Fence gap (gap in knowledge)

[PerranOak]

I lost some featherboard fencing and am left with a gap of 2725mm. I guess I need to put a post in between the existing two - both are very stable and not rotten and are 100mm square. So, a few questions:
Do I need a post of the same size for this gap or will a smaller one do?
How far down does it have to sit?
How do I best attach the cross members to the posts?
Thank you.

 

[Deluks]

You can get 3m arris rails which should bridge the gap without another post.
Sit a small 2x2 post in the centre of the bottom arris rail, sit it on a a half buried brick.

A gravel board should be used.
You can use a timber gravel board or a 3m concrete one.

 

[foxhole]

BPC Galvanised Arris Rail Bracket Timber 60 x 60 x 285mm | Tradefix Direct

Buy the BPC Arris Rail Bracket 60 x 60 x 300mm from Tradefix Direct. Strong galvanised steel for secure fence rail support. Fast UK delivery available!

tradefixdirect.com

 

[PerranOak]

Ah, sorry I should have said. I am "interfacing" with the already present remainder of the fence which has "arris rails" of beams that are 78x40mm. As this of the fence is directly opposite our (good) neighbour's conservatory I want it to look the same all the way along for their benefit.

Will the advice (Deluks) still apply and are there equivalents for the brackets (foxhole)?

BTW when I was talking to a (different) neighbour he said beware that when you order, say, a post that is 100x100mm it will really be more like 85x85mm! Is this true? Is it true of "beams" too?

Cheers.

 

[endecotp]

beware that when you order, say, a post that is 100x100mm it will really be more like 85x85mm! Is this true?

Timber is typically sawn first, and then optionally planed. Sometimes you are shown the dimensions for the sawn size before planing, which is a few mm larger than the finished size; sometimes both are shown. For example, here is a screenshot from the Wickes website for timber that they call 44 x 44 mm:

Note that this shows “thickness” and “width” of 44mm (top left) and also “nominal size” of 50 x 50mm (bottom right). So in this case the “headline” dimensions are the actual finished dimensions. I think that today that is most common, but you may still find sellers where the “headline” size in the description is the pre-finishing size.

 

[PerranOak]

Another thing is that the existing feather board is 1.8m x 150mm x 15mm-4mm.
I can't find these dimensions. Is it a weird size?