Shed Trusses vs Rafters: How to Build Them

Shed trusses are the triangles that hold up a gable roof, and how to build shed trusses depends on which kind you mean. A true truss is a triangulated frame made in a plant with metal plates. What most backyard builders make is a rafter pair with a 2x4 tie across the bottom and plywood gussets over the joints, built on a floor jig and lifted onto the walls.

This guide explains the difference, shows what the designer’s own roof looks like at four sizes, walks through the jig method as general practice, and says plainly where the model code stops covering you. If you want the cutting and sizing of rafters themselves, that lives in the shed rafters guide. For the bigger picture, start with how to build a shed.

Shed trusses vs rafters: what each one is

The two roofs look alike from the ground. Underneath they carry load in different ways, and the model code treats them as different products.

A truss is a triangulated frame

A truss is a frame of top chords, a bottom chord and web members, joined at each joint so that every member is pushed or pulled along its length. In a plant the joints are metal connector plates pressed into the wood. The Truss Plate Institute describes its ANSI/TPI 1 standard as setting “minimum requirements for the design and construction of metal-plate-connected wood Trusses”, and says it covers the connector plates and manufacturing quality assurance too (Truss Plate Institute, ANSI/TPI 1-2022). That is a factory process with a design standard behind it.

A rafter roof is pairs on a ridge board with a tie

A rafter roof is made of pairs of sloping boards. They meet at a ridge board at the peak and sit on the top plates of the walls. Left alone, the weight of the roof pushes the walls apart, so the pairs need ties across the building. The 2021 IRC says the roof and ceiling assembly must provide “continuous ties across the structure to prevent roof thrust from being applied to the supporting walls”, and that a ridge board must be at least 1 inch nominal and as deep as the cut end of the rafter (2021 IRC, R802.2 and R802.3). Where the ties do not do that job, the ridge has to be designed as a beam.

The designer builds this roof. It picks 2x6 rafters, a 2x8 ridge board one size deeper, and a 2x4 tie at every pair (the designer’s rafter logic). A 2x4 is never picked for the rafters themselves.

What builders call a shed truss

When someone searches for shed trusses, the plan they usually want is a rafter pair with a bottom tie, cut once, copied, and glued or nailed together with plywood gussets at the peak and the heels. It has the outline of a truss and the parts of a rafter roof. It is not a metal-plate truss and it is not the same thing as the frame in a truss plant. Keep that in mind, because it decides what the code says about it.

What the model code says about each

The model code is the International Residential Code (IRC). Your building department adopts a version, often with local changes, and its edition decides what applies to you. The section numbers below are from the 2021 edition.

Rafters have span tables and tie rules

Rafters have prescriptive tables. Rafter spans are measured along the horizontal projection, not along the sloped board (2021 IRC, R802.4.1). For spruce-pine-fir No. 2 with a 30 psf ground snow load, 10 psf dead load, no ceiling attached and a deflection limit of L/180, the tables give these horizontal spans (2021 IRC, Tables R802.4.1(3) and (5)):

The L/180 limit applies to rafters steeper than 3:12 with no finished ceiling attached; flatter roofs fall under L/240 (2021 IRC, Table R301.7). The code also fixes the tie heel joint: the number of 16d common nails at each end of a tie depends on the pitch, the spacing, the ground snow load and the span (2021 IRC, Table R802.5.2(1)). At 4:12 and 16 inches on center, a 12 ft span with 30 psf needs 3 nails, 24 ft needs 6 and 36 ft needs 9. That table assumes ties at the bottom of the attic space.

Wood trusses need design drawings

Trusses are a different section. The 2021 IRC says truss design drawings “shall be provided to the building official and approved prior to installation”, that wood trusses are designed “in accordance with accepted engineering practice”, and that metal-plate-connected trusses follow ANSI/TPI 1 (2021 IRC, R802.10). It sends bracing to the SBCA’s Building Component Safety Information (BCSI) guide when there are no specific bracing requirements. And it says truss members “shall not be cut, notched, drilled, spliced or otherwise altered in any way without the approval of a registered design professional.”

What that means for a site-built truss

In the R802.10 text we read, we found no span table or recipe for a plywood-gusset truss made on your shed floor. Rafters have tables and trusses have drawings, and a homemade truss has neither. So do not call a site-built truss code-approved. Where your shed needs a permit, ask the building department what they accept. Whether your shed needs a permit at all is a local question, so read our shed permit basics before you cut anything.

