How to Install a Terraced Window Well in Your Basement
Updated: May 01, 2023
How to build a series of low walls around your basement windows to let in more light.
Bring a flood of light into a dreary basement with a terraced window well. You also get an emergency escape route, planting beds and a view. Build this well as part of an egress window project or simply landscape an existing window well. In this article, we'll walk you through the construction process and tell you how to deal with the all-important issue of drainage.
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Design for safety
A terraced window well dramatically increases natural lighting in a lower level or basement. These egress windows satisfy Building Code rules.
The primary rules for designing a window well are the code requirements for an egress window. The bottom of an egress window well must be at least 3 x 3 ft., the well can’t interfere with opening the window, and you have to provide a permanent ladder if the walls are more than 44 in. high. Terracing our well eliminates the ladder requirement.
Otherwise, you’re only limited by safety and drainage issues. Any window well is inherently dangerous because of the potential for falls. Covered window wells are safest, but you can’t let the cover hinder the window operation and it must be easily removable (without tools) to provide easy egress and emergency escape.
Terracing minimizes the danger of a fall. We recommend that you construct the widest terraces and lowest steps that your site will allow. You could also put a railing around the egress window well, a feature we recommend if you have a walk nearby. The egress window well we’re building opens onto a spacious yard, away from walkways.
Still, we took the extra precaution of adding a curb about 8 in. above grade plus flower beds and low bushes to keep folks from stepping into it. We constructed our terraced well with 4×6 treated wood timbers because they’re long lasting, easy to cut and level, and economical. To dress up the timbers, we stained all sides before installation with an oil-based semi-transparent redwood stain.
Decorative concrete wall block is another good choice for terracing. This block will last forever, but it’s about twice as expensive as wood and harder to cut and assemble. Another option is to buy a manufactured well. These range from the corrugated steel culvert style (available in most home centers) to manufactured terraced wells.
Good drainage is critical
If your basement hasn’t suffered any moisture or flooding problems, digging a window well or expanding an existing well is unlikely to create water problems, as long as you follow the instructions in the photos. The two sizes of gravel we show provide a path for water to easily flow down into the soil without filling up the well. The landscape fabric acts as a filter to prevent soil from clogging the gravel drain field.
Soil with a high clay content, however, poses another problem. This soil tends to trap water instead of letting it drain away. For yards with clay soil or poor drainage, or for damp basements, consult a professional before undertaking the project. (Check the Yellow Pages under “Waterproofing Contractors.”)
One option to ensure good drainage is to tie the well’s drain field into the existing drainage system around the footing of your basement (Fig. A). If you don’t know if you have foundation drains, you’ll have to call the builder or the city department of inspections, or dig down to the footing at the bottom of the wall and look for a gravel bed with a plastic or other type of pipe. If you find the pipe, simply fill the hole with 3/4-in. gravel to connect the two drain fields.
Some homes have drains on the interior, under the basement floor, and rely on a sump pump to get rid of water. Connecting to this type of system is more complicated and difficult. We suggest that you leave this work to a pro.
Be sure to show your plan to your local building inspector and obtain a permit. Ask about any special requirements and discuss drainage issues. The inspector should be familiar with local soil conditions.
Step 1: Dig and prep the hole
Photo 1: Lay out the dimensions and start digging
Drive a stake for a center marker and mark an outline for each layer with spray paint. Dig each shelf about 17 in. deep and 12 in. wide (Fig. A). This will leave room for about 8 in. of gravel behind the wall. Protect your lawn with a tarp.
Photo 2: Excavate for drainage
Dig out the bottom level 12 in. deeper than the bottom of the rough window opening. Slope the bottom 1/2 in. per foot away from the foundation. Along the edge of the base farthest from the wall, dig a 12-in. wide x 6-in. deep drainage trench.
Photo 3: Add landscape fabric
Line the entire well with landscape fabric. Use fabric spikes to hold it in place and fold it into the corners to get complete coverage. The fabric acts both as a weed barrier and as a filter to keep soil from clogging your gravel drainage system.
Photo 4: Level a gravel base
Dump in a layer of 3/4-in. gravel, then add pea gravel until the surface is 6 in. below the rough window opening (Fig. A). Use a 2×4 to flatten and level the gravel. If you haven’t cut the opening yet, mark the rough opening clearly so you don’t overfill the hole.
Photos 1-4 walk you through the steps for digging the well. Fig. A shows you the exact dimensions we used, but you can easily adjust them to fit your site. Your layout doesn’t have to be precise; a stake centered on the window will be accurate enough for digging (Photo 1).
