Screed drying! When can I tile ?

So, you've had a new concrete screed laid, or perhaps you're planning one. The surface looks smooth, level, and ready for that beautiful new flooring you've picked out. But hold your horses! One of the most critical, yet often underestimated, stages is allowing the screed to dry correctly. Covering it too soon can lead to a host of problems down the line.

But how long does it really take? It's one of the most common questions in flooring and construction, and the answer isn't always straightforward. Let's break it down.

What Exactly is Screed?

First, a quick reminder: screed isn't usually the structural concrete base of your floor. It's typically a thinner layer (often sand and cement, or a calcium sulphate-based mix) applied over the concrete subfloor (or insulation boards, particularly with underfloor heating). Its job is to provide a smooth, level surface perfect for receiving the final floor finish – tiles, vinyl, wood, carpet, etc.

Why is Correct Drying So Important?

Concrete screed contains a significant amount of water used during mixing. This water needs to evaporate naturally. If you lay a floor covering (especially impermeable ones like vinyl or resin, or moisture-sensitive ones like wood) before the screed is sufficiently dry, you're essentially trapping that residual moisture. This can lead to:

* Adhesive Failure: The trapped moisture can break down flooring adhesives, causing tiles to lift, vinyl to bubble, or wood to detach.

* Damage to Flooring: Excess moisture can cause wood flooring to warp, cup, or swell. It can encourage mould growth under carpets or vinyl.

* Mould and Mildew: Trapped dampness creates the perfect breeding ground for mould and mildew, potentially affecting indoor air quality and causing health issues.

* Screed Weakness: While screed gains initial strength quickly, the full curing process involves hydration, which is linked to the drying process. Rushing can potentially affect long-term durability in some scenarios.

* Costly Remediation: Fixing these problems often means removing the new flooring, potentially treating the screed, and starting again – a costly and disruptive process.

Factors Influencing Drying Time

There's no single magic number for drying time because several factors come into play:

* Screed Thickness: This is the biggest factor. The deeper the screed, the longer it takes for moisture to migrate to the surface and evaporate.

* Type of Screed:

* Traditional Sand/Cement Screed: The most common type. Drying times are generally the longest.

* Calcium Sulphate Screeds (Anhydrite/Gypsum-based): Often dry faster than traditional screeds initially, but still require testing to confirm dryness. They are also more sensitive to moisture after drying.

* Proprietary Fast-Drying Screeds: Specially formulated mixes designed to dry much quicker, sometimes in days rather than weeks/months. Always follow the manufacturer's specific instructions.

* Environmental Conditions:

* Temperature: Warmer conditions generally speed up evaporation.

* Humidity: High relative humidity in the air slows down drying as the air can't absorb much more moisture.

* Ventilation: Good airflow across the screed surface carries away evaporated moisture, speeding up the process. Poor ventilation traps moist air, slowing things down. Our typical UK climate often means ambient drying can be slow.

* Subfloor Conditions: Is the underlying base dry? Is there an effective Damp Proof Membrane (DPM) preventing moisture rising from below?

The "Rule of Thumb" (and Why It's Only a Guide)

A commonly quoted rule of thumb for traditional sand/cement screed is:

* 1 day of drying for every 1mm of thickness up to 40mm.

* 2 days of drying for every 1mm of thickness above 40mm.

Example: A 65mm deep traditional screed would theoretically need:

(40mm x 1 day/mm) + (25mm x 2 days/mm) = 40 days + 50 days = 90 days

BUT BE WARNED! This is a very rough guideline assuming ideal drying conditions (e.g., around 20°C and 50% Relative Humidity with good airflow). In a cool, damp UK site with poor ventilation, drying times can be significantly longer. Never rely solely on this calculation.

How Do You Know When It's Dry Enough?

Visual inspection isn't reliable. The surface might look and feel dry, but significant moisture can still be lurking deeper within. The only way to be sure is through moisture testing. The most common method used by professionals in the UK before laying floor coverings is:

* Hygrometer Test (Relative Humidity - RH Test): This involves placing an insulated, sealed box (a hygrometer hood) or taping down an impermeable plastic sheet onto the screed surface for a set period (often 24-72 hours). A calibrated hygrometer (digital or analogue) inside measures the Relative Humidity of the trapped air pocket, which reflects the moisture level within the screed near the surface.

* Target Levels: Most floor coverings and adhesives require the screed to have an RH reading of 75% or less. However, some materials (like wood flooring) may require lower levels (e.g., 65% RH). Always check the specific requirements of your chosen flooring and adhesive manufacturer.

Other methods like the Carbide Bomb (CM) test exist but are less common for routine pre-flooring checks in domestic settings.

Can You Speed Up Drying?

While patience is key, you can help create better drying conditions:

* Ventilation: Ensure good, consistent airflow across the screed surface. Open windows on opposite sides of the room for cross-ventilation (ensure security).

* Heating: Gently heating the space (aiming for around 15-20°C) can help, but avoid aggressive localised heat (like fan heaters blasting directly onto the surface) which can cause cracking or curling.

* Dehumidifiers: Using industrial dehumidifiers can effectively remove moisture from the air, encouraging more evaporation from the screed. This is often the most controlled way to speed things up.

* Underfloor Heating (UFH): If UFH is embedded, it can be used to assist drying after the initial curing period (usually 7-10 days). This must be done carefully by commissioning the UFH – gradually increasing the water temperature over several days/weeks according to a set schedule (often provided by the screed or UFH manufacturer). Rushing this can damage both the screed and the UFH system.

The Bottom Line

Drying concrete screed takes time, and there are no real shortcuts if you want a long-lasting, trouble-free floor. While the 1mm/day rule is a starting point, environmental conditions heavily influence the actual time.

Always, always test the screed's moisture content using a reliable method like an RH hygrometer test before laying your final floor covering. Check the requirements of your flooring and adhesive, and if in doubt, consult your flooring supplier or a professional installer. Rushing this stage is a false economy you might regret later!