What the designer builds: rafters plus a tie

The designer does not do trusses. It has no prefabricated truss, gambrel or loft, and it says so. Its gable roof is rafter pairs with a ridge board and a tie at every pair. Here is that roof for four gable sizes at the defaults: 6/12 pitch, 16 inches on center, 6 inch overhang, 30 psf snow.

Four sizes, first-hand

Every rafter is a 2x6, and every tie is a 2x4. The ties are cut with their ends sloped to the roof, and the nails are 16d commons per heel joint, two heel joints per tie, counted from the IRC table above. The designer uses the 5:12 row of that table for a 6/12 roof, which means more nails and errs on the safe side.

Each rafter’s span is small next to what a 2x6 can do:

The span is half the shed width less 3/4 inch, and the check is bending with a deflection limit of L/180 on SPF No. 2. Its results match or sit slightly under the IRC tables above; it is a planning aid, not a design.

What 24 inches does on a 12x16

Change the spacing to 24 inches on the 12x16 and the roof drops from 26 rafters and 13 ties to 18 rafters and 9 ties. The allowed 2x6 span falls to 9 ft 8-13/16 in against a 5 ft 11-1/4 in span, so 64% is spare instead of 101%. The tie heel joint goes from 3 to 4 nails. The sheet list is 8 sheets of 7/16 inch OSB with H-clips between the rafters. The 2021 IRC lets a 24/16 panel of 7/16 inch span 24 inches on a roof, with or without edge support (2021 IRC, Table R503.2.1.1(1)).

The saving is real, but it shrinks as the shed gets wider. On the 16x20 at 24 inches the designer needs 22 rafters, 11 ties and 7 nails per heel joint, and only 23% of span is spare. Open the 12x16 at 24 inches in the designer to see its cut list.

Width sets the roof, depth only adds pairs

The roof gets tighter as the shed gets wider, not deeper. Depth only adds pairs. Width sets the rafter span, the tie length and the heel nails. Wide sheds are where builders start asking about trusses, and it is also where the tie carries the most load.

How to build shed trusses on a jig

This section is general practice. The steps below describe a method, not a code-approved design, and your plan or a truss designer decides the sizes. The full-size layout comes from the designer’s cut list for your shed, such as the 10x12 gable plan.

What the old plan sheets say

Land-grant universities published nailed and glued plywood-gusset truss plans decades ago. The University of Illinois Small Homes Council, working with Purdue, issued nail-glued designs. Their 3/12 plan covers spans of 20 ft 8 in to 28 ft 8 in and their 4/12 plan covers 29 ft to 34 ft 8 in. Both are for spans several times a shed’s, and the second one is the one that spells out the jig method. The 3/12 plan tells the builder to space the gusset fasteners “4 inches apart in two rows” and to glue the joints. Its testing found no glue-line failures and a failure load of 142 psf. The 4/12 plan has the builder use 6d nails 4 inches apart in two rows, and its three test trusses failed at an average of 115 psf.

Do not copy those numbers. They belong to those designs, at those spans, with those gusset sizes and glue. The plans themselves say responsibility for trusses built from them “shall rest with the user of the plans” (University of Illinois Small Homes Council). What they are good for is the method: the sequence a builder follows to get a dozen identical frames.

The jig method, step by step

The 4/12 plan starts with the span “out-to-out dimension of exterior wall plates” and builds a jig from “2 x 4 jig blocks for aligning chord members”. It has the builder “use members of first truss as a pattern to cut members of all trusses to be built” (University of Illinois Small Homes Council, 4/12 plan). For a shed, common practice runs like this:

1. Measure the wall plates. Measure across the outside of the plates, where the rafter heels will sit. Do not trust the plan drawing, and measure both ends.
2. Lay out the triangle full size. On a flat floor or a sheet of plywood, chalk the roof outline: the two sloped rafter edges, the peak, and the tie across the heels.
3. Build the jig. Screw 2x4 blocks to the floor outside each member so the rafters and tie drop into place and cannot move. Wedges hold them tight.
4. Cut one pattern set. Cut two rafters and one tie. Set them in the jig and check that the joints touch. The Illinois plan says to “recheck alignment on all members” and be sure they touch at the joints.
5. Test the pattern. Lift the pattern onto the walls, or at least hold it against the plates, before you cut the rest. An error caught now costs one set.
6. Cut the rest from the pattern. Trace the pattern onto every board so every pair is the same, and add a spare pair for mistakes.
7. Fasten the gussets. Glue the joints if the plan says so (“apply glue at gusset locations” in the Illinois plan), then nail or screw the plywood plates to both faces as the plan specifies. Turn the pair over and do the other side.
8. Stack flat. Keep glued pairs flat and out of the rain until any adhesive has cured. The Illinois 3/12 plan gives 16 hours at 50 to 70 F and 8 hours at 70 F or above for its glues.

