Applying Overcoat to a Concrete Roof
This application guide outlines the correct method for installing the Overcoat liquid STP roofing system on concrete flat roofs. It provides practical instruction on preparation, priming, application technique and curing.
Overview: Concrete Roofs and Common Failure Issues
Concrete roofs are frequently used in commercial, industrial, residential, and public buildings due to their strength, load-bearing capacity, and fire resistance. They are typically constructed using in-situ reinforced concrete, precast concrete components, or concrete decks finished with screeds or toppings to create drainage slopes.
Although concrete is a durable structural material, it is not naturally waterproof. Without proper protection, water can seep through the surface, joints, and microcracks, causing gradual deterioration of both the roof covering and the underlying structure.
Carbonation and reinforcement corrosion
Extended exposure to moisture can cause carbonation of concrete, which decreases its alkalinity and increases the risk of corrosion of embedded steel reinforcement. Once corrosion starts, the steel's expansion can lead to further cracking and surface spalling.
Failed or unidentified surface coatings
Many concrete roofs have previously been coated with substances that are no longer effective or whose ingredients are unknown. Applying new coatings to incompatible materials without proper testing can lead to adhesion failure and premature deterioration.
Water ingress and leakage
Concrete is naturally porous, and over time, moisture can seep through the surface, especially in areas where coatings have failed, cracks have developed, or construction joints are present.
Cracking and surface movement
Thermal movement, shrinkage, and structural loading can all cause cracking. Even fine hairline cracks can let in water if not properly sealed, while wider cracks may worsen if untreated.
Ponding water
Inadequate falls, settlement, or drainage problems can cause standing water, which puts extra pressure on surface coatings and accelerates deterioration when roofs are not built to withstand it.
Overcoat is designed to create a seamless, fully bonded waterproof membrane that allows for normal movement, seals cracks, and shields the concrete substrate from ongoing moisture ingress. Accurate assessment, preparation, and installation are crucial for ensuring long-term effectiveness.
System Overview: Materials Needed
The Overcoat system for concrete roofs includes the Base Primer and Overcoat Resin, which together create a fully bonded waterproofing layer. Optional products, such as RoofRinse fungicidal wash and Sharkseal detailing sealant, can be added if required.
A dedicated Overcoat kit specifically for concrete roofs is available, simply enter the approximate size of the roof in metres and you can order all the materials you need.
Core System Components
Overcoat Resin System
The hybrid STP roofing system comes in two sizes and is available in over 20 different colours.
Overcoat Base Primer
A moisture-tolerant two-part epoxy primer to prepare the concrete surface for resin application.
Tools & Accessories
Application rollers, detailing brushes and buckets are the only tools needed.
Optional Components
Sharksealer
A sealant for repairing any cracks in the concrete.
Roofrinse
A cleaning solution for preparing roof surfaces.
Overcoat Concrete Roofing Kits
Get everything you need for your concrete roof with our easy to order kits. Simply enter the size of your roof below and follow the link to the kits page.
Substrate Assessment and Pre-Works Inspection
Before starting work, it is best to assess the concrete roof surface to determine whether it's suitable for coating. This assessment should consider surface integrity, existing coatings, moisture condition, drainage performance, and any defects that may affect adhesion or long-term performance.
When the condition of the substrate, moisture content, or compatibility of existing coatings cannot be reliably assessed, a professional roofing survey may be necessary. Allbase can organise roofing surveys to evaluate roof condition and offer clear recommendations to assist with system selection and preparation requirements.
Where the existing surface composition or coating history cannot be identified with certainty, adhesion testing must be carried out before full installation.
Adhesion Testing Procedure (Optional)
Adhesion testing is required on concrete roofs where the surface composition or coating type is unknown. Test areas should measure at least 250 mm x 250 mm and be located in representative areas of the roof. Each test area should be photographed before application.
Use the adhesion test kit to prepare the test area in accordance with the surface preparation guidelines. Make sure to allow at least 7 days for curing before you conduct the test.
Make a cross cut through the coating to the substrate, press adhesive tape firmly, and then remove it sharply. The coating must stay fully adhered for the test to be considered successful, and it's recommended to take photos or videos to document the test.
Surface Preparation
Ensuring the roof surface is properly prepared is vital for the Overcoat system to work effectively. Gently remove any loose debris, crumbly concrete, surface laitance, and coatings that may have been applied.
The roof surface, including gutters and drainage channels, should be thoroughly cleaned to remove dirt, grease, biological growth, and general contamination. Cleaning may be carried out using RoofRinse fungicidal wash to help break down organic growth before power washing.
Once you've finished cleaning, allow the roof plenty of time to dry. If you notice cracks, splits, or minor defects, you can repair them with Sharkseal, ensuring all repairs are fully cured before applying the Base primer. The prepared surface must be clean, dry, sound, and free from any contaminants.
