The ties round the openings are the ones that get missed
Paragraph 2C8 of Approved Document A asks for two separate things and most take-offs only do the first. Ties across the field of the wall, at 900 by 450 or 2.5 per square metre. And then ties at no more than 300mm vertical centres, within 225mm of the vertical edges of every opening, movement joint and roof verge.
That second set is additional, not part of the grid. Which produces a result that feels wrong until you see why: putting a window in a wall increases the number of ties. A 1.2 by 1.2 metre opening removes about 1.4 square metres of field, worth roughly four ties, and adds two full-height runs at 300mm centres, which is ten. On a heavily glazed elevation the edge ties can be a third of the total, and an order worked off area alone lands short.
The two routes in the same paragraph do not agree
2C8 offers a choice: space them 900mm horizontally and 450mm vertically, or space them so there are not less than 2.5 ties per square metre. Presented as alternatives, as though they are the same thing.
They are not. A 900 by 450 grid puts one tie in every 0.405 square metres, which works out at 2.469 ties per square metre. That is about 1.2% below the 2.5 the other route demands. It is a small gap and nobody is going to fall down because of it, but it does mean the two are not interchangeable, and if you order off the grid while telling somebody you have achieved 2.5 per square metre, you have not quite.
The tool lets you pick either and tells you the density you actually achieve, so the number you quote is the number you have.
Length comes off the cavity, and the cavity is not always what you think
Table 5 is straightforward at ordinary widths: 200mm for a 50 to 75mm cavity, 225 up to 100, 250 up to 125, 275 up to 150, 300 up to 175. Past that the table stops giving figures and gives a rule instead, the cavity width plus 125mm to the nearest stock length. The 125 is 50mm of embedment in each leaf plus a bit, which is why a tie needing deeper embedment needs more length on top rather than the same length pushed further in.
The trap is note 1. Where face insulated blocks are used, the cavity width is measured from the face of the masonry unit, not from the structural face. That is frequently not the dimension written on the section, and getting it wrong is the kind of mistake that arrives on a pallet of several thousand ties.
What this is for
Ordering and pricing. It tells you how many ties and how long, so a quantity can go on a quote or an order without anybody counting squares off an elevation. It reports what Approved Document A says for cavity walls in coursed brickwork or blockwork within the scope of that section. It does not select a tie for design loading, which is a BS PD 6697 question, and it does not know about anything unusual going on in your wall.
Common questions
What is the spacing for cavity wall ties?
A horizontal spacing of 900mm and a vertical spacing of 450mm, or alternatively spaced so there are not less than 2.5 ties per square metre. That is paragraph 2C8 of Approved Document A. On top of that, ties at no more than 300mm vertical centres within 225mm of the vertical edges of every opening, movement joint and roof verge.
How many wall ties do I need per square metre?
Not less than 2.5 if you use the density route. Worth knowing: the 900 by 450 grid the same paragraph offers actually delivers 2.469 per square metre, because 0.9 times 0.45 is 0.405 and one tie in 0.405 square metres is 2.469. The two routes are presented as alternatives and they are about 1.2% apart.
Do windows reduce the number of wall ties?
No, they increase it. A window takes its own area out of the field, but both of its vertical edges then need ties at no more than 300mm centres, and those are additional. A 1.2 by 1.2 metre window removes about 1.4 square metres of field ties, roughly four ties, and adds ten edge ties.
How long should a wall tie be?
From Table 5: 200mm for a 50 to 75mm cavity, 225mm for 76 to 100, 250mm for 101 to 125, 275mm for 126 to 150 and 300mm for 151 to 175. Past 175mm the table stops and gives a rule instead, the cavity width plus 125mm taken to the nearest stock length, because the tie needs at least 50mm of embedment in each leaf.
Does insulation change the tie length?
It can. Note 1 to Table 5 says that where face insulated blocks are used, the cavity width is measured from the face of the masonry unit rather than the structural face. That is not always the dimension written on the drawing, so it is worth checking which one you are working from before ordering a few thousand ties.
What should wall ties be made of?
Paragraph 2C19: ties should comply with BS EN 845-1 and be material reference 1 or 3 from its Table A1, which is austenitic stainless steel. Selection then comes from Table 5 on cavity width, and from the design loading via BS PD 6697.