Damp Proof Courses

Damp-proof courses (DPCs) are critical components in construction, preventing dampness from rising through walls and compromising the integrity of buildings. This comprehensive guide will delve into the various aspects of DPCs, including types, signs indicating the need for one, and the ramifications of neglecting damp proofing.

Table of Contents

1. What is a Damp Proof Course?
2. Signs You Need a Damp-Proof Course and Area Commonly Affected
3. Consequences of Neglecting Damp Proofing
4. Damp Proof Course Regulations
5. Cost Considerations for DPCs
6. Damp Course in your projects

What is a Damp Proof Course?

Most of us are familiar with damp courses and immediately think of the DPC in both masonry skins of a cavity wall. Usually, the DPC edge can be seen within the mortar bed, approximately 150mm or more above ground level. Such DPCs prevent dampness from rising from the ground and entering parts of the building that should be kept dry.

Types of Damp Proof Courses

There are many types of DPC, in addition to traditional strip DPC, supplied in rolls. Such as:

• Damp Proof Course (DPC) Injection: This method involves injecting a chemical damp-proofing solution into the walls to create a barrier against rising dampness. It is commonly used in existing structures where traditional DPCs may be absent or ineffective.
• Electro Osmotic Damp Proofing Course: Electro-osmotic damp proofing uses an electrical charge to repel moisture and prevent its movement through walls. This technique is typically applied in situations where conventional DPCs are impractical or insufficient.
• Physical damp-proof courses like engineering bricks: Physical damp proof courses involve using impermeable materials, such as engineering bricks or slate, installed horizontally within the masonry to block the upward movement of moisture. These materials provide a physical barrier against dampness.
• DPC Trays: DPC trays are specialised components designed to redirect moisture away from vulnerable areas in building structures, such as window and door openings, roof intersections, and cavity walls. They are typically made of durable materials like plastic or metal and are installed horizontally or vertically to channel water safely out of the building envelope.

Signs you need a Damp-Proof Course and area commonly affected

If your building shows signs of dampness inside, indications include black spot mould, watermarks, peeling plaster, and deteriorating wood. The locality should be investigated.

Common problem areas include:

• Internal walls around window and door openings,
• Internal walls of rooms that are below a sloping roof that abuts another external wall,
• Areas above and around air bricks,
• External wall fans or metre boxes,
• Dampness in internal walls below parapets,
• In and around balconies,
• Wherever an extension has been added
• Dampness to bay window ceilings and adjoining areas.

Remember, a cavity wall has two masonry skins, and between the two is a gap – the cavity. The external skin will saturate with prolonged rain, and the cavity stops that wetness from reaching the inner skin. Every time you have a window or door in the outside wall, you create a bridge between both skins. Cavity trays are designed to protect across the opening, so gravitating dampness is arrested by the tray and water is safely redirected out of the wall via Caviweeps

Another critical area is when a typical single-storey extension is added to an existing cavity wall building. Above a flat-roof extension, there is an external wall of the original structure. However, once the extension is added, that often-saturated external wall skin becomes an inside wall of the extension. Accordingly, the wet has to stop its downward journey. This is achieved by inserting a row of horizontal cavity trays just above the extension roof level. Similarly, if the extension has a sloping roof, there are specific trays for sloping abutments.

Consequences of neglecting Damp Proofing

Neglecting damp proofing can have significant consequences for a building. It may lead to the degradation of plaster and decorative finishes, compromising the property’s aesthetic appeal. Moreover, the presence of moisture can accelerate timber decay, increasing the risks of wet rot and dry rot, which can weaken structural integrity over time. These structural issues pose safety concerns and have implications for the property’s market value. Therefore, prioritising effective damp-proofing measures is essential to safeguard the structural integrity and value of the building.

Damp Proof Course Regulations

Whilst there are various directives and rules regarding what extent a DPC cavity tray should rise within the cavity or extend beyond an opening width, the Building Regulations’ all-embracing requirement is that the built-in protection should prevent dampness from reaching those parts of the building that should be kept dry.

Since 1st January 2024, Properties being built in the UK under the NHBC warranty have been subject to a new NHBC directive stating preformed cavity trays should be used for all complicated junctions, build details, etc., in short – a move to eliminate on-site cutting, lapping and sticking and instead use preformed (that are correctly shaped every time) cavity tray components. In other words, to raise the standard of built-in wall protection by eliminating the variances of on-site labour fabricating.

Cost considerations for DPCs

•  The cost of using preformed cavity trays is usually similar or better (lower) than that of alternative methods. Additionally, the standard of build detail is raised, and site errors are eliminated. Wastage is similarly eliminated, and the builder benefits from accurate stock cost control.
• When a building is planned, the correct cavity tray specification will eliminate damp issues.
• Where a building exists that is problematic because it did not incorporate cavity trays when built, a surveyor can certainly be consulted. Cavity Trays Ltd, the longest-established specialist in its field, offers the service of visiting and giving an opinion. The service is offered without charge and is regularly used by architects and surveyors seeking a specialist’s viewpoint.

Damp Course in your projects

Properties without appropriate damp protection or with discontinuous or damaged courses may be affected by wet penetration. The use of approved DPC cavity trays built into cavity walls controls and manages the levels of wet penetration of the external leaf and ensures the internal leaf (and thus the building interior) is kept dry.

Learn More

See the full range offered by Cavity Trays Ltd. For more see the The Book of Wise Decisions and our Technical Manual Protecting the Building Envelope containing technical data and helpful guidance. Both are available in hardcopy form, without charge upon request. Alternatively, visit our resources page for more.

Remember, whilst standard products are suitable for most applications, some projects require bespoke solutions. Cavity Trays is able to manufacture custom ventilation solutions tailored to specific project needs. Unique architectural designs, historic building renovations, or new builds with unconventional roof structures. Contact Cavity Trays Ltd

Get in touch with Cavity Trays today

Call our team on 01935 474769 or email [email protected]