How to build a garden fence

A garden fence built from overlapped vertical boarding is an attractive addition to any home. Close-boarded fencing—as it is called—-is not difficult to build and uses pre-cut material, making it a realistic alternative to the cheaper but less durable ‘woven panel’ fencing now so popular for garden use. The tenon joints used in its construction do not have to be perfect, and the rail ends which slot in to them are simply rough-hewn to shape.

Three things make all the difference between a shoddy fence and an attractive, solid, long-lasting one. First and foremost the posts must be adequately supported. Many people try to save money by skimping on the length of the posts or the amount of concrete used to bed them, but this is false economy, as the first gale may quickly demonstrate.

Also, the timber must be adequately protected against rotting. Although fencing timber is sometimes sold pre- treated against rot, it pays to give it the extra protection of a good soaking in preservative before erection. Additionally, all cut ends and every joint must be well soaked in preservative before they become inaccessible.

Finally, the fence should be straight and its post tops level or, if on sloping ground, descending in steps of equal sizes. If your land slopes, you need to work out in advance whether to have a sloping or stepped fence, and whether you will use gravel boards or a plinth beneath it

Construction materials

Though oak is the traditional fencing material, purpose-cut, low grade softwood is used in preference nowadays because of its lower cost. This can be perfectly acceptable provided the timber is properly treated with preservative. The posts are of 75mm x 75mm sawn timber—sometimes available as prepared ‘fencing posts’—which are sunk at least 600mm into the ground and sleeved in concrete. Two or three arris rails—depending on the height Df the fence—fit into mortises in the Dosts. These are triangular in section asually 100mm on one side, 75mm on ;he other two.

The vertical boards are feather-sdged softwood, 100mm wide and iapering from 20mm at one edge to about 6mm at the other. When erected, ;hey are overlapped by about 25mm.

To prevent the bases of the boards rotting, a horizontal gravel board.s fixed between the posts just above ground level. Lengths of 100mm X 25mm timber are suitable, and should be fixed to 38mm X 38mm wooden bleats. As an alternative, you can use a masonry plinth—a dwarf wall—of Bither bricks or concrete walling stones set on a concrete pad.

You can make your fence even more iurable by fixing capping pieces and a weathering strip.


Fencing timber is particularly susceptible to rotting, especially the softwood type. The most vulnerable area is the 100mm or so at, and immediately below, ground level; sometimes this will rot through, leaving the wood above and below it more or less intact. This is why it pays to carry your supporting concrete at least 50mm above ground level and give it a finishing ‘crown’ to run off the water.

Pressure or vacuum-impregnated timber is best for fencing but is expensive and sometimes difficult to obtain.

If you cannot get pre-treated timber, treat the wood yourself by soaking it in a bath of exterior timber preservative. To make the bath you need about two dozen old bricks and a 3m wide sheet of heavy-duty polythene.

Find a suitable flat surface and arrange the bricks on edge to form a rectangle—slightly longer than the largest fencing post and wide enough to take all the timber. Lay the polythene over the bricks to form a bath then carefully pour in the preservative, taking care not to cut or otherwise puncture the sheeting. Steep the fencing materials for up to a week if possible. Cover the bath during this time if children and pets are around.

If you have to store timber out of doors for any length of time, lay it on a flat surface and cover to prevent warping. Leave plenty of ventilation space between individual boards.

As you erect the fence, paint preservative on all timber surfaces that you will not be able to reach later— for example, where the vertical boards overlap. Give all end grain an extra coat to protect it.

Initial preparation

Begin by using a line and pegs to set out a straight line for your fence. Nylon fishing line is best for this, and especially for some of the levelling you may need to do later, as it does not sag when damp.

Decide how far apart your posts are to be—but to save cutting, base this on the length of the arris rails. If your arris rails are 2440mm long, set the posts 2440mm apart, measuring from the centre of one post position to the centre of the next. If this means that one fencing panel must be narrower than the others, arrange for it to be at the least conspicuous end, or corner, of the run of fencing.