To build the size well we show here, you’ll be removing a small mountain of dirt (more than 8 cu. yds.). Save your topsoil, but get rid of the rest of the dirt by renting a 20- or 30-yd. trash container. The oversized container gives you a large base so you don’t have to pile the dirt so high.
Make sure the trash container has a gate on the back that you can swing open for the wheelbarrow. Tell the waste company that you’ll be loading dirt in the container so it’s aware of the weight involved. If you have to put the container curbside, you’ll probably have to get a permit from the police department.
We built our entire well first, then added the window. But it’s easier to cut in your egress window after you’re done with the initial digging (Photo 2). Then finish the well.
TIP: Cover the excavation during rainy periods to keep water out and keep the walls from eroding.
Before doing any digging, always call your local utility companies (Natural gas, Water, Electric, Phone and Cable) to locate buried lines.
Step 2: Set the first row of timbers
Photo 5: Cut timbers from opposite sides
Cut the 4×6 timbers with a circular saw set at an 11-degree angle. It will take two passes to complete the cut. As you cut the timbers to length, remember to adjust the length to alternate the overlap at the corners.
Photo 6: Apply preservative to cut ends
Treat the cut ends of the timbers with a wood preservative to stop rot. Then lay the wall timbers according to the plan in Fig. A.
Photo 7: Level the first row of timbers
Set the first row of timbers perfectly level on the gravel base. Be fussy. Screw the corners together. Assemble the first wall (See Photos 9 and 10) and backfill with gravel. Create a level base for each timber wall with about 6 in. of gravel (no compacting needed). After completing the first wall, use the timbers as a guide to level the base for the remaining walls.
With the bottom level and the window accurately marked, lay out your wall pattern. Then measure the lengths and angles of the timbers and cut them (Photo 5). Often, the cuts expose untreated wood, so treat raw ends after cutting (Photo 6). Use a wood preservative containing copper (available at paint stores, hardware stores or home centers). Set the first row of timbers perfectly level (Photo 7).
Step 3: Set and fasten the remaining levels
Photo 8: Position the next level
Use a framing square to establish the setback from the previous wall (12 in. on the side and 14 in. on the front). Tie the first course together (two side pieces and a front) with two 3-in. screws at the joints. Then tamp the course perfectly level with a hammer.
Photo 9: Overlap corners of each row
Set the second course in place. Make sure to alternate the joints so the timbers overlap at the corners. Screw the second course together at the corners. With a 3/8 x 12-in. drill bit, predrill the timbers for spikes at the ends and about every 3 ft.
Photo 10: Drive spikes to anchor the rows
Spike the timber courses together with 3/8 x 10-in. spikes and a 4-lb. hammer.
Photo 11: Backfill with aggregate and gravel
Backfill behind the timber walls with 3/4-in. aggregate, capping it with a 3- to 4-in. layer of pea gravel for the next wall. The larger aggregate will promote drainage.
The process for the second row is exactly the same as for the first (Photos 8 – 11). Driving the spikes takes some effort, so be sure to predrill with a 12-in. bit (Photo 9). One good reason to backfill with gravel is to avoid the need for compacting (Photo 11). Backfilling with soil would require a lot of tamping to keep it from settling over the next year or two. Complete and backfill all the levels.
Step 4: Anchor the walls at the foundation
Photo 12: Predrill for concrete anchors
Predrill 5/16-in. holes in 1-1/2 in. aluminum angle, hold it in place and predrill 3/16-in. holes into the concrete with a masonry bit.
Photo 13: Drive concrete screws
Apply a dab of caulk to 1/4-in. x 1-1/2 in. concrete screws and drive them home. Do not secure the stop to the timber wall. This allows the timbers to move slightly during freeze/thaw cycles.
Photos 12 and 13 show you how to anchor the timbers against the foundation. Once you finish the well, scatter decorative stone over the surfaces to spruce up its appearance. If you decide to add plants, confine them to pots nestled into the gravel. That way soil won’t clog up the drainage. Or encourage vines to grow over the top and down the sides. But leave an uncluttered pathway open for emergency escape. Have you heard about faux basement windows?
Required Tools for this Egress Window Well Project
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
3/8-in. drill bit, 12-in. long
5/16-in. masonry bit
Required Materials for this Egress Window Well Project
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here’s a list.
Originally Published: November 28, 2018