Concrete slab or a timber-joisted floor – which is best?

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If you’re planning on building a new home or simply doing an “alterations and additions” renovation to your existing house (such as adding a new first floor to your single-storey home), one of the earliest decisions you need to make is whether your new floor will be a concrete slab or framed with timber joists.   So what’s the difference, and what are the pros and cons of each?

There are many things to consider in such a decision, and some building sites or designs may be pre-disposed to one or the other, before your personal preference comes into play.   Let’s look at a few of the influencing factors:

1. Cost

Generally speaking, a concrete slab is going to cost you more money to build.  Whether it’s a slab-on-ground for your Ground Floor, or a suspended slab for your First Floor, a concrete solution is going to cost more money – in materials, labour, and time.  A slab will require formwork, steel reinforcement, pump hire on the day of the pour, and finishing costs (for example, unless you opt for a polished concrete finish, you’ll still need to build either a solid timber floor or lay tiles or carpet over your slab once it’s poured).

In contrast, a timber-framed floor (which might consist of bearers and joists that sit on brick piers at Ground Floor, or perhaps simple timber joists that span from wall-to-wall at First Floor level) can be easily installed by a carpenter; readily handled on site; cut to length as required; and is also more flexible for future alterations or renovations.  Timber is certainly the more affordable solution if your budget is squeezed.

 2. Performance & Durability

Concrete is clearly a stronger and more robust material.  Compared with a timber-joisted floor, a concrete slab will exhibit less bounce, vibration, and noise-transmission through the floor when subjected to foot traffic – and if you’ve got young kids who jump around upstairs whilst you’re trying to relax downstairs, then this is something you may wish to take into account!   If your floor features stone or ceramic tiles, then we’d also recommend a concrete slab – a timber joisted floor will be more prone to bounce and deflection, which risks cracking the tiles.  Depending on other features of your house, a concrete slab can also offer better performance in terms of thermal mass and insulation.

Timber is a natural product, and it needs to be treated accordingly.  Depending on your design and the features of your house or the site, waterproofing, ventilation, and durability need to be considered.  The majority of timber used for framing in the construction industry these days is softwood – typically radiata pine.   Whilst this is cheap, renewable, and readily available, it’s also entrée, main course, and dessert for termites!   The termite treatments and barriers in use today are vastly improved and more effective than in yesteryear, so this isn’t necessarily cause for alarm, but it’s certainly something you need to take into account if your area is a known termite hotspot.

Mind you, concrete doesn’t get a complete tick here either – the steel reinforcement within the concrete needs to be placed and cast in with appropriate cover for protection.  In aggressive  environments, such as suburbs near the sea or Sydney Harbour, airborne salts & moisture can penetrate the concrete and lead to corrosion of the steelwork.  As the steel rusts and expands, it can cause spalling (breaking off) of the concrete, requiring expensive remediation and repair work.  (This is the defect many people refer to as concrete cancer.  We’ve previously written about concrete cancer here.)  Of course, providing your engineer specifies the correct grade of concrete for your site, and providing the builder places the reinforcement with adequate cover at the time of pouring the concrete, you won’t have a problem.

Finally, whilst it’s not a pleasant thought, some people also give consideration to what happens in a house fire.  Needless to say, a concrete slab is less combustible than a timber floor.

3. Site-specific considerations

A timber-joisted floor at Ground Floor level needs to have adequate ventilation and crawl-space underneath the joists.  This is fine if your new Ground Floor level is sitting up above the level of the natural ground.  However, if your design involves sinking the floor into the ground (such as what typically might occur when a house is benched into a sloping site), then you’ll need to allow for the extra excavation costs.  Furthermore, if your site or existing house is directly on rock, then some of the savings afforded with a timber floor may be lost in the extra cost of having to excavate out the rock to create the necessary ventilation and crawl space underneath the joists.

As we’ve already explored, some sites may also lend themselves to a specific material.  For example, timber-framing may be an issue in an area with high termite activity, and the use of concrete might be an issue if you’re in an extremely aggressive environment near the breaking surf.

