User:Septerverius2/Repointing

BACKGROUND TO REPOINTING OF STONE AND BRICKWORK

When the FR buildings were first constructed they would have been built using stone freely available in the immediate vicinity or by transporting stone to the site from the quarry or the mine where the stone would have originated.

Although the buildings of the FR are built with a variety of stone types the most predominant stone used is either slate as random rubble (eg Blacksmith's Shop at Boston Lodge), coursed random sawn (eg Building 97a Boston Lodge - Telephone Exchange Store).

Sawn random coursed slate (Building 97a - Telephone Exchange Store at Boston Lodge.
Hewn random coursed slate (raised and rounded mortar - probably original. The pointing is generally sound with little or no repairs being required. Some localised failures would need to be repaired to match the present pointing- Building No 44 - Minffordd Station House
Random rubble with traditional lime mortar "buttered" pointing in good condition. Note the complete contrast in the stonework to the photos on the left(Building No 23 - Coed y Bleiddiau.

Whatever the type of stone used, they all share on thing. They would have been built using lime mortar to seal the joints in the facework. Lime may also have been used for bedding stones in the core of the walling but as making lime mortar was an expensive exercise, more often than not, stones in vernacular buildings would have been bedded using silty clayey soil which would have been freely available and is an ideal material for such use. The soils are pliable when damp and wet and harden when dry.

The main problem with soils used for bedding is that for the walls to remain stable they must remain dry. This is the function of the lime mortar sealing the joints on the outside of the building. Lime mortars are inherently soft and relatively weak compared with modern cement mortars. This is an advantage when used with weak stones as if the mortar is much stronger than the soft stones, the erosion of the stones will accelerate.

Erosion due to hard mortars is less of a problem the stronger the stones become. However, that isn't the only consideration. All of the walls associated with the heritage FR buildings are solid and typically between 450mm (18") and 600mm (2'-0") thick. For these walls to function correctly ie to keep the interiors warm and dry, the walls must be able to withstand penetrating damp from wind drive rain and persistently damp conditions. They do this by 'breathing' ie any moisture that gets into the walls through tiny cracks, and fissures in the stones, through absorption and condensation to evaporate freely through the mortar joints.

Lime mortars allow the moisture to evaporating rapidly which hard cement mortars prevent. They effectively act as a dam allowing moisture to build up in the wall which causes the walls to become damp and drop in temperature thus reducing their heat retention capabilities.

Sandstone (?) in chimney stack. Note harsh appearance of cement 'ribbon' pointing. This will cause accelerated decay of the stone (Building 23 - Coed y Bleiddiau).
Random coursed squared slate blocks some sawn others worked other hewn dry laid with some lime bedding. This needs no repointing (Old water tank base at Tan y Bwlch).
Brickwork to chimney stack showing some open joints some of which have vegetation beginning to grow in them. This is because the open joints invite water to settle and this provide ideal growing conditions for some plants(Building No 26 - Tan y Bwlch Old Station Building).

SPECIFICATION for REPOINTING EXISTING STONE WALLS

1 RAKING OUT JOINTS: Carefully rake out soft and perished lime mortar to areas to be agreed on site to a minimum depth of 2 x the width of the joint by using hand tools only. NO POWER TOOLS SHOULD BE USED.

2 WASHING OUT JOINTS: Wash out raked out joints by saturating with clean water using a hose pipe and allow water to soak into the remaining mortar and brickwork to prevent the new mortar from drying too quickly (this should take no more than 1/2 hour although the time will depend on the prevailing weather conditions). Ensure that the joints are absolutely clear to enable lime to adhere to the brickwork or stonework.

3 REMOVING HARD CEMENT POINTING: - Prepare hard mortar for removal: Where the stonework has been pointed with a hard cement mortar which has been "buttered" over the face of the stones it will be necessary to carefully remove this mortar without damaging the face of the stones. This may be undertaken by chasing along the top and bottom arris of the stonework with a sharp cold chisel. - Removing the hard mortar: Following this select an appropriate sharp chisel to fit the width of mortar joint. The chisel should be tungsten tipped with cutting edge wide than the shank of the chisel. DO NOT USE AN ANGLE GRINDER.

4 REPOINTING SAND: Sand for repointing mortars shall be sharp, durable, clean and free from coal, iron pyrites, soluble salts, clay and other impurities all in accordance with BS1200, and shall be well graded. The sand shall be thoroughly washed to remove all organic matter which may weaken the mortar, or cause efflorescence. Muddy sand which strains the fingers and balls up when rubbed should be rejected. The sand shall be carefully selected to produce a colour to match the existing and remaining mortar. Ideally, samples of the original mortar should be analysed for aggregate type and proportions and a similar aggregate mix shall be used.

