Concealed Post Base: Knife Plate, Hidden or Through-Slot, 6x6 to 12x12 Posts

Hidden or through

Hidden in a mortise. The knife plate goes into a slot plunged into the post's end, with wood left at both ends, so only the bolt heads and nuts show. Its top corners are clipped to clear the radiused corners a mortiser's bar leaves. With a standoff plate, the knife plate widens below it into a foot as long as the standoff plate, and the standoff plate is slotted over the knife plate and sits on the foot's shoulders.

Through slot. The slot is sawn right across the post's end, so the knife plate's edge shows on two faces. The knife plate can be the post's full width, or a little short of it so a post that shrinks doesn't leave steel standing proud. With a standoff plate, it is two halves, one each side of the knife plate, on webbing.

Either way, nothing is welded on top of the standoff plate, where the post bears. Its welds are underneath.

Cutting the slot

The configurator tells you the slot to cut: 1/8" wider than the knife plate, 1/8" deeper than it is tall above the post's end, and, for a mortise, 1/8" longer than the knife plate at each end. Center it on the post both ways, within 1/16". Lay it out from one face of the post, and cut the post's end square first: an end 1/16" out of square across 5 1/2" leans a 10-foot post about 1 3/8".

A mortise's corners. A chain mortiser's bar leaves its cut rounded at each end, to the radius of the bar's nose. Half the length of a single plunge, measured along the slot, is that radius. Choose a corner clip at least as large as your bar's nose radius, and cut the mortise the 1/8" deeper the configurator asks: the knife plate can then sit anywhere along its mortise and still clear the corners. The configurator shows the largest nose radius each clip clears. Match the knife plate's thickness to the slot your tool cuts, and if the slot is more than 1/8" wider than the knife plate, shim it tight. Check that your tool plunges as deep as the slot the configurator gives.

A sawn slot. A circular saw leaves the slot's top curved where the blade stops. Carry the cut off both faces, finish its top square with a handsaw or chisel, or use a bandsaw.

With no standoff plate, the knife plate's welds run along the base where the post bears, so chamfer the slot's mouth 3/8" at 45 degrees to clear them.

The standoff plate and webbing

The standoff plate lifts the post's end off the base, so its end grain can drain and dry. The gap under it is at least 2", or half the reach in from the plate's edge to the knife plate where that is more, so the welds under it can be made. Four 1/2" drain holes through the standoff plate, beside the knife plate, let water through to the base. Set the standoff plate a little inside the post's faces, about 1/4", so water running down the post drips past it rather than onto a ledge.

Webbing is two upright plates welded under the standoff plate, one each side of the knife plate. It makes the standoff plate stiffer. Whether your post needs webbing, and how thick, is for your engineer. A through slot's standoff halves always have it.

The International Residential Code's rules on protecting wood from decay (section R317) are why builders look for a standoff. Whether your post needs one, and what else it needs, is for your local code and building official to say. With no standoff plate, end grain sits on steel, so that style suits covered or interior posts. Don't seal around the base, and keep the slot and the drain holes open: end grain that stays wet is where posts rot.

Anchors, and the nut under a standoff plate

The base has four anchor holes, one near each corner. Their holes are the rod's diameter plus 1/16", or larger, as you choose, for cast-in rods that don't land exactly where the template says.

For a new pour, choose deformed bar anchors instead: two, one each side of the knife plate, or four near the corners, 1/2" x 8-1/8" or 3/4" x 12-3/16", each welded under the base with a fillet all around. Hold the base at its height and place the concrete around them. With no nuts, nothing has to clear the post or the standoff plate.

Where an anchor is clear of the post and the standoff plate, a socket reaches its nut from above, and you can set the base and tighten it before the post goes on. Where an anchor is under the standoff plate, a wrench has to come in from the side, and the nut needs room to be started: the configurator keeps the gap tall enough for a washer, a nut started on the rod's tip, and 1/4" of rod past it. It gives the window the rod must end in, measured above the concrete. Use a threaded rod that takes a washer and nut, ending inside this window, and set and cut it to length before setting the base. A taller gap widens the window.

The anchor's type, size, depth, and distance from the concrete's edge are for the anchor maker's instructions and your engineer.

Putting it in

• A hidden knife plate: fit the post over the knife plate on sawhorses, drill through the post from both faces using the knife plate's holes, and bolt it. Then stand the post up and anchor the base.
• A through slot: set and anchor the base first, then lower the post over the knife plate and bolt it.
• Measure the delivered part, not the drawing, before you drill: the bolt holes are cut where the configurator shows them, and the knife plate is square to the base within 1/16" over its height.
• Shims or grout under the base lift it: measure an anchor's projection from the base as set. A base 1/16" out of level across 8" puts a 10-foot post about 15/16" out of plumb.

The welds

Every joint is a fillet weld on both sides. You choose each joint's fillet, from 1/4" up to the thinner plate it joins: the knife plate to the base, the standoff plate to the knife plate, and the webs. A larger fillet takes more passes and costs more: 1/4" is one pass, 3/8" and 1/2" are three, and 3/4" is six. The cart line and the shop's paperwork list each one.

Sizes it is made in

• Post: 5" to 12" each way, for 6x6 to 12x12 posts, surfaced or rough-sawn.
• Knife plate: 1/4", 3/8", 1/2", or 3/4", 2 1/2" to 12" long, and 3" to 18" into the post.
• Standoff plate: 1/4", 3/8", 1/2", or 3/4", its gap 2" or more, its size up to the base's.
• Base plate: 3/8", 1/2", or 3/4", up to 24" each way, with four anchor holes for 1/2" to 1" rods, or two or four deformed bar anchors welded under it.
• Bolts through the post: 1/4" to 1", in one row or two.

Sizes are set to the 1/8". The configurator opens on a rough-sawn 6x6 in a mortise, on a 1/2" standoff plate 2" up, with a 1/4" knife plate and two 5/8" bolts, on a 9 1/2" square base.

What a post base doesn't do

A post base holds the bottom of a post in place. A post base doesn't brace a post: it doesn't keep it from leaning, however many anchors and bolts it has. A post that stands alone, like the post of a carport or pergola with no bracing, needs to be braced some other way. Ask an engineer.

Questions

Can I see the knife plate in a through slot?

Yes: its edge shows on the two faces the slot runs out through. Choose the hidden style to keep it out of sight.

Is the post treated?

If your post is preservative-treated, ask your building official what finish the steel needs where it touches the wood. Galvanizing is by quote, and it adds substantially to the lead time.

What connects the top of the post?

A post cap seats a beam on the post. For a plate in a slot between two timbers, see the splice and knife plates.

My design is too large to buy online.

Send it through the quote form. Bases outside the configurator's range are made to order from your numbers or your plans, and custom design costs nothing extra: you pay for the steel and its welds.

The basics

• A36 steel plate, laser cut and welded in our ISO 9001 shop in Texas.
• Holes are cut to the sizes you choose. Bolts and anchor rods aren't included; deformed bar anchors, if you choose them, come welded on.
• Enter your timber's real size, not its name. How to measure a timber.
• Bare steel, or matte black powder coat added in the cart. Galvanizing is by quote.
• Ships free in the lower 48 states.
• No published load ratings. Ask your engineer, and see what to give them.