Some of the work in a basement conversion is similar to any other type of home-improvement project: plumbing, wiring, wallboard, finish work, flooring, painting. For techniques and more information about these skills, refer to web guides or books on specific home-improvement projects.
Whether you hire a contractor to do all the construction or do some or all of the work yourself, there are some general principles of basement construction that are helpful to keep in mind.
- If new or rebuilt stairs are required, rough them in as soon as possible. Add the hardwood for the treads or lay the carpeting only at the end of the project.
- If you are installing a large window, delay closing up the opening until most of the work is completed. Use the opening for access during construction.
- If there is no door at the top of a stairwell, seal the opening with a tarp or plastic sheeting to control dust.
Preventing moisture problems
A basement should be completely dry before you convert it into living space. Although basement moisture problems are common, they are not inevitable if a basement is built properly in the first place. You can't rebuild your basement to solve moisture problems, but you may be able to make some improvements. It will be helpful to review the following techniques used in new construction to ensure a dry basement.
- Install a waterproof membrane underneath the basement floor.
- Apply a thick coat of water-proofing material on the outside of the foundation walls.
- Lay drainage pipes near the footing to collect subsurface water and discharge it far from the house.
- Place crushed rock or plastic drain mats against the wall to divert water down to the pipe.
- Compact backfill around the drain rock or drain mat.
- Extend downspouts to divert runoff water from the roof at least 10 feet away from the house.
- Grade the soil around the house so it slopes away for at least 6 feet, ½ inch per foot.
All these techniques are preventive measures: They keep water from reaching the basement walls and floor. Preventive measures are also the best way to cure moisture problems in an existing basement, al though superficial damp-proofing, such as coating the inside surface of the floor and walls with a sealing agent, may be enough in some cases. The first step is to find out where the moisture comes from.
Diagnosing and curing water problems
With some detective work you should be able to find the exact cause of basement moisture problems, al though the process may take a few weeks, or even several months. Following are remedies for some of the most likely culprits.
Surface water: If the grading around the house does not slope away properly, water can collect near the foundation. It eventually seeps into the basement, most noticeably after rainstorms. This problem can develop suddenly due to settlement of the soil, especially in homes 40 to 50 years old. Walks and patios next to the house may also settle over time and slope toward the foundation.
The remedy for this problem is to re-grade the soil around the house. It may require a small bulldozer, which you can rent if you want to do the work yourself. Where high ground surrounds part of your home, excavate a shallow depression, or swale, to create proper slope away from the house. It should drain easily to lower ground (as long as it's not the neighbors’ property). If surface runoff is severe, line the bottom of the swale with a concrete gutter to minimize ground saturation, or bury a drain me under the center of the swale. A concrete patio or walk is more difficult to re-grade. You can re-slope the surface with a special patching compound if the original concrete is sound and you paint or seal the patching material to prevent a freeze- thaw cycle from lifting it off. If the concrete is cracked from settling, which is likely, the best remedy is to remove or replace it.
Roof runoff: Gutters and down- spouts should not leak or get blocked by trapped debris. Even if they are in good condition, the water discharged from the downspouts can still seep into the basement unless it's carried at least 8 feet away from the house.
There are two ways to lead the water away from the downspout. You can use surface conduits, such as plastic pipe, a long splash block, or a flexible perforated tube to disperse the water. Alternatively you can in stall underground drainpipes to carry the water to a city storm sewer, a drywell dug in the yard, or to low ground away from the house.
If the downspouts are already connected to underground drainpipes, don't assume everything is taken care of. The drainpipes may be clogged or misaligned, causing water to back up and saturate the ground near the house. To check, observe joints in the downspouts during a rainstorm to see if water backs up, or test with a hose during dry weather. If drainpipes are clogged, clear them with a drain auger or replace them.
Window wells. A window well collects water, which will seep into the ground next to the house if it's not carried off. The most effective drain age system is a concrete floor with a drain that carries water away from the house. If the well does not have a concrete floor, you can dig a trench around the outside to a depth lower than the floor, install 4-inch-diameter drainpipe, cover the pipe with drain rock, and backfill. The drainpipe should terminate in a drywell, storm sewer, or low land.
Subsurface water: If treatments for surface runoff don't cure basement moisture problems, the cause may be permanent underground water. A high water table is a serious threat to basements, and you will need professional help to deal with it. The common procedure is to dig a trench around the foundation, waterproof the walls from the outside, install a 4-inch-diameter drainpipe at the footing, cover it with drain rock, and backfill. This is an expensive cure and may not work if the water table forces water up through the floor.