5 COARSE STUFF READY MIXED): Mortar for pointing shall be manufactured using ready mixed non hydraulic lime putty and sand 'Coarse stuff' (approx proportions 1:3 lime:sand -) prepared using a sample of the existing mortar for matching the colour of the existing and (see 10 below) obtained from one of the following suppliers.

Ty Mawr Lime Co, Ty-Mawr Farm Llangasty, Brecon, Powys, LD3 7PJ Tel: 01874 658249)

The Traditional Lime Company, Church Farm, Leckhampton, Cheltenham, Gloucestershire, GL51 5XX (01242 237727).

Cy-Pres, 14 Bells Close, Brigstock, Kettering, Northamptonshire,01536 73431

6 ORDERING COARSE STUFF: Arrange delivery of the coarse stuff as soon as practicable after confirmation of appointment to give maximum maturing time for the lime. The coarse stuff should be ordered to be as stiff as possible and not too wet. A stiff mortar can be "knocked up" to give more workability but in pointing works a stiff mortar is more manageable than a wet mix.

7 COARSE STUFF (MIXED ON SITE): Coarse stuff may be mixed on site using non hydraulic lime putty and sand. The lime putty may be obtained from one of the suppliers stated above or from other local suppliers. The mortar is mixed by hand to obtain a stiff consistency. A tumble mixer can only be used if time is allowed for water to evaporate afterwards to allow the mix to stiffen which can take weeks rather than days. Ideally a roll pan mixer should be used but these are expensive and difficult to obtain.

8 STORING COARSE STUFF: The coarse stuff shall be stored in a sealed container and shall not be allowed to dry out.

9 MIXING POINTING MORTAR: The coarse stuff will require to be knocked up, ie rammed and beaten before use, to make it workable and to drive off excess water. Mechanical drum type mixers should not be used for this task and as small amounts only will be required for pointing manual knocking up will be the appropriate method to be used.

10 MORTAR MIX: Carry out mortar analysis of the existing lime mortar to obtain the correct specification for aggregates.

11 SAMPLE PANEL OF POINTING: Before proceeding with any pointing of the parts of the building specified to be repointed, prepare a sample panel in a position to be agreed and approximately 1m x 2m to be approved by the Surveyor or Conservation Officer. Obtain the written consent of the Surveyor to the sample panel of brickwork before continuing with the whole of the work.

12 POINTING: Repoint using a mix to the approval of the Surveyor or Conservation Officer. The mortar should be firm and mixed by hand as 9 above or by using a mortar mill to grind the lime and sand course stuff into a soft putty. Using the correct size pointing iron for the width of joint being pointed insert mortar into the joints. (do not use a POINTING trowel).

Pointing irons.
Pointing using a pointing iron - from www.grovsvenorconstruction.co.uk).
Samples of mortars using different mixes of sands and lime can be made in small dishes with caps to protect them for future reference.

- Ensure joints are fully filled with mortar leaving no voids. For very thin joints, feather out mortar on a hawk and pick up the desired thickness on a small key and insert thin end of wedge into joint. Fill joint flush with the brickwork and leave WITHOUT ANY FURTHER TREATMENT for 24 hours. This is most important.

13 TREATMENT OF JOINT AFTER INITIAL SET: Following the initial set, clean off edges of mortar and cut back mortar face behind face of stonework. Work mortar into joints driving home with a churning brush. Complete process by gently brushing surface of pointing with a soft brush to expose the course aggregate in the mortar.

14 Conditioning the Mortar: THIS IS VERY IMPORTANT. If warm and windy protect the new mortar pointing with damp hessian and polythene draped from the scaffolding to prevent early dewatering. Spray wall several times a day and cover at night until the mortar has been allowed adequate time to cure. If frosts are experienced then hammer home the mortar after thawing and be prepared to renew the mortar if failures are experienced.

15 FINISHED PROFILE OF THE MORTAR JOINT: The finished joint shall be kept back from the face of the stones as directed on site by the Surveyor. The joint shall not be "tooled" but shall be lightly brushed to reveal the coarser aggregates in the mix. In warm or in mild windy weather keep the mortar well wetted until fully cured to allow the mortar to achieve its full strength. Leave face of stonework to dry to prevent lime mortar from sticking to the face of it during pointing works.