Floor Construction: Lewis Plates Review

A guide to using lewis plates with your underfloor heating installation.

floor

A Lewis Plate, or Lewis Dovetailed Sheeting, is rolled, dovetailed, galvanised steel sheeting used for constructing lightweight concrete floors. The floor is usually laid above a timber frame.

Lewis Plates are ideally used in rooms such as bathrooms, utility rooms, kitchens etc, wherever water-resistance is a cause for concern and the preservation of the timber frame more difficult to achieve. Lewis Plates are fire-resistant, sound-proofing, and water-resistant, and therefore are used for constructing flooring in buildings such as apartment blocks, within which numerous dwellings are located, and greater noise pollution and risk of fire must be dealt with. The steel element of the flooring provides high durability, and the floor itself, once complete, is relatively shallow at a minimum depth of around 38mm in a domestic environment. A minimum depth of 50mm is required for any separating/party floor, as specified by building regulations.

Construction of the Floor

The floor is created by firstly nailing or screwing the Lewis Plates to the joists of the timber frame. The underfloor heating (UFH) pipework is then laid into the grooves of the steel sheets and poured over with screed, creating an even high load-bearing floor. The depth of screed necessary for a floor of this type is relatively low because the dovetail sheets provide the reinforcement of the screed once it is set. The amount of screed required depends upon the diameter of the pipes used to supply the heat. It is recommended that a depth of somewhere between 20mm and 25mm of screed is laid over the top of the underfloor heating pipes. Therefore, if a pipe of 14mm diameter were used, then the overall depth from the bottom of the Lewis Plate to the top of the screed should be 50mm; a pipe diameter of 8mm would prescribe a top-to-bottom depth of 44mm. The relative shallowness of the screed layer causes it to take little time to heat to the desired temperature, and using a screed mixture as opposed to a dry sand and cement mix or aluminium plates improves efficiency by achieving a higher thermal mass. Additionally, Lewis Plate flooring can be used within both new builds and renovation projects.

Lewis Plates and Underfloor Heating

Lewis Plates easily accommodate for underfloor heating pipes within the grooves of the design and provide an even, comfortable heat across the floor area. They also provide upper floors with thermal mass, which is difficult to achieve when using other forms of upper-floor heating system.

Lewis Plates and Heat Pumps

Using heat pumps with an underfloor heating system in conjunction with Lewis Plates can achieve water temperatures of 30ºC to 40ºC when used with timber joists and screed. The low temperatures of around 40ºC provided by both ground and air source heat pumps make concrete and screed a good combination because as the low temperature water is pumped through the pipework it causes the thermal mass of the floor to act as a heat store.

Image: commons.wikimedia.org

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