Cup Head Carriage Bolts or Hex Bolts for Timber

This is a pretty common scenario, maybe your building a structural frame for your decking or even framing a house, whatever your project you suddenly find your self with a pile of structural grade timber and no bolts or fasteners to put it together … and maybe even less idea of which ones to to use. To make the right decision firstly you have to understand the basics of standard bolts, there are loads of different types and bolting technology is a massive subject but here we’ll just be talking about the basic types which mostly fall into 2 categories  :  high tensile and low tensile (or mild steel as they are often termed).

For our application, in timber, we’ll use low tensile and this would be the case 99.9% of the time, the reason for this choice is simple for 2 reasons, firstly, although high tensile bolts are of a much higher strength (as the name implies) there strength properties makes them less flexible than mild steel and as timber is an organic product that’s constantly moving, we want fasteners that will accommodate this movement and roll with the changes (so to speak), if the bolted joint isn’t allowed to flex it could snap a stiffer fastener or the local timber around the bolt. The second reason is a mild steel bolt is very strong and its pretty much guaranteed that most timber will break, way before the bolt will,  so using high tensile bolts for timber is complete overkill and wasted expense.

So OK low tensile it is, now which ones, well the main 2 choices are hexagon head bolts or cup head bolts aka coach bolts aka again carraige bolts. The wise choice is coach bolts as they are designed specifically for bolting timber sections together or timber to steelwork, the industry designation for these is actually ‘Cup Square Hex Bolt and Nut’ and the reason is all due to the shape, the head is a rounded cup shape with a square lug under it and a hex nut on the other end, its this lug and large cup head that makes this bolt so useful for timber as when you tighten them up the lug bites into the wood and locks it to stop it turning while you tighten up and the large surface area cup head stops it pulling through. You’ll find if you try and use a hex bolt its actually pretty difficult to fully tighten them, the head doesn’t lock so the whole assembly often turns, you can hold both sides with a spanner of course but you’ll quickly find the head burying into the timber and pushing the spanner off and leaving you in no mans land with with a half tight assembly that you can’t undo or tighten. click the pics for bigger images if you want a close up …

So there you go, for bolting timber together or timber to steel, use a low tensile (mild steel) or stainless steel (for maximum corrosion protection) coach or carraige bolt in either zinc plated for standard timber or hot dipped galvanised for treated timber, it what its designed for. Hex bolts are perfectly acceptable but you should use a washer both sides and be aware you will need access to both sides of the joint to hold and tighten.

There’s lots of horror stories around with bolted joints so please use common sense when building anything structural, don’t just guess a size or use anything lying around for convenience and of course for any major structural application you should always consult a structural engineer to confirm your connections are adequate, and be aware that other forces may be applied to the bolts other than just the timber you can see. As a tip, to recognise a mild steel hex bolt it will have 4.6 stamped on the head (its the grade of steel), carraige bolts are not always stamped, dependant on the manufacturer.