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Floor Structure Guide

ABOVE: Screedflo dB is a masonry screed for intermediate floors that is applied onto a timber suspended floor (screedflo.co.uk)

Floors require a little thought — and a lot of pre-planning. The first major consideration is what type of floor you will build. There are basically three choices: solid concrete (ground floors only); precast suspended concrete; or timber, which is always suspended, i.e. its joists set into walls. On ground floors, it’s now very unusual to use timber joists (though it’s common in Scotland): precast suspended concrete beams have more or less superseded timber joists, as they are quick to install and give a good working platform. They are infilled with either standard building blocks or, increasingly, with insulated forms like Tarmac’s Heatsave Plus and Hanson’s Jetfloor.

With intermediate floors, the decision is a little more complex. Traditionally, intermed - iate floors were built off timber joists – solid floors were never an option – but the introduction of precast concrete beams in the 1970s gave the homebuilder a choice. A large number opted for concrete floors, even though they were more expensive and required a crane to lift into place. Concrete beam and block floors offered a feeling of solidity and, crucially, improved soundproofing. However, timber floors can be improved to make them perform as well as concrete ones — at an extra cost. There are now products on the market which enable you to pour a masonry screed onto a timber suspended floor, such as Screedflo dB, which describes itself as an ‘acoustic floor system’.

The cost implications are thus: you can have a standard timber suspended floor for around £30/m², or you can have an improved floor for something like £50- 60/m². The improved floor can be either precast concrete or timber based.

The Options

Solid Concrete: 1. Concrete block/ innerwall; 2. Outside wall; 3. Hardcore infill; 4. Damp-proof membrane; 5. Sand blinding; 6. Min 100mm concrete

Suspended Concrete: 1. Concrete block; 2. Outside wall; 3. Concrete block/ inner wall; 4. Sleeper wall; 5. Concrete beam

Suspended timber flooring diagram

Suspended timber: 1. Metal joist hanger; 2. Herringbone strut; 3. Outside wall; 4. Concrete block inner wall; 5. Concrete oversite; 6. Wooden joist

Underfloor Heating

Most self-builders now fit underfloor heating into their homes. Just how well this works depends, to a great extent, on the floor construction. Concrete floors are ‘high mass’ and will act like a giant storage heater, staying warm for many hours without any heat source. In contrast, a timber-based floor is relatively ‘low mass’: underfloor heating in a timber floor therefore tends to heat up (and cool down) fairly rapidly, more like a conventional radiator system. So, to benefit from the slow, passive heating of concrete floors, you have to actually be in occupation, preferably 24/7; if you are there for just a few hours a day, you’d be better to opt for a quick-response heating system.

Buildability

Consider how you plan to run cables and pipes through your floor void. With a precast floor, you have little option but to hang a suspended ceiling below the floor. Timber joists can be drilled out or chipped away (with caution) but it is not straightforward. Step forward the timber I-joist and its cousin, the posi-joist. These are timber equivalents to the precast concrete beam, and consist of thin sections of timber strengthened by a webbing of either OSB (I-joists) or metal straps (posi joists). They are more expensive than traditional timber joists, but they are much faster to fix and, critically, they are far easier to run services through.

Floor Levels

The decision of what floor you will build needs to be made early on in the design process. This is because there is one crucial part of the design – the finished floor levels – that has to be resolved as early as possible, because so many other factors are derived from it, i.e, the staircases are set by it; so too the room height and the door thresholds.

Insulation

It is now a requirement of the Building Regulations to insulate ground floors. The standard method is to lay sheets of polystyrene or similar above the ground floor and then lay a screed or a timber floor over. But some systems incorporate insulation within the floor itself, such as Tarmac’s Heatsave Plus (tarmac.co.uk).