Acceptable limits of decking

Acceptable limits of decking

Introduction to decking limits

The limitations of a decking board and the understanding of these limitations is vital to the success of the installation. The consideration here is to appreciate that there are various limits that sometimes get overlooked.

  1. What are the limitations of the decking board?
  2. What are the limitations of the tradesperson?
  3. Does the warranty cover what has been built?
  4. Caveat Emptor (buyer beware)

What to expect from YOUR decking

Your decking may not be the same as your neighbours or friends. It will have more than likely been designed and built by someone else. There should be one common attribute that binds the two-separate project together and that’s an industry code of practice. There are two ports of call here, one being the manufacturer of the decking. The next is the Association of Professional Landscapers where they promulgate Industry guidelines for the structure. The can both advise on what are the acceptable limits of decking

Your decking should be in line with all of the following.

  1. Fit for purpose
  2. The structure is designed to EC5 (Euro Code 5 for timber construction) by a professional
  3. Installed to be by a professional and in accordance with the manufacturer’s installation instructions.
  4. Covered by an insurance-backed or manufacturer’s warranty.

What the manufacturers say

Most manufacturers sing from their own hymn sheet. They all have their own methods and techniques; all of which have to be followed. They each give reference of how their own brand of decking has to be fitted.

Decking “GOLDEN RULE No1” is to follow the manufacturer’s instructions otherwise your decking will most certainly NOT be covered by their insurance.

Decking “GOLDEN RULE No2” is to understand that if your installer does not follow the manufacturer’s installation instructions THEY become liable for any defects for what they have built.

The ability of your tradesman

The ability of your decking to perform for as long as the manufacturer has said it would is down to the installation. If it is correct then it will last… if is not correct then it probably won’t. The other important element to understand is that is the decking should be finished in a straight line, then it should be exactly like that.

Nobody wants to see a wonky edge or a poorly finished detail. Curves should flow perfectly, especially when one curve runs into the next. This where the tangent of the vertex of one curve meets the next…  and not a wobbly arrangement of several curves, this looks awful. Mitres are another area of your decking that will stand out if not done correctly. Mitres are not always at 45 degrees to one another, they may be a series of different angles. All of which require the given angle to be divided equally to form a perfect mitre.

So, notwithstanding a full van of tools, a portfolio of lovely images and a promise of a great installation one should always zoom in and look closer at the examples of work put before you.

Decking “GOLDEN RULE No3” if it doesn’t look right it probably isn’t. In these instances, you should always challenge the installer. Don’t be fobbed off by “It’s okay love, it’ll straighten out when it settles” (It certainly won’t)

Scribing in with precision

Scribing in is where a decking board needs to be cut in to match a certain profile. This should be done with precision, is a slow process and done right will add tremendously to your installation cost.

There are many instances when a decking platform is not a perfect square or rectangle. You may even have an inlay or pattern within your deck. For one reason or another, your deck board has to be scribed into the adjacent surface. The art of making this detail look good is complex and should be done with precision. If the standard gap between your decking board is 6mm then the scribing in should match this gap exactly. This gap should be consistent throughout the whole pattern, inlay or line you are scribing into.

Decking “GOLDEN RULE No4” your scribed in detail gapping should be consistent at the same gap as between the decking boards.

Acceptable limits of decking should be appreciated by the tradesperson and also the client.

Minimum board dimensions for cutting

In decking platforms, there may be areas where small pieces of decking are cut to fit. If you read the majority of manufacturers’ installation instructions you should note that they cite the minimum dimensions of such small pieces. As a rule, they say that the smallest piece of decking should not be less than to span three joists.

The reasons for not allowing such small pieces are due to a couple of sensible factors. How would these small pieces of decking board be securely fitted without using the appropriate clips or screws? If they were screwed they will more than likely split and if they were glued, is the glue compatible with the decking material? The second main point is that manufacturers have minimum dimensions for cut boards due to the expansion and contraction of the boards. Some boards need a certain amount of fixings to hold the boards in place and without will certainly fail. If the decking structure is built using timber then common sense would suggest that small, thin and tiny details will not last as long as one would expect. Screw fixing timber is always advised and when minute pieces of timber form a pattern they will fail or have a reduced service life.

Why is decking fitted to joists?

Decking is fitted to joists as this is the simplest method of creating a platform. It is this beam and joist structure that has been calculated to perform to a certain load bearing.

Joists are constructed in a standard fashion to allow decking boards to be fitted to allow the following to happen.

  1. Air to flow freely underneath and around the boards
  2. Rain water to run off the boards efficiently
  3. Allows the boards to dry out
  4. Allows the decking boards to expand and contract

Considering the principles of decking frame construction, it should be said that double and triple joisting is not considered a sensible practice. This being that water will become trapped between the joists and cause early rotting. You can avoid this if you use the correct joist tape over the surface to prevent ingress of water or create a 10mm gap between joists.

Other considerations are when a pattern has been designed and it requires a multiple joist arrangement or a sheet of ply or similar. One may be able to construct a platform in this manner to the correctly loadbearing but it is considered bad practice. Also think about where will the water go? There are very few boards that can be installed directly to a flat surface. whilst the flat surface may be protected for the external environment the decking boards can’t. In this situation, the decking boards will be sat in an environment that holds water and doesn’t allow undisrupted airflow around the boards. This will cause decking boards to expand and move in an uncontrolled fashion and cause the decking pattern to fail prematurely.

Decking “GOLDEN RULE No5” Your decking boards have acceptable working limitations, make sure they are followed. The longevity may be compromised if you build outside these limitations.

Is your investment insured?

Your contractor may say it’s warrantied, but is it? Do you have an insurance indemnity certificate? Was your project registered with the manufacturer in that you received an email directly from the manufacturer about what your warranty from them covers…


If you are unsure about the acceptable limits of decking you should contact the manufacturer for written advice or email the Decking Network

Decking Regulations and codes of practice

Further advice on decking regulation and best industry practice can be found here  

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