Constructional Timber

There is a common misconception that C24 is vastly superior to C16, however when you consider the pros and cons, you may come to a different conclusion!

The critical difference in structural performance between the two grades is the difference in "span". This refers to how far the length of timber can span unsupported between joists. Although C24 is more expensive then C16, the difference in span is minimal, especially when you consider the standard length of floor joists.

C16 performs just well in every other situation. Apart from the performance, there are other factors to consider. Price: C16 can be around 10% (or more) less expensive. Treatment: due to the nature of the raw timber normally used in C24 productions, it is harder to treat to UC3, meaning C24, can only be used when covered (see below), whereas all of FORT's C16 stock is UC3. Product miles: timber is renewable, and therefore a very "green" method of building. Given that C16 is mostly grown, cut and treated within the UK, it does an awful lot less miles to get into our yard. Quality: Due to the speed at which it grows, C16 contains less fibre in volume terms (you can see the annular rings on the end are further apart); this means that C16 material has a higher tendency to bow and twist. The easy answer is to fix it in place promptly.

Timber Treatment

There are 2 main ways to preservative treat timber (double vacuum and high pressure vacuum). Constructional timber is almost always treated under high pressure vacuum. The advantage is that it is more robust, but it does slightly distort the timber profile. It usually has a green tinge due to the copper in the chemical mix.

There are 4 basic grades of preservative treatment (you can get higher for items such as bridges and motorway fencing). They are called User Classes (UC). We've set out below how they work;