Underground water does not al ways come from a high water table. It may be seepage from higher ground, running just a few feet below the surface. If so, a simple flashing system installed in the ground may be enough to divert the water away from the basement. Dig a shallow trench along the entire length of the problem wall, 2 to 3 feet deep and 3 to 4 feet wide. Lay a sheet of 6-mu plastic or special waterproofing membrane in the trench, lapping the edge up the wall and attaching it with roof mastic or caulking. Fill in the trench, sloping the backfill away from the house at the surface.
Installing a new concrete floor, whether to gain headroom or to re place a damaged floor, is a major project. You should seek advice from an engineer, builder, or architect who specializes in basements, especially if the project involves any shoring or there are moisture problems. Because most of the work is demolition and excavation, you may want to do it yourself.
If the existing concrete floor is sound, but you would prefer a more resilient surface, you can install a wood subfloor over the concrete. This is a lot easier than installing a new concrete floor.
Lowering a basement floor
Provide temporary shoring to support main floor beams so existing columns or posts can be replaced with longer ones. Place temporary posts on both sides of each column.
Break a hole in the concrete floor at least 2 feet square and dig to 6 inches below the new floor level.
Place a wide timber, thick wood pad, or precast concrete pier on the bottom of the hole.
Place a jack column or post with screw jack or “jack pier” under it, on the pad. Jack the post up until it fits snugly against the beam.
For safety, nail cleats or metal connectors to both the post and beam.
Tighten the jack periodically to counteract compaction of the soil.
Remove old columns.
Remove concrete and excavate. Be cause of the limited access to basements, this is pick-and-shovel work. For extensive digging, consider renting an electric jackhammer with a pointed bit for concrete and a spade bit for heavy clay, a small bulldozer if your basement has a grade-level en try, and a conveyor belt.
You should also plan ahead of time how you will dispose of the dirt and broken concrete, whether hiring someone to haul it away, hauling it away yourself, or distributing it around your yard.
Outline the limits of the excavation on the old concrete floor. Do not dig closer to walls than 2 feet or you will undermine them.
Break up the old concrete with a sledge hammer or jackhammer and remove it. (Wear gloves, safety goggles, and dust mask.)
Excavate to a depth of 6 inches below the new finish floor.
Build perimeter walls. The low re taming walls will stabilize the soil supporting the foundation and pro vide a finish surface for the basement space. If they are above a certain height, usually 3 feet, they must be designed by an engineer.
Excavate a footing trench 12 inches wide and 6 inches deep.
Build vertical forms for a wall 6 inches wide to the height of the original concrete floor.
Place horizontal rebar (#4) at the top and bottom of the wall, at least 3 inches away from the soil and 1½ inches away from exposed faces. Place vertical rebar as required, usually 18 inches on center and fill the forms with concrete. Vibrate the concrete, screed it off at the top, and let it cure for five days before removing the forms.
Pour a new concrete floor: Install all underfloor plumbing or drain lines.
Lay 2 to 4 inches of sand or gravel and compact it with a power tamper. Install rigid insulation, if needed.
Using plastic at least 4 mil thick, install a moisture barrier, lapping it 2 to 3 inches up the walls. Overlap any joins in the plastic by about 6 inches. Place material against the walls for isolation joints, such as rigid insulation, beveled siding boards, or special isolation joint fillers.
Install steel reinforcing mesh; use 2-inch cement blocks (dobies) or wire “chairs” to center it in the concrete. Tie it to any rebar protruding from the wall footings.
Place, vibrate, screed, and finish the concrete to a minimum thickness of 4 inches. Cure by covering with plastic sheeting or spray-on curing compound. (Avoid using compound you will be adhering tile, vinyl, carpet, or other materials directly to the concrete floor.)
Resurfacing a concrete floor
If the floor is cracked, broken, or damaged, it's possible to resurface it. You can either level the existing floor or you can add a new floor on top of the old one.
Self-leveling underlayment com pound. If the floor is sloped and rough, but structurally sound, level it with one of several liquid mortar products available from concrete suppliers or construction materials companies. Simply mix up the com pound, pour it onto the floor, and spread it out. It levels itself and hardens to a smooth, water-resistant surface without troweling. If you want it thicker than ½ inch, add aggregate. The surface can also be feathered to blend into the original concrete floor. Always apply compound according to manufacturer’s instructions, which usually specify a primer applied at least a day before.
New structural slab. If the floor is not structurally sound and has severe cracks and sunken areas, it may be possible to pour a new concrete slab on top, providing there is enough headroom. First seek professional ad vice to determine the cause, because problems caused by unstable soils, a high water table, or shifting foundation walls require more elaborate treatments. If no such problems exist, do the following to install a new slab.
