Garden Arch

Garden arch - basic assembly A simple timber arch in the garden is an ideal feature for helping to separate different areas or for training climbing plants. In this article we describe building a simple and decorative timber arch.

The description and materials list is for an arch 1.2m wide, 0.9m deep and 2.4m high, however the design is such that the dimension can easily be changed to suit individual areas in the garden.

Materials.

Timber - use pressure treated timber, it will cost more but will last longer. All sawn edges of the timber should be treated with a similar wood preservative before assembly.

  • A - Posts 4 off, 75x75mm 2.4m long
  • B - Lintels 2 off, 140x35mm 1.5m long
  • C - End "rafters" 2 off, 140x35mm 1.2m long
  • D - Middle "rafters" 3 off, 140x35mm, 0.9m long (these will need to be trimmed to fit)
  • E - Braces 4 off 90x35mm, 0.3m long
  • F - Side braces 6 off, 95x35mm, 0.9m long

Hardware - use stainless steel screws/bolts to avoid corrosion.

  • 4 off 75x75mm post sockets inserts
  • 4 off 6x100mm coach bolts plus large washers
  • Assorted screws and nails

Securing the uprights.

There are a number of different options for securing the upright, the one you choose will just be a personal choice, there's no 'right' or 'wrong'. The options are:

  • Just stand the uprights on concrete, bricks, or paving slabs - not very permanent so the arch can be moved in future if necessary, but the arch will not be firm and may move in the wind etc especially if a climbing plant is grown over it.
  • Use post sockets embedded in concrete with the bottom of the uprights clear of the ground - permanent fixing which should reduce the speed of the timber rotting as the lower ends are held off of the ground, there are limited choices of socket sizes which will influence the size of timber used for the uprights.
  • Use short metal stakes embedded in concrete (or just driven into the ground) with the uprights of the arch bolted to them - permanent fixing with the same advantages as post sockets above plus the advantage that they can be used with any size of square posts.

Laying out the post foundations.

This will vary depending upon the chosen method for securing the uprights - however the rest of this article is based on the assumption of using post sockets, we make suggestions for the other methods on this other page.

Working out the post holes The footprint of the arch is 1.2 x 0.9m, markout the required position of the posts on the ground.

Measure out from these marks about 300mm in each direction and drive a small wooden peg (that is 8 wooden pegs).

Stretch string-lines between the 8 pegs to define the sides of the arch position. Check that the diagonal measurements are equal to ensure that the 'box' is square.

Mark the ground below the crossed strings - this is the centre for a 300 x 300mm hole to take the post socket. Dig out the hole to about 300mm deep.

Mix up a dryish concrete mix (1:2:4 cement: sand: coarse aggregate), part fill the hole with the mix and set each post socket in place with its centres under the crossed string-line - try also to keep the top of the sockets consistent, use a batten and spirit level to check between them.

Finish filling the hole with concrete and check that each post base is still under the cross-line and vertical.


Erecting the posts.

The postsOnce the concrete has gone off, place the posts (A) into the four post sockets and temporarily secure them using a spirit level to ensure that they are vertical.

Use a batten and spirit level to check the height of the tops, if they are not the same, use the shortest post as a datum and transfer this height to the other posts using the batten and spirit level.

Mark posts and the sockets so you know which post goes where, mark a face of each post to show its orientation. Take the posts down and cut to length. From the top of each post, measure down 140mm on the appropriate face to take the lintel. Mark out a cutting depth of 35mm and cut out the waste material.

Refit the posts in their appropriate socket, check with a spirit level that they are vertical and that the measurements between the outside of the posts are 1.275 and 0.975m. If the dimension are slightly out, this can be allowed for by adjusting the length of the lintels and rafters.

Cutting and fitting the lintels

Lintel/rafter end shapesCut the timber for the 2 lintels (B) to the same length and mark the end shapes - two sample end profiles are given to the right, choose one (or make one up of your own) and make up a cardboard template so that the profile will be consistent.

Cut the end shapes; use a jigsaw for a curved shape, for the straight edged end, a handsaw can be used.

Lintel Along the top edge of one lintel, establish and mark its centre - from this mark, measure out each side half the width of the arch measured to the outside of the posts (this should be 0.632m but may need adjusting if the posts are nearer or further apart). From each of these marks, measure outwards a further 35mm (the thickness of the end rafters), and make another mark. From these marks, mark across the lintel to the centreline. Cut out each of these slots in both lintels.

Position the lintels in the cut-outs at the top of the posts so that the inside edge of the lintel slots line up with the outsides of the posts - clamp the lintels to the posts, drill through the centre of each post and lintel and secure the lintel in place with one coach bolt using a large washer under each nut.

Cutting and fitting the end rafters

End raftersCut and shape the ends of the 2 end rafters (C) in a similar way to the lintels.

Along the bottom edge of each end rafter, establish and mark its centre - from this mark, measure out each side half the depth of the arch (this should be 0.482m but may need adjusting if the posts are nearer or further apart). From these marks, measure 35mm IN towards the centre of the rafter and make another mark. From these marks, mark across the end rafter to the centreline. Cut out each of these slots in both end rafters.

From above the arch, slot the end rafters into the lintels using the slots cut in both. Secure the end rafters to the posts using screws.

Bracing the frame

BracesCut the 4 braces (E) from 90x35mm timber with a plain 45° mitre at each end so that they will fit between the posts (A) and the underside of the lintels (B).

Before fitting the braces, drill through the mitre to give clearance screw holes. Then secure them to the posts and main lintels using screws.

Cutting and fitting the side braces

Side bracesCut the side braces (F) from 90x35mm timber to suit the width of the side of the arch.

Before fitting the side braces, drill two clearance holes at each end within the width of the posts.

Then secure them to the posts using screws.

On each side, secure the upper brace about 300mm from the top, the lower brace about 300mm from ground level and space the middle brace equally between the upper and lower braces.

Cutting and fitting the middle rafters

Measure the distance between the main lintels and trim the three middle rafters (D) to size.

Equally space these rafters between the end rafters and secure them using nails driven in from the outside of the lintels into the end grain of the rafters.

Finishing off

To finish, either paint all the timber with an exterior paint, oil or other timber treatment - alternatively allow the treated to weather naturally.

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