Two notes on the outline. A truss needs no ridge board because the gusset at the peak joins the two top chords, but the pairs still have to be braced against each other once they stand. And wide gussets need to stay inside the top and bottom edges of the chords so the roof sheathing and the ceiling lie flat.

The heel joint is where the load goes

A gusseted pair still pushes outward at the heels, and the bottom tie is what stops it. For a rafter-and-tie roof the code’s number is the heel nail table above, and that is what the designer counts (3 nails per heel joint at 10x12, 5 at 16x20). The table counts nails through the tie into the rafter. It does not give credit for plywood gussets, so treat gussets as extra stiffness and handling strength, not as a replacement for the heel joint nails. Toenail or connect each end to the top plate as your roof plan says: the designer lists a rafter-to-plate connector at every bearing, and the 2021 IRC’s uplift rule in R802.11 applies to rafters and trusses alike (2021 IRC, R802.11).

Trusses vs rafters: which is less work?

Whether it is cheaper to build your own comes down to local prices, and prices change by store and week, so there are none here. What you can compare is the count of pieces and the number of hands-on steps.

The count of pieces

The designer’s 12x16 has 26 rafters, a 192 inch ridge and 13 ties. A gusseted-pair version has the same 26 top chords and 13 bottom chords. It drops the ridge board and adds plywood for the gussets, glue or fasteners, and lumber for the jig. Pair after pair is the same cut, so the gusseted route has a slightly different shopping list, not a smaller one.

The labor

The difference is how the work is spread. With rafters, you stand on the walls, hold each pair against the ridge and nail it in place, over a heap of cuts. With gusseted pairs you build every pair at floor level and then lift 13 of them. That is more upfront work and more lifting, but every joint is made where you can see it and clamp it.

When to order manufactured trusses

Order trusses when the shed is wide enough that you want a design behind every member, when your building department asks for truss design drawings, or when you would rather not build a dozen frames by hand. A manufactured truss set comes from a plant and comes with the drawings the code asks for.

What you get

The 2021 IRC wants “truss design drawings” given to the building official and approved before installation (2021 IRC, R802.10). A truss supplier produces those. ANSI/TPI 1 covers the plates, the lumber and the quality assurance for metal-plate-connected trusses (Truss Plate Institute). If you need a paper trail for a permit, this is the cleanest one. Ask whether the drawings carry a design professional’s seal, because what a jurisdiction requires differs.

Handling and bracing are part of the job

Trusses are slender frames and they need to be stood up and braced properly. SBCA calls its BCSI guide “the component industry’s guide for jobsite safety and prescriptive truss restraint and bracing recommendations”, covering handling, installation, restraining and bracing (SBCA). The model code points to that guide too. Follow the truss designer’s bracing notes on the drawings first, and do not cut, notch or alter a truss to fit.

What to give the supplier

Give the plate-to-plate width (outside of the wall plates), the pitch, the overhang, the spacing you want, your ground snow load from the building department and whether a loft or storage will hang from the bottom chord. The designer’s numbers help you sketch the shed, such as 6/12 and 6 inch overhang, but the supplier’s drawings, not ours, are the design.

Trusses, rafters and a loft

A loft changes the tie

If you want a loft, a rafter tie can help, but a tie placed high for headroom is a different design from the one in the heel-joint table, which assumes ties at the bottom. The shed loft guide covers what a loft needs. On manufactured trusses, a bottom chord that carries storage is a question for the truss designer, not something to add afterward.

Next steps

Pick the pitch first, because it sets the rafter length and the tie nails; the shed roof pitch guide walks through it. Then sheathing and covering: the how to build a shed roof guide takes it from framing to shingles. Try your own width in the shed designer, or open the 12x16 gable plan for the full cut list.