Install any new piping or plumbing drains. Add extenders to floor drains. Lay waterproof plastic membrane over the existing floor.
Place special joint fillers or ½-inch rigid insulation for isolation joints around the perimeter.
Place steel reinforcement, such as 6 by 6, #10 wire mesh. Use cement blocks or wire “chairs” to center it in the concrete.
Pour 4 inches of concrete; use the isolation joints for screeding it level. Smooth the surface and cover with polyethylene sheeting to cure.
Installing a wood subfloor
To create a more resilient surface for installing carpet, resilient tile, sheet vinyl, or wood flooring, use the following procedure for laying down a wood subfloor. It is also a good way to moistureproof a floor subject to mild dampness.
Be sure severe moisture problems are solved. Apply one coat of asphalt emulsion sealer and spread 6-mit polyethylene sheeting over it, over lapping sheets 6 inches. Lap edges up the wall 6 inches; hold with tape.
Use a powder-actuated fastening tool (stud driver) to attach pressure- treated 1 by 3 or 1 by 4 sleepers to the concrete, 16 inches on center. (Follow safety precautions carefully for using the fastening tool.) Space nails 12 to 16 inches apart. Level the sleepers with cedar shims.
If insulation is needed, lay ¾-inch thick panels of extruded polystyrene between the sleepers. (For thicker insulation use 2 by 4 sleepers.) In very cold climates glue foam panels directly over the asphalt sealer with #11 mastic and install the sleepers over them.
Attach 5/8-inch tongue-and-groove plywood or ¾-inch particle board (underlayment grade) to the sleepers with construction adhesive and 1½-inch ring-shank nails. Leave a 1 gap between panels and 1/2 inch around the walls. Trim the plastic vapor barrier flush with the floor.
Alternative method. If you don't have access to a stud driver, set the sleepers in mastic.
Seal the floor with asphalt emulsion and spread a layer of #11 asphalt mastic over it.
Install polyethylene sheeting, as described above.
Install pressure-treated sleepers, 16 inches on center. Use mastic to adhere them to the plastic. Attach plywood subfloor, as above.
88 Subfloor installation: Construction adhesive; Alternative materials; Cold climate alternate
There are different ways to install a basement ceiling, depending on available height and the types of obstructions. If headroom is sufficient, install a continuous suspended ceiling. Otherwise, nail a wallboard ceiling directly to the floor joists, enclosing pipes and ducts in boxes.
The sequence of installing a wall and ceiling depends on the type of ceiling. If it's a suspended ceiling, finish the walls first. If it's a conventional wallboard ceiling attached to the floor joists, install the ceiling be fore doing the walls.
Installing a suspended ceiling
A suspended ceiling has the advantages of making utility lines accessible, overcoming sagging or uneven joists, making it easier to install light fixtures, and providing sound insulation. Components come in many colors and styles, and instructions for installation are usually included.
Be sure any wall and ceiling areas concealed by the new ceiling are insulated wherever necessary, especially the rim joist area. Insulate any cold water pipes to prevent condensation and dripping problems.
Mark level lines around the walls of the room at the height of the finished ceiling at least 7 1/2 feet above the finished floor. Use a long level, a hydrolevel, or take measurements up from the floor joists as long as they are all level.
Nail a metal molding strip around the perimeter of the basement using the level marks as guides.
Suspend main runners from the floor joists with screw eyes and wire. Space them 4 feet apart.
Fit cross-tees between the runners to complete the grid. Use the slots in the runners for proper spacing.
Slip the ceiling tiles and lighting panels into the framework. The tiles are usually 2 feet by 4 feet and are held in place by their own weight.
Installing a wallboard ceiling
A conventional wallboard ceiling can be suspended below pipes and duct work, if there is enough headroom. It does not allow access to the utility lines, but it makes a smooth, continuous ceiling possible.
Insulate, as described above. Nail 2 by 4s around the walls at the height of the finished ceiling.
Install a new 2 by 4 ceiling joist below each floor joist. Use plywood scraps or 2 by 4 cleats to connect them, spaced 24 inches apart.
Install light fixtures. Attach wall board, tape, and finish the joints.
Framing around ductwork: If you are installing a conventional ceiling of wallboard nailed directly to the floor joists, you may have to conceal ducts or utility lines by boxing them in. There should be at least 80 inches of headroom below the box.
Construct the two sides of the framework on the floor. For a long duct, build the framework in sections and splice them together after installing them. Lift them into position and nail them to the floor joists.
Nail crosspieces between the bottom plates of the sides.
