|
In this section we work our way out of the ground and into areas where the progress of construction can begin to be appreciated. It is most important that foundations are built accurately and carefully. Materials The materials will vary according to the type of foundation that's to be built. We will limit our discussion to poured footings and concrete-block walls, since we are recommending a crawl space for your extension rather than a full basement wall. The list of materials follows. For footings:
For concrete-block walls:
Tools: The tools required for foundation work are quite varied. Many of them have already been used for site preparation. In addition, masonry and carpentry tools will be required.
In order to keep the following discussion relatively simple, we aren't including an explanation on the construction of stepped foundations. The following discussion applies to houses with a crawl space or slab on grade where the new foundations will naturally align with the old ones. All foundation work must be coordinated with the floor framing plan. Be sure you know exactly the heights of each wall and pier. Foundation work starts by reestablishing the building lines on the batter boards. This is done by placing the mason’s line in the grooves cut in the batter boards and on the nails on the existing building. A plumb bob is dropped at the intersection of the batter boards to locate the building corner (which is also the foundation-wall corner), and a stake is driven at that exact location as a marker. When all corners are located, the extension dimensions are checked against the distances between the stakes to make sure every thing is still as accurate as when originally measured prior to excavation. The next step is to locate and build the footing formwork. === MIXING MORTAR: Mortar is a mixture of cement, lime, sand, and water. There are various types of mortar with different material contents, such as straight-lime mortar or straight-cement mortar. A very popular type is known as masonry cement mortar. This is a mixture of masonry cement which is manufactured for mixing only with sand and water, thus minimizing the variables in the mixing process. The recommended proportions for mixtures vary with the function of the masonry; for example, mortar mixtures to be used for foundation walls vary from those for non-load-bearing partition walls. For this reason, you should always consult the recommendations made by organizations such as the Portland Cement Association or by the local building code (which often specifies the mortar proportions for the various localities). The most widely used proportions are one part masonry cement to three parts of sand (1:3). When using Portland cement, the proportions are generally one part Portland cement to one-fourth part hydrated lime to three parts sand (1:1/4:3). The water recommendation is trickier since water increases the workability of the mixture but decreases its strength. The water content shouldn't be such that the mortar slides off the trowel when picked up or falls off the ends of the brick when laid up. The consistency of the final mixture shouldn't be too soft or too stiff. Mortar can be mixed by hand or with a mixer. The power mixer saves time and effort. When using a power mixer, the procedure starts by adding a little water to the drum. Next, about a third of the sand is added, and finally all the cement (and lime if required). While the machine turns, the remainder of the sand and the water is added until the right consistency is obtained. With all the ingredients in, the mixture continues to be mixed for a few more minutes. To mix mortar by hand, about half of the sand is placed in a metal mortar box. The cement is laid over the sand (and the lime if required). The rest of the sand is then added and the dry ingredients are thoroughly mixed together until the mass is even in color. Water is added by making an indentation in the center of the pile of dry ingredients, pouring water into it, and mixing until the desired plasticity is obtained. Mortar must be constantly mixed fresh. If it's more than two or three hours old (depending on the weather), it becomes hard to handle and must be discarded. === FOOTINGS Building the Footing Forms—Outside Perimeter Footing forms can be trenches dug in the soil or built-up wood forms. Pouring footings in a trench is possible if the soil is firm and capable of holding a sharp cut. In loose soils, such as sand, gravel, etc., this method can't be used. Footings must be poured on undisturbed earth. If the ground has been over-excavated, the footing should be made thicker. Watch for any loose fill at locations that may have previously been backfilled, such as near the existing foundations. A labor-saving approach is to make the foundation-wall height a multiple of concrete-block courses (including the joints). The depth of the footing can be increased as necessary so that the wall will be made up of the desired number of blocks. E.g., for the top of the foundation wall to be level with the first floor, it needs to be 4’-2” high. Concrete blocks are 8” high (including the thickness of the mortar joint). Six courses of concrete block will result in a 4’ wall, leaving 2” to be built up. By increasing the depth of the footing by 2”, you will have an even concrete-block module to work with. Instead of making the footing 10” deep, increase its depth to 12”. The stakes just driven represent the corners of the foundation walls. Footings, however, project a distance outside these walls, and their location has to be established. The extension-corner stakes are used as reference points. Let us assume that the extension has foundation walls that are 10” thick and 4’-0” high resting on footings 12” high (using the above example) and 20” wide. From one corner stake (we’ll call it stake A) measure the distance the footing projects outward from the foundation walls—that is, 5”—and add to this dimension the thickness of the form board you