The cost of a concrete slab (particularly a raft slab-on-ground) can also increase dramatically if your site is known to be underlain by highly-reactive clays, as the size and depth of the footings required to resist foundation movement increases dramatically.

4. Construction considerations

One of the beauties of timber is that you can frame your entire house in it – the floors, the walls, and the roof.  You can utilise timber studs for your walls (we’d always recommend appropriate insulation!) and it becomes a very effective, efficient, and economical way to build.

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Suspended concrete slabs, on the other hand, need strong walls to hold them up!  Adding a First Floor concrete slab to your existing single-storey house won’t be possible if your downstairs walls are all timber stud!  You’ll need brick walls to hold up the concrete, which is another reason why concrete slabs cost more to build – it’s not just the cost of the slab, but the cost of more expensive masonry walls to support them.

Another factor to consider is services such as plumbing and air-conditioning.  Plumbing pipes and/or air-conditioning ducts can often typically run in the same plane as the floor joists, which means you can minimise your floor depth zones.  You can then fix your gyprock ceiling directly to the underside of the joists.   In contrast to this, a concrete slab would need to have all the air-conditioning ductwork suspended underneath the slab, and then a false ceiling suspended under that.  The end result might be a significantly deeper floor zone, which will impinge on the ceiling height in the rooms below.

Another factor that might drive your decision will be what other features you’re adding to your house.  For example, hydronic heating is becoming increasingly popular, and this almost always drives the decision to opt for a concrete slab, so that the water heating pipes can be cast in with the concrete.

point-piper-1024x768 The purple pipes visible in this photo are the hydronic pipes, tied to the underside of the reinforcing mesh.[/caption]

5. Design considerations

Your architectural layout or floor design may also influence the decision as to which material to build out of.   Generally speaking, and depending on the spans involved, concrete slabs will be thinner than their timber-joisted counterparts, which means you can maximise your ceiling heights.  As an example, if a concrete slab at First Floor level had to span five metres over your Ground Floor lounge room, you might be able to achieve this with a 200mm thick slab.   To achieve the same span in timber might require joists of between 240-300mm thick, resulting in a thicker floor zone.

Concrete slabs can also work with rounded or curved floor shapes, which timber joists would struggle with!  Concrete can also easily accommodate setdowns and folds in the floor (as might occur for bathroom wet areas or split-level floors).  Timber construction can also be made to achieve these features, but occasionally with a bit more framing, baggage, and additional structure.

6. Environmental concerns

In an era when we’re all more mindful of our impact on the environment, many people give consideration to issues such as sustainability and our carbon footprint when building a home.   Given that supplying a timber-joisted floor requires us to cut down trees, it may seem counter-intuitive, but – generally speaking – timber-framing has far less impact on the environment than a concrete slab, and certainly carries a smaller carbon footprint.  Most timber used for framing in the construction industry today is renewable, plantation timber grown specifically for this purpose.  Concrete, as we’ve seen already, requires formwork to build (which is timber sheets, planks, and beams), followed by reinforcing steel (with all the industrial processes required to produce it), and then finally the materials and processes required to produce concrete – sand, cement, water, and admixtures.

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So what’s the conclusion?   Ultimately, it’s up to you and what you want from your house.  And also what you can afford.  The construction industry is definitely one field where you get what you pay for.   For us, as engineers, we love both materials and we love detailing them to meet the designs that architects present us with.  If you’ve any doubts or further questions about the merits of one over the other, we’d be delighted to sit down with you and your architect and talk it all through at design stage.

Cheers,

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COMMENTS

Just say Your opinion.

  • Amanda Drew

    3 years ago

    That’s really interesting that concrete slabs tend to be thinner than timber-joint ones. I want to put in a patio out of some slabs of something. I think that I’d like to use the concrete ones, so I’ll just need to find somewhere to get one.

    Reply
  • sidah

    2 years ago

    Great information thanks Andrew.

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