Next, prepare your posts. The mortises are cut to take 100mm X 75mm X 75mm arris rails. Cut slots 75mm high X25mm wide, inset 20mm from the face of the post, for the gravel boards.

For a fence on level ground, begin the upper mortise 250mm from the fence top, and the lower one 300mm above ground level—that is, 900mm from the bottom of the post if this is bedded to a depth of 600mm.

If the fence is to be more than 1200mm high, add an extra mortise at the mid-point to accept the third arris rail which is necessary to strengthen the construction. Mark out the mortise positions with a try square and pencil, drill from both sides with a brace and 25mm bit, and use a 19mm or 25mm mortise chisel to square the holes neatly.

For a stepped fence on sloping ground , the mortises on the ‘uphill’ and ‘downhill’ sides of the post will have to be offset, and drilled only halfway through the post.

If you do not intend to use post caps , cut the top of each post so that it slopes away at 45° on the side to which you will be fixing the boards. This ensures that rainwater runs off the vulnerable end grain.

Next, using a small hand axe, taper the ends of the arris rails enough for them to jam into their respective mortises. For a level fitting, try to leave the face of the longest side intact and cut instead into the top edge, bottom edge and back. An exact fit is not necessary, but the neater you are the easier the assembly becomes. Remember to dab preservative around the insides of the mortises and the cut ends of the rail. Also, try to make all the cut ends of the arris rails of equal length—this greatly enhances the overall appearance of the fence.

Making the post holes

Once you have checked that your rails go halfway through the posts—and that your proposed post spacing is therefore accurate—you can mark the positions of the post holes.

Post holes need to be wide enough to allow for a decent sheath of concrete around the posts, but not so wide that they waste material. A narrow hole 600mm deep is hard to dig with a spade; a better tool is a post hole borer—sometimes called a ground auger—available from a tool hire shop. To use it, you just drive it in like a corkscrew.

Use the spade to trim the hole to about 200mm square. If you have the strength and the ground is soft enough, you may be able to ‘pile drive’ the post into its final bedding depth. Remove any loose material from the bottom of the holes.

If you are planning to build a masonry plinth , dig the footings at this stage.

Next prepare shuttering from scraps of timber or plywood so that you extend the concrete sleeve above ground level and so protect the post at its most vulnerable point. Quite how far you extend it depends on whether the fence is beside loose soil, gravel, or concrete, and whether you are using a plinth. Do not go too far or it will be difficult to fix the cleats for the gravel boards.

Erecting the fence

When installing posts, ramming material around the base of a post loses much of its effect unless the bottom of the hole itself is really hard. So use a half brick or lump of concrete as a sole pad in each hole.

Erecting fence posts is much easier if there are two people, because one can hold the post plumb while the other rams in the concrete.

But if you are alone, stand the first post upright, fit a timber brace on each side, then use a plumbline or spirit level to check that it is vertical. When it is, fix the other ends of the braces to pegs driven into the ground.

Use a concrete mix of one part cement to six of all-in ballast. Pour it into the hole a little at a time and ram it well down with the end of a length of timber , checking as you go that you are not knocking the post out of plumb. Slightly over-fill the shuttering with concrete, then use a trowel to slope it to a smooth finish like the flaunching around a chimney.

Leave the concrete to set enough to hold the first post firmly and stand the second post on its sole pad. Fit the first set of two arris rails between the two posts. Check that the top rail is level. If it is not, scrape out more dirt from the second post hole or pour a little concrete under the sole pad. Then pour just enough concrete to steady the second post while you check that it is properly vertical. When it is, pour, ram and trim off the rest of the concrete.

Continue in the same way until you come to the final panel. Then measure off the last arris rails to the required length and erect your last post.

Installing gravel boards

The next stage is to install the gravel boards or build a plinth , sawing it into 100mm squares. These are secured with galvanized nails.

Also useful is a weathering strip along the tops of the boards. You can make this by cutting 38mm X 25mm battens to length. Nail them on at the thick end of the board edges, carefully, to avoid splitting.