are using—that is, 1 1/2” (a 2” board is actually 1 1/2” thick). Be sure to make this measurement (6 1/2”) in both outward directions at each corner and drive a stake at each. Repeat this procedure at all corners. Follow this step by marking the height of the footing on one of the outside stakes—that is, 12” —and securing one end of the line to that level. Repeat this at a second stake (stake B) and stretch the line tightly between them, making sure to check for levelness with a line level. Continue this process until all stakes are joined, delineating the perimeter of the footings. Intermediate stakes are then driven a fraction of an inch away from the stretched line. These stakes should be spaced no more than 2½’ to 3 1/2’ apart. (Concrete is heavy. It is better to have an over-structured form than one that collapses during the pour.) When all stakes are driven, the mason’s line can be removed. It is time to nail the form boards to the stakes. (Make sure that the top of the board is in line with the previously marked height.) Nail one form board to the inside of the end stake and fasten the other end to an intermediate stake. Check the top of the form board for levelness with the carpenter’s level. Butt a second board against the first and repeat the same procedure. All the remaining boards are attached in this fashion until the outside perimeter is completed. Again check for levelness. Once all the boards are nailed, cut the stake tops flush with the top of the forms. This is done to facilitate the leveling of the top of the footing when the concrete is poured. Building the Footing Forms—Inside Perimeter To find the exact location of the inside footing form, measure the width of the footing—that is, 20”—and add to this dimension the thickness of the form material—that is, 1 1/2”. Measure 21 1/2” in from the outside perimeter forms and drive a stake where the side and end measurements intersect. The steps that follow are similar to those described for exterior-perimeter formwork. Measure and mark the footing height at an end stake, stretch a line between two corner stakes, locate intermediate stakes, and nail the boards. Using a carpenter’s level, make sure that the inner and outer perimeter forms are level with one an other. The final product should look like a U. Preparing to Pour There are two remaining tasks before pouring the concrete: first, preparing the ground surface to receive the concrete, and second, placing steel reinforcement bars in the formwork or trench. (These procedures must be done in that order. Once the steel bars are in place it's impossible to successfully prepare the ground.) Steel reinforcement bars aren't always used for house footings. Unless specifically required by the local building code, footing reinforcement is optional. We recommend its use, however, as reinforcement provides insurance against settling problems. Immediately before pouring the concrete, wet the base of the footing forms with a garden hose. Be sure not to overwater the forms and see to it that there are no remaining puddles when the concrete is poured. Wet down the soil bed (or, for that matter, any surfaces that come in contact with wet concrete or cement) to prevent the dry soil from absorbing the moisture in the concrete, which would cause it to lose strength. Tamp (press) the earth down firmly with a square shovel and wet again. Reinforcing bars are usually placed in the lower half of the footing. They must be held up in some way at their final positions (2” from the bottom of the trench) until the concrete is poured. This temporary support may take a variety of forms. The most professional support is a small metal device called a chair. A more primitive way of accomplishing the same end is to support the bars on small, thin pieces of masonry such as concrete block or brick. The recommended diameter of reinforcing bars is 1/2”, unless otherwise specified by local codes. When you have to join two lengths of reinforcement, the bars should overlap about 20” (40 times the diameter of the bar). It is crucial that the bars are solidly joined where they overlap. Tie them securely together with wire. You are now ready to pour. Find out whether the work must be inspected by a building official before pouring; if so, schedule a date with an inspector. When planning the date of the pour, keep two things in mind. First, you will not be able to continue with the foundation work for a couple of days after the footing is poured. The mixture needs sufficient time to set before the foundation walls can be started. Second, plan to pour in weather that's neither too hot nor too cold, since extreme temperatures can alter the set ting process of concrete. Once these two factors have been taken into account, you can call the ready—mix company and have them deliver the quantity and quality of concrete needed. A few tips on concrete pouring: Concrete is difficult to work with because it sets so fast. Have all the formwork ready before the truck arrives. Should the forms be located in such a way that the truck can't reach them all, have one or two wheelbarrows, planks, and extra people available. The concrete can be poured from the wheelbarrow into the forms before it sets. The concrete is “worked” while the wheelbarrow goes to get an other load. (Some ready-mix companies have a pumper which hooks on to a concrete truck and has a long hose that can deliver concrete up to 50’ or more away from the truck.)