Liquid Screed
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Preparation Guide

Prior To Commencement

The building should be weather tight prior to commencement. The roof should be covered and all external doors and windows in place.

Alternatively all openings are to be made weather proof with clear polythene. Remove all dust and debris from the floor surface and leave free from contamination.

Insulation

Usually the screed will be installed on top of some form of insulation. This is often Celotex, Kingspan or expanded polystyrene.

Ensure that all high/low points are removed from the sub-base and that insulation boards are laid flat. The boards must be laid so that they are stable when walked on and should be tightly butt jointed with staggered joints.

Where there are service pipes running across the floor, such as gas pipes, the insulation should be grooved on the underside to allow the pipes to sit within the insulation, rather than leaving the boards resting on top of the pipes.

Edge Strip

Once the insulation is installed, any vertical surface coming into contact with the screed, such as walls, stairs or pipes, should have a foam insulation edge strip stuck to it.

This is to allow for any minor expansion once the screed is dry. There are various types of strip, but they are typically 8mm-1omm thick and come in rolls.

External Perimeter Edge Strip

A 25mm foil backed insulation board is recommended on external perimeter walls, especially when used in conjunction with underfloor heating.

This helps to reflect the heat back in the building. Strips are usually cut at 25mm above finished screed height and are held in place by the floor insulation board being butted against the strip, any excess is cut back once the screed has dried.

Tanking Membrane (Essential When Using Liquid Screed)

The tanking membrane consists of reasonably heavy (500 gauge) polythene, which is placed on top of the insulation.

The tanking membrane is not there to act as a damp proof membrane, but rather to stop the screed from escaping when it is poured. Care should be taken to make sure that all thejoints are overlapped by at least 10omm and that all abutments are 100% water tight and fully taped.

The polythene should be taken at least to the height of the edge strip up the face of any walls, pipes etc. and taped fully along the top edge. Ensure that all air pockets and voids are removed.

Lay the polythene membrane ensuring that it is free from punctures and creases and ensure enough slack is given where the floor meets the wall and into the corners to allow the screed to form a sharp right angle.

Exposed edges such as door openings or level changes should be provided with temporary formwork. Please note that when installing liquid screed, it is imperative that the preparation is carried out to the highest standard.

The underfloor heating pipes should always be on top of the tanking membrane, not beneath it. The screws or staples used to hold the pipe or cable down will pierce the polythene, but seal the holes themselves, and do not cause any leaks.

Regardless of which system is used to hold down the pipes, it should be installed so that there are no unfixed lengths of pipe or cable greater than 30omm long. If left unfixed, all under floor heating elements will float to the surface of the screed.

Warm water systems should be filled prior to the application of our screeds to reduce the risk of pipes floating when the screed is poured. When setting out the floor areas consideration must be given to the minimum recommended thicknesses for a liquid screed as listed below.

  • Un-bonded on slip-sheet/polythene laid direct to substrate 30-35mm.
  • Floating on insulation (Domestic) 40-5omm.
  • Floating on insulation (Commercial) 45-5omm.
  • Cover over top of conduits/underfloor heating pipes 3omm.
  • Please make an allowance for tolerances and any isolated high spots in the substrate. These will dictate the minimum thickness of the screed and therefore impact upon the overall depth and material usage.

Setting The Levels

We will work from a place called a datum point. Typically, this will be from the stairs, but can also be from other areas such as a doorway or threshold.

We will then set out leveling gauges called ‘tripods’ and, using a laser, will set them to the correct depth to make sure that the finished screed surface is level throughout the building.

Installing Screed

The screed will then be pumped in and poured to the depths set by the tripods. Once this is done, the tripods are removed.

To make sure that it is smooth, without any tide marks or air bubbles, we will then agitate the screed using a dappling bar. The installation is then complete.

Drying The Screed

The screed should now be left untouched until it is solid, which usually takes 48 hours. During this time, all the windows and doors should be kept shut and nobody should walk on the screed.

Once the screed has become solid, the windows should then be opened to allow evaporation to take place helping the final drying stages.

Sub Floor Preparation

  1. Scrape off any debris or mortar splashes from the sub-floor.
  2. Brush the sub-floor, leaving a flat clean surface, on which to place the insulation

Installing Insulation

3. Place the insulation in 2 layers where service pipes are to be installed. Place the first layer between any service pipes on the sub-floor, ensuring it is laid flat.

4. Fill any voids around the pipes, with dried sand and level off, then place the 2nd layer of insulation over the 1st layer, ensuring it remains flat, with no rocking.

Installing Edge Strip and Plastic Membrane

5. Install the edging strip around all walls, taking care to ensure it is neatly placed into all corners at 90 degrees. Tack the edging strip to the walls using a staple gun as required to hold it in position.

6. Place 500 gauge plastic over the top of the insulation ensuring it is pulled tight with no creases and lap joints at a minimum of 100mm. Tape all joints and then tape the 500-gauge membrane to the plastic skirt of the edging strip.

Pipe Installation

7. Install the underfloor heating pipes, ensuring they are appropiately fixed at every 400mm along the length of the pipes more frequently on the bends.

8. Fill the under floor heating pipes with water and check for any signs of leaking. Perform a pressure test to ensure the pipes hold water under pressure.

Expansion Joints and Final Check

9) Install expansion joints across doorways and where the aspect ratio is greater than 1:6. Cut away notches on the underside of the joints to allow it to sit level over the UFH pipes.

10) Create V notches on the top of the joint, this allows the screed to flow through the joint keeping the same level either
side.

11) Complete a final depth check to ensure there is at least 25mm cover over all pipes, paying particular attention to the
corners of rooms.

12) Brush or vacuum out all debris from the surface of the plastic. Any material left will float once the screed is poured and this will result in blemishes on the surface once the screed has hardened. Inspect the whole tanking system for any tears or untapped joints, make good ready for screed to be poured.

Un-bonded Screeds

Un-bonded screeds can be placed directly on to the membrane being floating or floating with under floor heating.

Bonded Screeds

For bonded screeds the substrate (floor slab / unit) may require mechanical treatment to remove Iaitance and other adhered material (construction spillages). Shot blasting or scabbling would be appropriate. After treatment the substrate surface should be swept or vacuumed to remove any dust.

Prior to installation, a primer/ sealer should be used in accordance with the manufacturer’s instructions. In general, bonded screeds are not recommended.

Control Joints

As liquid screed has minimal drying shrinkage, control joints are not usually needed at less than 4om lengths. In the case of long narrow sections, where the aspect ratio of the floor is greater than about 1:6, a joint orjoints are recommended to reduce the ratio.

Control joints will also be needed between any areas of heated floor and unheated floor. They should also be used 6 doors and similar restrictions in plan dimension. AII structural joints in the sub-base should be carried through the screed or reflected.

Quality Control

Only approved contractors can lay anhydrite liquid screed. Liquid Screeds Ltd will arrive on site and set up the pump, check and setup levels with multiple, evenly placed tripods and agree a datum level with you.

We will then measure the floor to ensure that the materials we have ordered are sufficient to fill the designated areas which are to be screeded. Once the mixer truck arrives with the screed we will commence pumping the liquid screed. The screed is then dappled in two directions to level the surface and then left to dry.

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