Temperature and Weather
Before starting any Overcoat roofing project, it is crucial to check the weather conditions, as they directly affect the success of the application. Check the weather forecast and try and schedule your roof repair around a few days of good weather. Overcoat can be applied in cold temperature The weather should be dry, with temperatures ranging between 5°C and 35°C. Rain, frost, or excessive moisture can compromise the bond between the liquid membrane and the roof surface.
Primer Application
Once the surface preparation is complete and the roof is dry, it's time to apply the Overcoat Base Primer. The primer comes in two parts; Part B needs to be added to Part A and mixed thoroughly to activate. Apply to the roof at between 130g and 200g per square metre, depending on the surface texture and allow 12 hours for a full cure.
It's important to note that once you have applied the primer, you must use the Overcoat resin system on that surface within 5 days, or you will need to reapply the primer.
Primer Curing times
Curing times are based on 15°C; this will vary due to temperature and on-site conditions.
Touch Dry
Overcoatable from
Full Cure
Application
This section outlines the practical steps for applying Overcoat to a concrete roof, from detailing work to coating the main roof area.
Detailing
To ensure the best results, it's important to apply Overcoat carefully and systematically, aiming for the right coverage and thickness. It's a good idea to start with the more complex and sensitive details, then move on to the main roof area.
Concrete roofs often include parapet upstands, vertical returns, drainage outlets, service penetrations, construction joints, and changes in plane and require more careful attention than open surfaces. Use the chemical-resistant brushes to penetrate corners, edges, joints, and surface irregularities, paying particular attention to repaired cracks and where horizontal and vertical surfaces meet.
Resin Application & Coverage
Once the detailing is finished, move on to coating the main roof area. Overcoat should be applied at around 1.2 kg per square metre, so planning material quantities in advance helps ensure an even coating across the roof and avoids thin or over-applied areas.
While this will never be 100% accurate due to surface texture, you can use it as a rough guide for measuring out the materials. A 7 kg container of Overcoat will cover approximately 6 m² when applied at the correct rate, so for larger roof areas, it is recommended to mark the surface into 6 m² sections using chalk lines or temporary markers. The full contents of one container should then be applied evenly across each marked section, ensuring complete and consistent coverage.
Where smaller or irregular areas are being coated, coverage should be controlled by weight. The roof surface should be marked into 1 m² sections, and 1.2 kg of Overcoat measured out per square metre using calibrated scales. Each measured quantity should be applied evenly to its designated area, maintaining consistent thickness and avoiding over-extension of the material.
Second Layer
If a second coat is required then this needs to be applied at least 24 hours after the first layer. Application is the same as the first layer by roller or brush at a coverage rate of 1.2 kg / per m sq, to the full roof area.
Resin Curing times
Curing times are based on 15°C; this will vary due to temperature and on-site conditions.
Touch Dry
Overcoatable from
Full Cure
Curing, Access, and Protection
Curing times for Overcoat vary depending on temperature and weather. Under typical conditions, light foot traffic can usually resume after about 24 hours, although it may take longer in cooler weather. Complete curing generally requires at least 7 days, during which the coating gains strength and durability.
Frequently Asked Questions
This section addresses common questions that arise during the planning and application of the Overcoat system on concrete roofs. The guidance below is intended to clarify practical considerations that may not be fully covered in the main application steps.
Concrete is porous and can vary significantly in surface condition, and the Base Primer is used to seal the surface and promote adhesion between the concrete and the Overcoat membrane. Applying Overcoat without priming can result in uneven absorption and reduced bonding.
Coverage should be controlled by measuring material rather than judging by appearance. Overcoat is typically applied at 1.2 kg per square metre, with a 7 kg tin covering approximately 6 m². Marking out roof areas and working to known quantities helps maintain consistent thickness.
Overcoat can be applied to existing coatings on concrete roofs if they have been cleaned and are well-adhered. If the coating type cannot be identified or its condition is uncertain, it should be flagged at the survey stage, and adhesion testing should be carried out before any work begins.
Overcoat is an STP-based system that can be applied to damp surfaces, provided there is no standing water. The concrete should be surface dry, with any water removed before priming and coating.
Although the system tolerates residual moisture, excessive moisture within the substrate or active water ingress can compromise adhesion and curing, especially in cooler conditions. When moisture levels are uncertain, additional drying time or further assessment may be necessary before application.
Overcoat usually does not need reinforcement across the entire roof area on concrete substrates. However, localised reinforcement can be added at high-risk details, joints, or repaired areas where extra strength or crack-bridging is necessary.
Heavy equipment, plant, or follow-on works should not be placed on the roof until the coating has fully cured, which can take more than 7 days. While lighter foot traffic may be possible earlier, the surface should not be loaded until fully cured.
The Overcoat system can accommodate normal substrate movement and bridge minor surface cracking when applied at the correct coverage rate. Structural cracks or areas subject to ongoing movement should be assessed and addressed before waterproofing is installed.
Yes. Rough or heavily textured concrete surfaces may require more careful working of the material and can increase material usage. Proper priming and controlled application help ensure consistent coverage and reduce the risk of pinholing.
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