NOTES ON THE PREPARATION OF LIGHT WEIGHT MORTARS FOR TORCHING.
Traditional buildings did not have bituminous underfelt beneath the slate or tile roofs. Instead, a soft sticky mortar mix was used both to help secure the slates and also prevent draughts. This mortar and the process is called torching. It is common for the torching to deteriorate and for pieces to fall away from the inside of the roof. This can create a potential danger if sections of plaster can fall upon people below. The problem is most serious in churches.
It is often mistakenly assumed that the only means of repair is to completely re-slate the roof with new underfelt. However, this is very expensive. A much simpler cure is to repair or replace the existing torching. It is also possible to increase the thickness of the torching plaster using lightweight fillers to provide an acceptable standard of thermal insulation. The mortar I have used for torching in traditional buildings is produced as follows:-
1). Make a good creamy lime paste from hydrated lime beaten into water and soaked for at least two weeks, stirring and beating occasionally to form a thick creamy paste. This can be done in large plastic tubs with a paint mixer head on an electric drill.
2). Mix ordinary sand or half & half sand and lime sand (i.e. powdered limestone waste), with Portland cement, in the ratio of about 5 sand to 1 cement.
3). Add a fibre to improve the strength and texture. Many different types of fibre can be used, traditionally horse hair or straw, but vermiculite, glass fibre or rockwool (shredded) can be used. The light weight filler can double the sand / cement volume.
4). Add sieved fire ash to the mix, no greater than one tenth of the volume.
5). Thoroughly mix the dry ingredients, then add lime paste to form a thick pliable mortar. As a guide, you will need about the same volume of thick lime mix as Portland cement.
As with all lime based mortars it is not possible to exactly specify mixes by volume, as there is no knowing what the density of the lime paste is. Also the exact proportions are not so critical as with cement mortars, and different mixes can be used for different purposes. It is therefore important to have an experimental attitude!
The area to be covered should be brushed to remove any loose or flaking material, and then sprayed with water or a thin glue size to bond the existing torching. Any melanex reflective sheet insulation should then be stapled into place with a hand powered tacker. It is desirable to secure this insulation with 1" chicken wire mesh secure to the inner edges of the rafters by staples. Adhesion is also improved if 2" nails are driven into the edges of the rafters as shown in the drawings. These will be embedded in the new torching mix.
For torching, quite a wet sticky mortar is needed to adhere to an underside surface, but it must also be dry enough to stay put and trowel out smooth. For this reason ash is added. The best texture can be described as like a bread dough. A lump should keep its shape on the mortar board, and should not stick on the trowel.
A fist sized section should be cut with the trowels edge, and then formed in to a neat lump. This lump can then be thrown up into the ceiling space. The impact forces the contained water against the slates and battens, and helps the torching to adhere. This 'splat' is quite important to master! It is important to have the water more concentrated on the top surface of the mortar than on the lower surface.
Once a small area has been spotted with splatted fistfuls, then it can generally be evenly trowelled out to an even smooth finish. With plenty of light weight fibre, a thick layer of torching of 20-30mm can be laid in one coat. However, if the roof structure is questionable it may be better to lay a thin initial coat, and then a second coat a few days later, when the first coat is secure. The only problem that I have experienced is with access to awkward corners out of easy reach.
If the room is to be heated, then it is necessary to consider the insulation value of the torching. A thickness of 30-50mm, filled with vermiculite, pearlite or rockwool will improve the insulation. However, I would also consider the ease of lining the entire ceiling with insulated boards, or polystyrene panels with additional rockwool insulation in the void between the joists. It is important to ensure there is some ventilation between the underside of the slates and the top of the insulation fill. This is necessary in order to evaporate any condensation. Two inch thick polystyrene can be obtained from most builders merchants, and can be cut easily with a sharp knife to be a tight fit between the rafters.
White Portland Cement can be used to maintain a white colour throughout the whole mortar mix, but this is expensive. The underside of the torching can be coated with a special thin mix without the insulating fillers and made up with White Portland Cement or Snowcem. The underside can alternatively be painted with a cheap traditional lime wash made as follows:-
1 1/2 gallons unslaked
lime, (i.e. fresh hydrated lime)
3 1/2 gallons water
1/2 lb melted tallow
Boil the water in a large old can, add the lime through a sieve to remove any small lumps. Stir well to create a thick creamy syrup. Then add the melted tallow. Keep stirring, and add more water if required to achieve a usable consistency. Boiled linseed oil can also be added to further improve the weather resistance. This white wash is best applied at once, but can be stored ready for use in well sealed cans, and then beaten up again before use. Do not expect it to be fully opaque when first painted on. It dries opaque. Several thinner coats are better than one thick one. It should be well brushed out to ensure that all little holes are fully filled. This will last for 1 to 5 years before needing a new coat. Any simple pigments can be added to change the colour or to give a lasure finish.
Whilst I do not mind using commercial 'Snowcem', it is also very easy to make your own cement based paint for exterior use:-
50 parts Portland cement 2 parts calcium chloride.
25 parts clean fine sand 1 part calcium stearite (powdered)
50
parts water
Sieve and mix all the dry ingredients in a clean plastic tub, then add the water and beat. Pour through a fine screen. Use White Portland cement for a white colour, or add pigments. Apply with two coats, keeping the first coat moist as it dries out (i.e. do not let it dry out too quickly in hot sun or it will crack). Allow at least 12 hours between coats.
Further information may be obtained from:-
JOHN NICHOLSON. Tanrallt, Rhostryfan, Caernarfon, Gwynedd, LL54 7NT.
(0286) 830312.