Photography / Travel

How to Build a Picture Window-Sized Window Box (on Vinyl Siding)

by Tim Darling (email) - July, 2012.

Build and secure the mounting brackets on vinyl siding

(1) Build and secure the mounting brackets

The hardest and most critical part of creating a window box on vinyl siding is mounting it securely to the house. Especially for a 9' long window box which will ultimately be heavy enough (150 lbs+) to require securing to the wall studs.

Windowbox component Weight (lbs)
30' of wood, painted 40
Water (3 * 2.5 gallon reservoirs) 70
2 bags of potting soil 70
9' of dense fully grown flowers 20
Total 200

To secure the window box to the house, wood mounting brackets and shelf supports can be affixed first; the window box is then secured to the mounting brackets. Mounting brackets also create 1" or more of space between house and the windowbox for movement of air.

To secure the mounting brackets to the studs, I had to use 6" long deck screws (they have to first go through 1" of the mounting bracket, 1.5" of siding/wood furring/Tyvex moisture barrier, 2" of insulation, then the studs). So it was critical that the mounting bracket conformed to the siding's shape so there were no gaps that water could seep into.

Deciding to drill holes and run screws through the siding and moisture barrier is a personal decision: it will void your siding's warranty and if a waterproof/weatherproof seal is not created, it could create serious problems. I was able to create a strong seal by first cutting the mounting brackets to complement the shape of the siding.

Create a paper cutout of the shape that the mounting brackets will be cut to by squashing molding clay against the side of the house where the brackets will sit (1" down from the bottom of the windowsill, 18" long). Then the clay can be laid on paper and the shape of the siding traced and cut. When measuning where the mounting brackets will go (and this determining how wide the window box will be), the end brackets should be slightly wider on both sides than the end of the window and the window box itself 3-6" wider than the brackets on both sides.

Next, the location of the mounting brackets has to be identified. By using a stud finder on the inside walls, I found double-width studs on either side of the window. I decided to put a full mounting bracket with 6" screws going from the outside into the studs in both of those locations.

I believe metal studs are used underneath large windows to support their weight, so while I decided to add 2 smaller mounting brackets in the middle of the windowbox, I used smaller screws (3") and only affixed those to the wood furring. The majority of the weight is supported by the end brackets with the middle brackets being used for additional support and to prevent sagging.

The size of the mounting brackets and shelf supports depend on the size of the windowbox. Mine was built using 8" boards (8x8) which is relatively thin and narrow, but helps to balance the weight of the 9' length. 12" wide and 16" tall allow plants' roots to sink in; however, they are for much shorter boxes. With the right irrifation system, discussed later, and lots of water every day, 8"x8" can be as successful as 12"x16" in producing bright healthy plants over a full season.

18" brackets were used on either side (8" at the top to cover the 8" height of the window box and 10" at the bottom to cover the 10" shelf supports I bought). The middle brackets were just 8" with no shelf supports. The shelf supports I used were 8" deep, to support the 8" depth of the window box.

Cutting the mounting brackets to complement the shape of the siding took about 30 minutes per bracket with a jigsaw. After tracing the paper outline on both sides of the pressure treated 2x4s, I clamped them securely to a bench and used a 2" deep fast jigsaw blade to cut through the 2x4 from each side.

The brackets and shelf supports (which were not pressure treated) were then primed and painted twice. The shelf supports were screwed onto the brackets using 1-1/4" exterior screws (not the interior screws they came with). 2 holes in the top and bottom of each bracket for the screws were made and countersunk.

To secure them to the house, the exact location of the studs has to be translated to the outside. While a 3/16" pilot hole can be made in the siding, the 6" Deck Mate screws are best when no pilot hole is made in the studs. Once the pilot hole has been made, I squirted a few seconds' worth of expanding foam into the hole as an extra weatherproofing step. Then the mounting brackets were lined against the side of the house and the screws driven all the way in.

The final step is to caulk all around the seal between the bracket and the house as well as around any screw heads using exterior silicone caulking.

(2) Build the windowbox

This is the easy part. As mentioned above, I built it to be 3-6" wider on both ends than the mounting brackets.

4 1"x8" (9' long) pressure treated planks were primed and then painted white on one side. One of the planks was cut into 2 7.75" lengths that were used as end pieces. They were glued together using wood glue and held in place with C-clamps and many 1-1/2" galvanized steel nails every 6" along all joints. A final coat of paint was added to the assembled box.

For a shorter window box, I would have considered making it deeper: 11" wide and 16" tall to allow the soil to retain water and let the roots sink in. For one this long, it had to be balanced against excessive weight and overall size. An irrigation system was critical to keep the plants watered enough.

