Wall plate, rafters and ridge: rods, angle brackets and plates
In a seismic zone of intensity 8 the load-bearing roof joints are designed by a structural engineer, and we cannot replace that. But we can show the lower bound of the fixings: what holds the wall plate, how to tie the rafters, and what from our stock is primary here and what is auxiliary.
Wall plate to the ring beam
Under the timber goes a waterproofing layer (two layers of roofing felt or a waterproof tape) and a preservative, otherwise the wall plate rots. It is held by M12 threaded rods cast into the ring beam during concreting — in a seismic zone this is the most reliable. Spacing no more than 1 m, plus a rod at every joint in the timber no further than 250–300 mm from the joint and at every end of a wall. On windward sections and where the engineer requires it — M14–M16. Length of a cast-in rod: 250–300 mm embedment with a bent foot, 150 mm of timber, 17 mm for washer and nut, 25 mm projection — about 490 mm; a 2 m bar (M12 — 5.84 ₾, pack of 20) gives 4 rods. Nuts, oversized washers or 40×40×4 plates and lock nuts come from a fastener shop.
If no rods were cast in
Then a chemical anchor into the finished beam: an injection resin approved for cracked concrete and seismic loads, an M12 rod, a Ø14 hole, depth at least 110 mm. The hole is cleaned blow — brush — blow; without this the anchor loses half its capacity. A 300 ml cartridge does about 21 M12 points. The rod for a chemical anchor is about 310 mm, 6 pieces from a bar. The resin, brush and SDS-plus Ø14 bit come from a chemical-anchor supplier; from us — the M12 rod and the blow-out pump (67.90 ₾). Distance to the edge of the beam and between rods — at least 55 mm; beam width in practice from 200 mm.
Rafters to the wall plate
The lower joint is a birdsmouth bearing plus a rigid tie against uplift: two skew nails on each side and metal angle brackets on both sides. The proper option is reinforced rafter brackets with ribs and anchor plates with a European approval: such a bracket has a working shear load of about 1.8 kN, and they come from a timber-connector supplier. Our perforated angle brackets are 30 mm wide and 2 mm thick, without ribs and without a data sheet: we use them as auxiliary — a second pair next to the designed connector, on battens and secondary ties. Wind uplift on the lower joint is roughly 1 kN, so one thin bracket without a data sheet is not enough for that joint. The 100×100×30 bracket costs 1.53 ₾, pack of 50.
Ridge, collar ties, struts
Rafter pairs on a ridge beam or butted with cover plates: 2–3 nails 4×100 through each rafter and a perforated 195×30×2 plate (1.42 ₾, pack of 50) on both sides. Collar ties and struts — M10–M12 bolts with washers: M10 and M12 rods are cut to length on site, nuts and washers are bought separately. We do not stock Ø4 ring-shank nails or 4.2–4.8 mm screws for the brackets — any fastener shop has them; fill every hole in a bracket or plate, not two “for show”.
Perforated strap
Strap 12–20 mm wide, 0.55 mm thick, in 25 m coils — for hangers, pipe clamps and temporary ties. It is not counted as a load-bearing rafter tie: it is lashing, not a joint.
Example: a 10×10 house, 20 m of wall plate, 26 rafters
M12 rods: 20 by spacing + 2 at joints + 4 at wall ends = 26 pieces — 7 bars of 2 m; a pack of 20 bars — 116.80 ₾, 7 bars individually — 40.90 ₾. 100×100 brackets — two per rafter, 52 pieces, two packs — 153 ₾. 195×30 plates at the ridge — 26 pieces, one pack — 71 ₾. Blow-out pump, if you do not have one — 67.90 ₾. About 409 ₾ from stock in total, about 333 ₾ if the bars are sold individually. Separately: 2 cartridges of chemical anchor if no rods were cast in, about 80 nuts and washers, about 600 nails, about 7 m² of waterproofing under the timber.
Mistakes
- Rafters “just resting” on the wall plate with no bearing notch and no uplift tie — wind and earthquakes shift the roof.
- Rods not cast into the ring beam: drilling later costs more, and in a seismic zone the resin needs an approval for cracked concrete.
- A chemical anchor in an uncleaned hole — dust instead of bond.
- A wall plate on concrete without waterproofing — the timber rots in a few years.
- Perforated strap instead of a bracket at a rafter joint.
- Two nails in a plate “for show” — the joint does not work.