Understanding a decking specification… a contractors point of view

Timber decking, a specification from another world…

“Now, I want a decking board that is durable and very hard, perhaps Ipe. I would like it very light in colour, in fact the colour must homogenous across all boards. It should be 25 mm thick; the width needs to be 225 mm as it will then match the interior flooring. I require the terrace to be completed in 5.1m single length pieces and the project is for 150 square metres. If we can have a side fixed system as I don’t want to see any fixings that would be great”.

To a lot of companies installing decking this sounds all too familiar, so what can we advise?

The timber in short.

Ipe, is a great choice, Lapacho, Tajibo, it has many names but Ipe is a good standard in the UK. The standard size for the majority of installations is 21 mm thickness by 140 to 145 mm or so, this may vary by a few millimetres either way. The Tabebuia Serratifolia has 6 sub species with texture, grain, colour and density that will be similar and depending how close you look one may argue that they are all completely different too. The colour will be from a dark yellow merging into green in places with hues of brown flowing through grey and into vibrant pinks and reds.

Lengths will vary from 1200mm to 5100mm maybe more but only for the longer lengths and a hand of boards for every 3 cubic metres purchased. Less and less suppliers will supply at all of the same length unless you pay a premium.

Decking board width

Ipe has various standard sizes and depending which supplier has been specified it is usually best to stay with the parameters of what is already available. The board widths are historically there for reason. Most countries supplying the raw materials will do in Imperial sizes, 4”x1”, 6”x1. This would produce 90 x 21 and 140 x 21 and to deviate from this would require further machining at a costly premium. It is feasible to produce 225mm x 56mm thickness boards but they would have an enormous cost and availability would be strictly limited.

The other reason for these sizes is because this is what works as decking, boards wider that 145 or so will suffer from excessive movement and cupping, they will also require tighter joist spacing and 3 screws per joist…

Deck thickness

This is determined again by the raw materials but is generally in proportion to the board width. If we stray much thinner than 20mm the boards will not have the same structural support and would require an enhanced substructure. If we went much thicker we would be subject to increased delivery costs, bigger screws would be required, more tooling and the cost of the raw is more than double for double the thickness… after machining the standard thicknesses would be 21, 28 and 35mm


This is dictated by the tree, for Ipe the trees are up to 1.8m in width and have a usable “bole” (above the roots and below the canopy) of up to 18 metres. These are then cut to the maximum size to fit into a container so 6.3m would be the longest, but once you select and remove the splits, knots, fissures you end up with much less.


Yep, this is a simple one folks, if you want a different colour use a different timber. Ipe is what it is and you get what you get, so any colour matching should be done with tinted oils…

The impossible…. Or is it

The cost of 225 x 50, grain and colour matched in 5.1m lengths would cost over £400 a square metre…

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