Attach the wallboard and tape and finish the joints.
Finishing the walls
You can leave masonry or concrete walls exposed or cover them with special waterproof coatings or paints. There are also plaster mixes available that create a waterproof and durable textured coating. However, it's generally desirable to cover the basement walls with wallboard or paneling. The advantages are twofold: improved appearance and insulation.
If the concrete or masonry walls are in good condition, attach finish materials directly to them. If they are rough, uneven, or not plumb, build a false stud wall in front of them. This way you can use fiberglass blanket insulation, which is less expensive than rigid foam panels. This type of wall will reduce both the length and width of the room by about 1 foot.
Attaching to masonry
Make sure any moisture problems are completely solved. Also, if you live in termite country, consult with an ex terminator about applying pesticides.
Clean the walls thoroughly to re move dirt and any loose paint. Then remove any trim or moldings around windows and doors. Caulk the joint between wall and floor, and coat the wall with a masonry sealer. Use a sealer that's compatible with the insulation adhesive.
Using wood furring strips the same thickness as the rigid insulation panels, cut pieces to fit the top and base of the wall, around windows, and in corners. Apply construction adhesive to the backs of the furring strips and nail them in place using concrete nails or a stud driver (powder-actuated fastening tool).
Install all electrical wiring, then attach insulation panels horizontally, starting at the bottom. The manufacturer’s instructions usually specify applying adhesive to the panels in ribbons, sticking the panels to the wall, removing them for a few minutes, and reapplying then.
Fit the panels tightly together, staggering vertical joints. Score panels with a utility knife and snap them to make cuts.
Do not insulate over water pipes or they might freeze. Try to place loose insulation behind them. Fit the rigid panels tightly against them. Use foam insulation from a spray can to seal any openings not covered by the rigid panels.
Foam insulating panels are flammable. Keep them away from flues. Use fire-resistant fiberglass instead.
Install wallboard panels vertically. Use 1/2-inch-thick panels for a painted wall, 5/8” thick panels if you plan to cover them with wall paneling.
Apply wallboard adhesive to the backs of panels, place them in position, and nail the top and bottom with wallboard nails spaced approximately 7 inches apart.
Cover exposed wallboard edges around windows and at corners with metal edge strips.
Provide access panels for all shut off and drain valves, cleanout plugs, meters, and electrical junction boxes by cutting an opening into the wall board. Cover with a plywood panel cut slightly larger than the opening.
Tape the wallboard joints and finish the walls as desired.
Building a false wall
If it's not feasible to attach materials directly to the masonry walls, build a stud wall in front of them. This type of wall is sometimes used to conceal moisture problems, but it will only forestall, not eliminate, them. They should be solved beforehand.
Framing the wall
The wall can be framed using standard stud-wall framing techniques, with the following differences.
- Cover the foundation wall with a plastic moisture barrier stapled to the mudsill. Cut the sheets long enough so that they will lap under the sole-plate of the new wall.
- Use a single top plate. Nail the top plate to the floor joists above with 16 penny (16d) nails.
- If the foundation wall is out of plumb or not flat, use shim stock or 1 by 2 blocks behind the wall.
- If there are pipes that run along the foundation wall, attach 1 by 2 or 2 by 4 blocks to clear them.
- Use pressure-treated 2 by 4 lumber for the soleplate.
- If the floor is concrete, attach the soleplate with 2-inch concrete nails, lag screws and lead anchors, or powder-actuated fasteners 16 inches on center. If fastening the soleplate to a new subfloor, 10d nails are sufficient.
Staple fiberglass blankets between the studs. Insulate above the top plate in the space between the floor joists.
If there are pipes behind the walls, insulate behind and beside them. Do not let the insulation separate the pipes from the heated space.
Install a vapor barrier over the insulation, if required. Nail wallboard to the studs and finish as desired.
Adding or enlarging windows in a basement is usually more complicated than it's upstairs. Any opening in the foundation wall is a serious structural change that should be at tempted only with the advice of a competent architect, engineer, or builder. It is safer and easier to lower the sill rather than to widen the window. If you lower the sill the header does not have to be changed. Widening a window requires specialized tools and techniques and is not a casual weekend job. However, you may be able to excavate for the light well or perform the finishing operations if you are not able to perform all the following steps.
Shore up any floor joists that terminate above the new window.
Excavate a new light well outside the basement wall. Be sure to provide a drain and build a retaining wall for the light well.
Breach’ the concrete or masonry basement wall. Add a header the correct size for the total floor and roof area that are bearing on it. Install the header across the top of the window opening.
Install the window frame in the opening and patch around it filling any cracks with mortar. Then install the window in the frame.
