moulding is very instructive.

• I assume the metal reinforcement grid is stainless.
• If I copied that I think I would raise the grid 20mm above the bricks below for increased strength.
• I like the rear overhang detail, I now reckon I should form the rear of the mould using PIR that will permanently sandwiched by the finished cill.
• I notice @Construction Channel solved the potential for leakage down the facing bricks by squashing some foam between the brickface and front moulding.
• He well and truly cramps the front moulding to the inner block wall with long cramps, not sure of their correct name.
• The taper at each end will look good in the final cill and also helps the release I assume.
• And he used cable to form the drip channel.

[Construction Channel]

19 hours ago, epsilonGreedy said:

 

@Construction Channel's photo of the cill moulding is very instructive.

• I assume the metal reinforcement grid is stainless.
• If I copied that I think I would raise the grid 20mm above the bricks below for increased strength.
• I like the rear overhang detail, I now reckon I should form the rear of the mould using PIR that will permanently sandwiched by the finished cill.
• I notice @Construction Channel solved the potential for leakage down the facing bricks by squashing some foam between the brickface and front moulding.
• He well and truly cramps the front moulding to the inner block wall with long cramps, not sure of their correct name.
• The taper at each end will look good in the final cill and also helps the release I assume.
• And he used cable to form the drip channel.

Couldnt have explained it better myself. But the mesh was just standard galvanised and was raised a little while i filled the mould

[Onoff]

I'm going to make an upside down mould like this if I can figure the "pyramid" maths to cut the bits. I'll then fill from the top and vibrate it so it hopefully fills the voids:

 

[PeterW]

Pyramid maths is easy ....

 

how high do you want the peak in the centre ..??

[Onoff]

3 minutes ago, PeterW said:

Pyramid maths is easy ....

 

how high do you want the peak in the centre ..??

 

I'll get back to you.

[dpmiller]

no local precasts the right size?

 

[Gus Potter]

On 10/05/2021 at 19:59, PeterW said:

Pyramid maths is easy ....

 

how high do you want the peak in the centre ..??

It can be if you know how to go about it. You use basic trigonometry to do it.

 

Onoff.. you can cheat and model it in 3D to check the maths.

 

As an aside having visited a few precast outfits they use an almost dry sand / cement toner mix and mechanically whack that into the mould in layers. It's not like concrete we pour for founds say. If you want to copy what the precasters do then I think getting a really sturdy mould (shutter) is one key. I have seen the precasters using the expensive pigmented mix next to the shutter and no pigmented stuff in the middle.

 

That said I have cast chimney copes say in ordinary concrete hand batched in a half bag Belle mixer and carried up in pails with a drip and a good DPC under.. I passed one the other day and it's still looking good after 25 years..but my glasses are a lot thicker these days.

 

If your DIY precast cill is a bit permeable then make sure you spend time getting the DPC all neat and water tight.

 

 

[PeterW]

16 minutes ago, Gus Potter said:

It can be if you know how to go about it. You use basic trigonometry to do it.

 

Or one of the online calculators ...

 

https://www.calculatorsoup.com/calculators/geometry-solids/pyramid.php

 

 

[Onoff]

29 minutes ago, Gus Potter said:

Onoff.. you can cheat and model it in 3D to check the maths.

 

Thanks. I think I did start to model it in AutoCAD...might have given it to my lad to model in Fusion...can't remember. Sometimes even if I can do something easier/quicker I'll give it to him to excercise his CAD skills. 

[epsilonGreedy]

On 11/05/2021 at 23:44, Gus Potter said:

It can be if you know how to go about it. You use basic trigonometry to do it.

 

 

Just a hipped roof without a ridge mathematically speaking. I can do hipped roof maths, so let's do the trigonometry in this thread @onoff

 

How many bricks wide is this pillar?