Roof structure basics: rafters, trusses and purlins

Learn to identify rafters, trusses and purlins in a loft, and why you must never cut a truss without professional advice.

RS-01Get into the loft safely firstUse a proper loft ladder or a sturdy stepladder, and take a torch even if there's a light up there. Step only on the joists (the horizontal timbers) or loft boards, never on the ceiling itself, because plasterboard will not hold your weight and you'll go through it. Wear a hat or watch your head, nails often poke through timber.

RS-02Spot a rafterA rafter is a sloping timber that runs from the ridge (the top of the roof) down to the eaves (the edge, above the walls). It follows the pitch of the roof exactly. In an older 'cut roof' you'll see rafters spaced roughly 400-600mm apart with nothing joining them except at the top.

RS-03Spot a trussA truss is a whole pre-made triangle of timber: two rafters plus a horizontal bottom chord (which usually doubles as the ceiling joist) plus diagonal 'webs' inside the triangle forming a W or zigzag pattern. Modern houses (roughly post-1960s) almost always use trusses, spaced about 600mm apart, standing like a row of identical fences across the loft.

RS-04Spot a purlinA purlin is a thick horizontal beam running at right angles across the rafters, partway up the slope, supporting them from underneath. Look for a chunky timber (often 200mm or more deep) lying flat, usually with a diagonal strut propping it up onto an internal wall below. Purlins are common in older cut roofs; trussed roofs don't normally need them because the truss webs do that job internally.

RS-05Tell a truss roof from a cut roof at a glanceWalk along the loft and look at the pattern. If every rafter has a matching diagonal web pattern below it, spaced evenly, it's trussed. If the rafters are plain slopes with a purlin and struts holding them up, it's a cut roof, built timber by timber on site. Beginners often mistake the diagonal webs of a truss for random bracing, they are not, every piece is load-bearing.

RS-06Understand why you can't just cut a trussThe mistake beginners make is treating the open triangle inside a truss as wasted space and cutting a web out to fit a water tank, ductwork or a loft conversion opening. Every part of a truss is calculated to work together, cut one piece and you weaken the whole roof, sometimes with no visible sign until it fails. Any alteration to a trussed roof's structure needs an engineer's design and Building Regulations approval, this is a genuine legal requirement, not just good advice.

RULES

KNOW THE RULES

Structural alterations to a roof, including cutting or removing truss members, require Building Regulations approval and normally an engineer's calculations.

RS-07Check you've got it rightStand at one end of the loft and look down its length. You should now be able to say confidently: is this trussed or cut, where are the purlins if any, and which timbers are structural. If you can point to a rafter, a purlin (or explain there isn't one), and either a truss web or a cut roof's bare slope without hesitating, you've identified the structure correctly.

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