Cill Installation

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Installation Guide - Stooled Cills

Cast stone stooled cills are designed so each end of the  
cill appears to be supporting the masonry at each side of a window opening.  They are available as one piece units up  to 1500mm (dependent on unit height). Standard stooled cills are usually fitted as the build proceeds and protected from follow on trades, however some cills can be retro fitted.
To avoid stresses from differential movement the cill should be installed to the following guidance. 

For one piece cills up to 1500mm.

1. At the window opening sit the cavity tray 20mm wider than cill (with stop ends), on a 4mm mortar bed. (The window aperture will be narrower than the cill unit).
    
2. Weep vents should be positioned at each end ofthe cavity tray.
    
3. Place a shorter section of low bond DPC material on top of the outer leaf surface of the cavity tray to act as a slip plane. Place at each end under the stooled profile positions only.
    
4. On top of the second DPC layer place a further 4mm mortar bed under the stool end only and carefully place the cill in position.
    
5. When forming the masonry reveal above the cill ensurefree movement is maintained. Form a slip plane here by placing a 4mm mortar bed on top of the stool and smooth off. After hardening, place a square of low bond DPC cut to cover the mortared stool bed and side of the cill. Mortar on top of the DPC bed surface only and continue with construction of the masonry reveals ensuring bed joints remain in line and plumb.
    
6. At the end of construction insert a compressible backing cord into the gap between the bedded stool ends under the cill, and at the ends of the cill above the weep vents.
    
7. When considering an appropriate pointing solution, movement of different materials can be accommodated more effectively with a mastic or flexible joint sealant which would also prevent water penetration caused by mortar shrinkage.

Stooled cills exceeding 1500mm - Supplied in sections. 

1. For installation follow the general guidance for the one piece unit, however where the jointed sections meet a further section of low bond DPC should be placed on the external leaf of the cavity tray, enough to sit under 150mm of each bearing end and the mortared cill joint(s).
    
2. Place supporting mortar of a stiff consistency.
    
3. Apply a thin layer of morta on one central end of the unit and lay. Carefully insert the second section ensuring the cill is level along the full length.
    
4. Ensure joint/s where the cill units meet are fully filled with mortar.
    
5. Place additional weep vents at intervals no greater than 1000mm before inserting the backing cord.
    
6. When considering an appropriate pointing solution, movement of different materials can be accommodated more effectively with a mastic or flexible joint sealant which would also prevent water penetration caused by mortar shrinkage.

Note: To prevent cracking after installation only the ends and cill joints should be bedded on mortar. The joint below should be left open and pointed up after the brickwork is finished.

For further help and advice please contact the sales office on 01993 842391