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Damp-Proof Courses in Stone Work |
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New Build |
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Introduction
A damp-proof course (d.p.c.) in stone work is intended to provide a barrier to the passage of water from the exterior of the building to the interior, or from the ground to the structure, or from one part of the structure to another. The passage of water may be horizontal, upwards, or downwards. The advice given in relevant British Standards should be followed, this note is intended to elaborate some of the points particular to stone work.
For the purposes of this note, 'masonry' means stone, whether or not dressed to shape, laid in mortar, forming a solid wall in the outer leaf of a cavity wall and supporting its own weight at least. 'Cladding' means stone in the form of slabs, with the face dimension many times the thickness, attached to a wall or structural frame by mechanical anchorages, forming an aesthetic veneer and acting as dead weight on the structure.
Type Of D.P.C
There are numerous d.p.c. materials available.
Some granite masonry when laid in 1:1/4:3 (cement/lime/sand) mortar, does not need a separate horizontal d.p.c.; the masonry in naturally resistant to rising damp. However, this is not universally true, particularly with some imported granites. If in doubt, use a d.p.c.
In principle, d.p.c. bricks could be used in stone masonry, but are unlikely to be aesthetically acceptable. Copper should not be used, due to the danger of staining. Lead and slate are technically satisfactory, but need oversize joints which again may not be aesthetically acceptable.
Flexible d.p.c.'s are probably now most common, but the large dead weight of stone masonry must be recognised and the material selected accordingly to avoid it squeezing out. Flexible d.p.c.'s also need careful selection for random rubble masonry, to avoid puncturing. Some flexible d.p.c. materials react adversely with bitumenous or asphaltic compositions or with mastic sealants.
Horizontal D.P.C s Generally
Where a horizontal d.p.c is intended to prevent the upward movement of water due to capillary action. Joints may be lapped without sealing . However, Where water is moving downwards ,the joints in the d.p.c should be sealed .Bitumen d.p.c materials with cold applied roofing felt adhesive, pitch and bitumen polymer materials, with adhesive in accordance with the manufacturer’s instruction , polyethylene, with double-sided adhesive tape and lead and zinc with welts or soldered joints,
Care must be taken to achieve continuity of the d.p.c. at ground level with the damp-proof membrane in the floor.
Where a d.p.c is used in an external wall on a sloping site, it must be more than 150mm but less than 300mm above the ground everywhere.
Slates d.p.c.s should consists of two courses, laid to break joint,each fully bedded in 1:1/4 :3 (cement :lime: sand) mortar. The total thickness of the slate, plus mortar should be kept as small as possible and certainly less than 30mm
Flexible d.p.c.s. should be laid on full even bed flushed up level using the same mortar as in the course below ,with 100mm minimum laps in runs and with full laps at angles .It is not practicable to expect lapped d.p.c.s to be accommodated in the normal bed joint, which must therefore be thickened.
If laid at the same time .the weight of the course above the d.p.c will help to develop good sdhesion between the masonry units, mortar and the d.p.c.
Where there are separate d.p.c.s in each leaf of a cavity wall, the edges should not project in to the cavity. On external faces the horizontal d.p.c preferably should project slightly by a minimum of 5mm,and the edge should never be covered with mortar.
With jointed ,previous ,or timber sills, a d.p.c should be provided underneath ,turned up at the back to prevent moisture coming in contact with inner leaf or finishings ,with the leading edges projecting 5mm from the wall face below sill, otherwise water running off the window may penetrate through the sill to any part of wall or finishes in contact with sill
The joints in a coping or capping are also a source of weakness; a continuous d.p.c underneath will prevent moisture penetrating in to the main portion of the wall .The d.p.c should be bedded in fresh mortar and the coping or capping immediately laid above in fresh mortar in order to maximize the bond. Ideally the d.p.c should project 5mm from the wall face
Horizontal D.P.Cs In Cavity work (And Cladding)
A rigid support must be provided for flexible d.p.c spanning a cavity greater than 25mm nominal width
Where flexible d.p.c.s span across cavity bridges e.g over door and window openings ,ensure that the following apply.
a) Wherever possible they are used in continuous length to form cavity trays ( If a joint cannot be avoided seal it appropriately and provide a rigid support of suitable material at the position at the joint to ensure solid bonding)
b) They are fixed securely across the cavity .stepped up at least 150mm towards the inner leaf and prevented from sagging
c) They are extended beyond the ends of the lintels and other cavity bridgings by at leaf and prevented from sagging .
d) They are provided with stop ends to direct water flow to weepholes provided at about 1m centers
Fixing Through D.P.C.s
With stone work, and particularly cladding, it is almost inevitable that fixings for panels, support angles, copings etc will somewhere have to pass through a d.p.c. Rigid d.p.c. materials should be carefully slit and folded back, before the fixing is installed, and then dressed back over and around the fixing. Flexible d.p.c. materials present a greater problem and the only solution usually is to cut carefully and cleanly the minimum size hole in the sheet to accept the fixing, and then seal the hole around the fixing after installation with a compatible mastic.
British Standards
BS 5390: 1976 "Code of Practice for stone masonry"
BS 5628; Part 3: 1985 "Code of Practice for use of masonry Part 3 Materials and components, design and workmanship"
BS 8298: 1989 "Code of Practice for design and installation of natural stone cladding and fixing"
Other Data Sheets
Mortar and Pointing
Guide to the Tests for Natural Building Stone
Stone in Civil Engineering
Stone Sett Pavings
Considerations to Reduce Staining on Natural Stone Facades.
Laying Marble and Granite Tiles on to an Existing Timber Floor
Sealing joints in natural stone cladding
Information Leaflets
Natural Stone Flooring and Paving
Fixings for Stone Cladding
Care and Maintenance
Sandstone
Limestone
Marble
Granite
Slate
Stone Walling
Stone Cleaning and Surface Repair
Restoration |
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