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Jointing Design

Updated:2007-11-19 14:15:16

The specification, design and detail of joints and sealants should be done by qualified persons.

1 Loadbearing joints in vertical walls transmit loads to the stone below. Shims are used to provide the designed joint width until the cement mortar cures. Cement mortar is also used for pointing or grouting horizontal surfaces such as paving, steps, and copings, etc.

2 Sealants applied in joints of vertical walls accommodate movements of the stone cladding and movements of the structure which may be transmitted to the cladding. Sealants need back-up materials which compress easily and do not bond to the sealant. Most sealants require primers for good adhesion. Particular care is necessary to have clean joints to insure proper adhesion.

3 Expansion joints with sealants are designed to accommodate vertical, as well as horizontal, building movements. Expansion joints are needed in stone joints beneath supporting steel angles to prevent stress concentrations due to differential vertical movements between the stone veneer and the building structure, or due to deflection of the spandrel beam, thermal movement or sway. Utmost care is needed to make sure that shims, or any other rigid objects, are not left in the expansion joints. Vertical expansion joints shall be designed to accommodate thermally or otherwise induced horizontal movements of the stone veneer or its supports.

Adequate expansion joints are needed on roof parapets which are open to the weather on two sides, to roof copings, and to expansion joints between intersections of a stone base course and a horizontal sidewalk where lack of properly designed and executed expansion joints may cause serious failure.

When designing joint widths the fabrication and installation tolerances shall be considered. For prefabricated panels such as stone faced precast or stone on trusses, unitized curtain wall panels, joints between the stone slabs should be caulked in the shop under controlled conditions. Only the joints between the prefabricated units should be caulked at the jobsite and using the same caulking compound which was used in the shop. When designing joints the potential problems due to handling, loading, transportation, unloading and erection should also be carefully considered.

Joint sizes should be designed realistically. Aesthetic considerations should not be more important than the functional requirements. Designing joints too small, could create serious functional problems.

4 Sealants are classified as single component or multi-component.

Single component sealants have a slower curing time.

Non-sag type sealants are applied with a gun. Self-leveling type sealants are poured into paving joints and do not require tooling.

Silicones cure fast and resists ultraviolet light. Urethanes show good resistance to abrasion and are preferred for use in paving joints.     To avoid smears in critical areas masking tape may be used along the joint edges.

Part of the jointing design is the selection of the proper joint filler which controls the depth of the sealant in the joint and can also act as a secondary barrier in case of sealant failure.

Closed cell joint fillers are non-absorbent. If the ambient temperature is very high, some closed cell joint fillers may cause bubbling of the sealant. Puncturing or over-compression may also lead to bubbling of the sealant.

Open cell, sponge type joint fillers have water absorption characteristics.

Kerfs, or holes in the tops of stones must be filled with a high quality compatible sealant.

Sealant application shall be according to manufacturer's recommendation, and prior to the expiration of shelf life of the sealant.     If stone thickness and setting conditions allow, the use of double sealing (back and front) is recommended.

Gaskets are usually extruded or pre-formed for joints where pressure will compress the gasket for efficient water protection.

5 Water leakage may lead to such serious problems as damaging the anchoring system. Due to the effects of freeze-and-thaw cycles, water trapped in anchor slots may crack the stone and cause failure of the anchoring system.

Therefore exterior stone joints must be designed and properly sealed to prevent leakage.

After the support structure and stone slab supporting system is reviewed, the stone joints should be examined.

The taller the building, the more flexural deflection, shrinkage, and creep or thermal movement of the structure may be expected.

When designing joints between the cladding units, it is important to take into consideration the expected dimensional changes in the parts of the building to which the stone cladding is applied.

The larger the stone slabs the more stress is put on the small stone joints by the mechanical and thermal movements of the structure. However, reducing the stone panel size will increase the number of joints. Potentially, more joints also mean the greater possibility of human error and imperfection in installation, as well as more erosion of joint sealants due to exposure to the elements.

6 Applications shall comply with the specifications, with design details, and with the sealant manufacturer's recommendation.

The most common problem during the application is the change in the joint sizes, due to tolerances of stone dimension. Such field conditions may result in undesirable deviations from the jointing design and may lead to leakage and failure. Therefore, before sealant application begins, qualified persons should inspect joint conditions and either remedy improper joints, or re-design the joint treatment.

source: Building Stone Institute

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