Attach trim around the window or leave the edge of the frame exposed to be covered with wallboard.
Finishing a window
Basements present some unique finishing challenges for windows. The thick walls tend to make windows feel smaller. The high placement of most windows can interfere with a suspended ceiling. The outside window well has a strong impact on the view. These finishing techniques can minimize such problems.
Edge transitions. Instead of squaring off the window frame, bevel the inside edge to make a wide frame and reveal more of the window surface, Experiment carefully with the size of the bevel to avoid the mistake of calling too much attention to a small window. For a beveled edge:
- Nail or screw cant strips around the window frame. The angle will vary with site conditions, but is usually 10 to 30 degrees.
- Bevel furring strips and nail them around the window at a distance determined by the angle of the bevels.
- If you are building a false stud wall build a frame of 2 by 4s with beveled edges between the studs.
- Insulate and cover with wallboard.
Alternate detail: Instead of a beveled frame, use a stepped frame for the same effect. Frame in two or three steps using progressively thicker furring strips or stud material.
Window well: A terraced window well is much easier to build than a concrete retaining wall, is more attractive than a prefabricated steel liner, and creates opportunities for a miniature garden. Many designs are possible, including the following.
- Excavate the soil back away from the well at an angle of no more than 45 degrees.
- Install a drainpipe and a drain in the bottom of the well.
- Cover the bottom of the well with gravel or concrete.
- Build a series of small retaining walls, starting with the bottom one. Make them out of concrete blocks, bricks, timbers or wide boards, and hold them in place with pipe stakes or galvanized steel plates.
Creating an outside entrance
An outside entrance enhances any basement and is well worth the effort to build it. The most difficult step is breaching the basement wall, especially if it's reinforced concrete. In that case it's better to have the wall cut by a professional concrete sawing contractor. You can rent a roto-hammer or jackhammer to help you break through other types of walls.
Building the areaway. Although you only need enough space for a set of stairs, build as large an outside areaway as you can. It will make an extra window possible and create a more pleasant entry. A large, terraced opening creates a feeling of luxury and is not as difficult to build as an areaway with retaining walls, but if you don't have the space you can follow this procedure for a steep- walled entry.
Excavate a hole large enough for the areaway plus retaining walls, to a depth 4 inches below the foundation floor. If you plan concrete steps, leave a ramp along one edge of the hole on which to pour the steps.
Dig footing trenches for the retaining walls 16 inches wide to a depth that matches the foundation footings.
Set two horizontal rebars in the trench, at least 3 inches from the soil. Pour a concrete footing 8 inches deep in the trench. Install vertical rebar in the wet concrete if it's required for the retaining walls.
Install a drainage line around the exterior of the footing and tie it into the main foundation drain.
Build a concrete block wall to grade level or slightly higher. Start at the corners and fill in between.
Apply waterproofing to the outside surface of the block wall. Use asphalt emulsion, bentonite, a rigid membrane, or another method recommended for local conditions.
Breaching the foundation wall: Mark cutting lines on the inside of the basement wall for a hole large enough for the doorway and frame. Starting from the inside should re duce rubble in the basement.
Shore up floor joists for any opening wider than 3 feet, or as necessary.
Break through the wall with power tools, hammer and chisel, or sledge hammer. Make the edges as clean and smooth as possible.
Installing the door. Position an exterior door frame into the opening. Temporarily brace it after making sure it's plumb and level.
Fill the area between the frame and the wall with cement mortar or concrete topping before installing the door in the frame.
Building the stairs. Because of possible weather exposure and the need for a fire escape, the stairway should be concrete, masonry, or steel prefabricated for exterior use.
Concrete and masonry stairs can be incorporated into the retaining walls. Leave a wide ramp along one wall as you excavate and build the stairs on top of it, using standard forming techniques for concrete stairs. Prefabricated steel stringers are available for basement entrances; installation instructions are included.
Covering the areaway. For an all-weather entrance, cover the stairway with a hatch door or shelter. A pre fabricated greenhouse makes an attractive entry and provides extra light to the basement.
For either type of enclosure, form and pour a concrete cap on top of the block retaining walls. The final height depends on the height needed for the stairs and the type of enclosure.
Completing the project
Completing a basement conversion—building partition walls, hanging doors, finishing the walls and ceiling, installing trim and fixtures, and covering the floor—follows the same procedures as any other room. It is also the stage of construction that seems to last forever. But if your preparation is thorough and your commitment strong, you will be ready. Then you will not only have new living space for your home, but you will also have the pride and satisfaction of having taken a major part in the improvement.
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