There's a variety of subfloors you're likely to encounter when laying carpet.

We discussed the main subfloor types and typical preparations in the following two units:

• Inspecting and testing subfloors
  
  
• Subfloor coatings and toppings.

So in this lesson we'll summarise these issues by putting them into a set of questions that you should ask yourself before you begin the job.

Remember, as the flooring installer it's your responsibility to decide whether the subfloor is suitable and has been adequately prepared.

If you're worried that the substrate isn't suitable, or that there is an underlying problem that might cause trouble later on, don't ignore it.

Check with your supervisor or manager before going ahead.

Everyone will be thankful in the long run, even if it means that there'll be a delay while the problem is fixed.

Questions to ask yourself

General issues

• Is the substrate smooth and flat?
  
  AS/NZS 2455 has minimum standards for 'planeness' and smoothness'.
  
  The planeness standard says that when a 3 metre long straight edge is placed on the surface at any position, no part of the surface is allowed to be more than 5 mm below the straight edge.
  
  The smoothness standard says that when a 150 mm long straightedge is placed on the surface at any position, no part of the surface is allowed to be more than 1 mm below the straightedge.
  
  Indentations, ridges and dents can sometimes show through to the surface of carpet.
  
  Ridges also tend to create wear points in areas where there is regular foot traffic or movement of furniture.

• Is the surface free from dirt, oil, adhesive residues and all other contaminants?
  
  Dust and other substances on the surface will interfere with the strength of adhesive bonds in stick-down installations.
  
  Some contaminants will also degrade the carpet over time.

Concrete subfloors

• Are the relative humidity (RH) and alkalinity (pH) levels within the allowable limits?
  
  Excessive moisture in the concrete subfloor can cause the carpet to change shape over time, and also allow fungal spores to develop.
  
  There are limits set for RH and pH, both in the Australian Standards and in the flooring manufacturers' own installation instructions.
  
  The only time you should over-ride these specifications is when an approved moisture barrier is being installed.

• Is the substrate sound and free from loose, powdery or scaly material?
  
  If the substrate is concrete and you are planning to use adhesives, the surface must be sound and sufficiently porous to allow the adhesive to bond properly.
  
  If you're using nails to secure grippers, the substrate needs to be able to hold the nails without crumbling or cracking.
  
  If it's not in good condition, the affected layer will need to be removed and resurfaced.

• Are there expansion joints in the floor?
  
  Expansion joints need to be in good condition and free from dirt or obstructions.
  
  You'll need to keep them clear and finish them off with approved cover strips.
  
  Don't get mixed up with relief cuts that have been put into the concrete to stop it from cracking during the curing process.
  
  These will be a 5 mm wide saw cut, and can be filled with a suitable compound.

• Have heating elements been installed in the floor?
  
  Flooring manufacturers provide recommended limits for the temperature of the subfloor.
  
  Make sure the heating elements will not exceed this temperature, and follow the specific instructions relating to substrate preparation for heated floors.

Wooden subfloors

• Is the existing floor properly supported and well secured?
  
  Any structural problems or loose boards should be fixed before the installation begins, especially squeaky floorboards or springiness in the floor surface.

• Are there gaps between boards, protruding nail heads or other surface defects?
  
  Gaps, ridges, cupped boards, protruding nails, and other defects will 'telegraph' through to the surface of the floor covering.
  
  In general, structural floors made from plywood and strip flooring need to have a hard underlay placed on top to provide a flat smooth surface.

• Is the subfloor ventilation adequate and in compliance with the relevant standards?
  
  Check that the air vents provide sufficient ventilation and that the subfloor cavity meets the minimum requirements for clearance between the floor and the ground.

• Has the moisture content (MC) been checked and is it within the allowable limits?
  
  The moisture content of structural members and floor boards or sheets must all be within the allowable MC range.
  
  There must also be no evidence of plumbing or stormwater leaks that might have a long-term effect on the MC.

Old resilient floor coverings

It is permissible to lay carpet over the top of old resilient coverings, as long as the original surface is in good condition and free from defects such as cracks and bumps.

It is best to avoid using adhesives over an old resilient floor.

One reason is that you would be depending on the old adhesive under the resilient covering to remain sound and not separate from the subfloor.

Another reason is that the new adhesive may not stick well to the top of the resilient covering, especially if it has old polish or ingrained contaminants on the surface.