I covered the inside with 4mm thick clear polyethylene sheeting, nailed in place. Even though the wood is pressure treated, it's good toavoid extended contact with water and wet soil. Every 12 inches, I drilled 3/16" holes into the base for drainage. I plugged them with 3/16" metal grommets, such as are used in boat sails, to keep the sheeting sealed to the wood at the those spots. It worked OK - ideally longer grommets would have been better. Finally, a 3" crumpled stretch of burlap was stuffed between the reservoirs and the sides to cover the holes to prevent soil from dripping through them.

(3) Add an irrigation system

Window boxes need a lot of water. Because they are elevated and in a container, they dry out much faster than plants in the ground. This is especially critical for the root system as the roots will extend down to find groundwater. There is no danger of overwatering windowboxes, so they should be soaked daily until they overrun. In the parts list supplied, half of the cost of the entire window box is the irrigation system, but it is a critical component.

Drill holes slightly less that 1" wide into the reservoirs for the tubing to fit into. The original open top of the reservoir pipes were then sealed with a strip of plastic sheeting and rubber bands.

(4) Add timer-based watering

From the faucet, a Garden Hose Digital Water Timer ($30) connects to a standard garden hose that runs directly under the window box. It connects to a Garden Hose to 1/2" MIP adapter and then sections of 1/2" (inner dimension; ~7/8" outer dimension) PVC pipe run into the window box. There a section extends over top of the window box before being capped. From it, 3 1/2" holes were drilled into the underside and 1/2" brass threaded nipples screwed in. 3/4" rubber tubings (1/2" inner dimension) fit snugly on them and then run in 2-4" sections into the main tubes that run to the irrigation tanks (those 1" external tubes are 3/4" internal).

The timer is set to water for 2-3 minutes every 12 hours, at sunrise and sunset. The same exterior paint used on the window box helps to make the pipes less noticeable.

Parts list / bill of materials

Total cost (not including tools) $450


2x4 pressure treated, 8' long for mounting brackets (cut into 2x18" and 2x8" long sections)$10
1x8 pressure treated, 9' long x4 for the windowbox itself $7 x4
8" wide wood shelf bracket (like Mural 10" high link ) x2 $8 x2

Screws and nails

6" deck screws (e.g. FastenMaster HeadLok) (12 pack) for affixing the mounting brackets to studs $13
3" steel exterior wood screws (e.g. Deck Mate #9 x 3") (1 lb) for affixing the mounting brackets to wood furring only $10
1-1/4" exterior screws (e.g. Grip-Rite #6 x 1-1/4") (1 lb) for affixing the shelf supports to the mounting brackets $10
1-5/8" exterior screws (e.g. Grip-Rite #6 x 1-5/8") (1 lb) for affixing the windowbox to the mounting brackets $10
1-1/2" galvanized steel nails (e.g. Grip-Rite #15 x 1-1/2") (1 lb) for building the windowbox $5

Paint, caulking, and measuring

Exterior water-based primer and sealer (1 gallon) $10
Exterior semi-gloss ultra pure white exterior (1 gallon) $30
Exterior paintable silicone caulking (e.g. GE XST Silicone II) $7
Sandpaper (coarse) $5
Paint roller covers (3 pack) $9
Great Stuff gaps and cracks insulating expanding foam $5
Molding clay (like Sargent 1 lb link ) $8

Irrigation system and water-proofing

3x 32" x 6"W x 2"H reservoir (like Hooks and Lattice link) $200
1" external - 3/4" internal clear vinyl tubing (20 ft) $35
2" galvanized steel nails (e.g. Grip-Rite 2") (1 lb) $5
4mm thick clear polyethylene sheeting (3'x20') $10
3/16" metal grommets (e.g. from Sailrite, as used in boat sails) $2


2x 40lb bags potting soil $10
Burlap (3"x10') $5
Miracle Gro (5 lb) $10

Tools, bits, and blades

Jig saw that can do 2"+ cuts, 4+ amps (e.g. Makita 4329 link ) $70
10" compound miter saw (e.g. Ryobi TS1343L ) $120
Corded drill, 8+ amps (e.g. Milwaukee 1/2 in. Magnum Drill link ) $120
Cordless drill, 18V (e.g. Dewalt link ) $100
3/4" countersink drill bit $10
Fast cutting wood jig saw blades $7

Planting flowers

8" wide box can take really just 1 row at a time, though plants can be staggered every 6 inches: tall in back, low hanging spillers in front.

I water them daily with a 2 gallon watering can (2 gallons for each of the 3 reservoirs). Before adding the timer-based watering, I used a Pennzoil 1 quart metal funnel that has a 12" hose - the hose fits into the plastic tubing and 2 gallons empties in a few seconds. Every 2-3 weeks for the first 1-2 months, adding Miracle Gro fertilizer to the water appears to promote growth.

2012 - Spring/Summer



2013 - Spring/Summer


2015 - Summer


2016 - Summer



Your Comments

Nice work. Looks good.

-- joe fukuto, Aug 5, 2013
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All text copyright © 2012 Tim Darling.