Dealing with slopes

Whereas most other types of fence and, indeed, constructions of all kinds must be stepped on sloping ground, a vertically-boarded fence can have a sloped top if the ground below it slopes only slightly. To maintain a consistent slope at the top of the fence, the procedure is to erect and plumb the highest post first, then temporarily erect the lowest post, plumb it, and hold it in position with cross braces. Stretch a nylon line between the two posts and use this as a height guide for the intermediate posts.

Otherwise proceed as for a level fence. Posts and boards must both be vertical. The arris rails are sloped, but the slight step between successive boards which this creates is barely perceptible.

On steeply sloping ground it is still possible to slope the top of the fence, but a stepped top looks much better. Slightly longer posts are needed, however, and the first thing you need to know is by how much the ground falls away. It is no use trying to estimate this by eye, because ground falls are highly deceptive and usually greater than they look.

Start by temporarily erecting the highest and lowest posts. Sink them in the ground by only the usual 600mm and when plumb, use temporary braces to steady them.

Next, take a nylon line and fix it between the top of the lower post to an approximately level position, partway down the higher post. Measure the exact centre of the line and mark it with a dab of paint or tape. Then carefully level the line at the centre spot using a spirit level. Finally, measure the height from the line to the top of the higher post: this is the amount by which the ground falls between the two posts.

Now divide the total fall by the number of panels—not posts—your fence will contain between the highest and lowest posts. If, for example, your fence is to have eight panels, and the ground slopes by 1850mm, you will want eight steps of about 230mm.

To achieve this, make the mortises on the ‘uphill’ side of each post at the normal level, as described above, and those on the ‘downhill’ side 230mm lower. This means that, in this example, all the posts will need to be 230mm longer than standard.

At the bottom of a stepped fence like this, you must make some provision for following the slope of the ground, and you have a choice between sloping gravel boards or a masonry plinth.

Sloping gravel boards

On sloping ground, gravel board ends must be angled so that the boards follow the ground contour. This calls for slightly longer gravel boards than would otherwise be necessary and you will need vertical boards of varying length, which rules out the use of a standard fence kit. The only alternatives are to use two or more gravel boards one above the other—which looks ugly—or build a masonry plinth.

To mark the gravel boards correctly, start by running a nylon line along the length of the fence and about 150mm above the ground. Lay each board against a pair of posts, aligning its top edge with the nylon line. Then use the posts themselves as marking guides while you scribe each board end to the correct length and angle. Number each board to ensure that it goes in the right place.

Fix the gravel boards to their cleats, as described, and then stretch a line across the post tops in the normal way. Stand each vertical board against the gravel board with the top just brushing the nylon line, then mark the correct length and angle. Use the spirit level to check the plumb of every other board.

Masonry plinths

A masonry plinth is almost as easy to construct as gravel boards,and certainly more durable. On sloping ground, it has the advantage that the top of each section can be level, so you do not need to scribe a lot of boards to varying lengths.

A plinth can be constructed of brick or walling stone. It is laid only between the fence posts , and because it carries no weight, it needs only the lightest of foundations—concrete 75mm thick if the ground is reasonably firm.

To build a plinth on anything but dead level ground, you need to step the foot between post positions. Make the height of each step equal to one course of the building materials you choose to use, and the length a straight multiple of the brick or block length.

When you do this, do not forget to allow for a mortar bed between the foundation and the bottom course of masonry—and a double thickness of

I concrete where one step joins the next—or you will have nothing to bind the two steps together.

To avoid wasting material, cut the trench with a garden trowel or bricklayer’s trowel—the average spade is too wide—and use the soil itself as shuttering. A couple of pegs driven into the bottom of each trench length and levelled with the spirit level will help to keep the foundation concrete level. And an offcut of timber hammered into the ground can be used to retain the end of each step.

If the soil is of a badly uneven consistency—topsoil patches, clay patches, rocky patches—and settlement seems likely, you can stabilize the wall by incorporating a length of galvanized expanded-metal wall reinforcement into the mortar joint between concrete and masonry.

While the concrete is hardening, stretch a nylon line across the fence posts above. Then, as you lay the masonry, use a gauge rod to keep each course at a constant height.

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