* “Working” concrete is the term used to describe the process of helping the mixture compact itself and preventing pockets or voids from forming. With the aid of a flat hoe or a spade, move the mixture at the center of the forms (the sides can be handled by lightly tapping the forms with a hammer). Be careful not to work the concrete excessively since this will tend to separate the materials within the mixture. Never add water to concrete to make it more workable after the initial mix. The water will seriously affect the strength of the concrete. *Be sure that the concrete isn't poured so that it “free-falls” more than 3’ or 4’. It is important that the concrete be evenly distributed within the forms. This is done by pouring the mixture at several points and spreading it to where it's needed with a garden rake or a hoe. Work the concrete into the forms by using a pointed trowel, moving it around to fill any depressions and get rid of any excess. To help the mixture flow into gaps and compact itself, tap the sides of the forms lightly with a hammer. The concrete at the top of the footing is leveled with a screed board (this can be any straight piece of wood that's longer than the footing is wide). With a person positioned on either side of the footing, move the board along the top of the form. To create a tie between the footing and the concrete-block wall, vertical reinforcing bars are used in the footings. These bars are placed at regular intervals (usually 2’, but they vary according to code). The steel is inserted in the footing forms while the concrete is still plastic. FOUNDATIONS Erecting Concrete-Block Foundation Walls Construction begins by reestablishing the building corners and snapping a chalk line between them on the footing as a guide to locate the first block course. In order to visualize any shifting or cutting that might be needed, set one row of blocks without mortar all around the footings. As you are doing this, keep in mind that the joint thickness is approximately 1/8”. Most likely, you will find that some blocks need to be cut. Reposition the blocks until they successfully fit the wall length and mark the joint location on the footing with chalk. Then remove the blocks, leaving only the chalk mark behind. The most important step is to carefully place the corner blocks. It’s essential that these blocks be positioned correctly since they will act as a guide for the remaining wall construction. Lay a full mortar bed at the footing corners. Take one concrete block, stand it on end, and apply mortar to the end surface. Picking it up by the webs, set it on the mortar bed. To ensure levelness, place a carpenter’s level over the block and tap the block lightly with the trowel handle until the level’s bubble is centered. Also see to it that the block is plumb and that it's directly at the building corner. Lay the remaining corner blocks in the same manner. To check for squareness, measure the diagonal distances between corner blocks. They should be equal. If they aren't , adjust the corners accordingly. Most likely, relocating a corner block that might not be directly under the string intersection will be the solution. *A story pole is a straight piece of wood used to check or transfer key dimensions easily. === === LAYING CONCRETE BLOCK: The first course of block will be set directly over the footing and should rest on a full bed of mortar to ensure complete bonding. The second course will be set over block, and instead of being laid in a full bed, it's laid in a face shell bedding. This means that mortar is applied only to both edges of the block, leaving the core voids and webs undisturbed (below). Blocks must be bonded to each other not only by horizontal but also by vertical edges is referred to as buttering. The buttered block is placed in position (with the thick-webbed side facing up) by pushing downward and forward into the mortar bed (below). The top block should be level with the mason’s line. Should the block be much too high over the line, either there is too much mortar or the mixture is too stiff. On the other hand, if the mixture gives too quickly and the block sinks in, the cause is probably too much water. After a few blocks have mortar joints. Applying mortar to the vertical been placed, the correct alignment is checked with a level or straightedge. The excess mortar squeezed out of the side should be cut off (upward motion). The last step in laying block is the tooling of the, joints. This means compressing the mortar squeezed out of the joints tightly back into the joints and taking off the excess. Since tooling isn't only for appearance but also for weather-tightness, a concave joint is generally used. === === The second stage of construction is to build up the corners of the wall by laying three or four courses of block in half lap. With the aid of a carpenter’s level, make sure that the corners are plumb, level, and aligned. A story or course pole* might come in handy for checking whether the tops of the masonry courses are at the planned height. If they don’t line up, the joints might have become excessively thick or thin. Every built-up corner must be the same height. Once all corners are ready, the side walls are constructed with the aid of both a mason’s line and the chalk line a! ready drawn on the footing. Secure the mason’s line to one built-up corner with a clip attached to the block (be careful that it's not attached to the mortar joint) and stretch it tightly between two of them. This line serves as a guide to leveling the top edge of the block. (If the line is left overnight, watch out for sagging due to moisture.) The placement of the block on the line is gradually worked out by each individual. A good start, however, is to tip the block toward you so that you can see the edge of the block below. This way you can line it up with the new block by eye. Don’t forget that every other course should be provided with horizontal joint reinforcement. The last course is made up of regular block with the cavities filled with concrete. To prevent the concrete being poured in the cores from falling all the way to the footing, a piece of metal lath is placed two courses before the last. Anchor bolts which go through two courses (tying the foundation wall to the extension’s structure above- ground) are inserted in the filled block cores at approximately every 4’ while the concrete is still plastic. They extend down into the foundation wall about the depth of two courses of block and sufficiently above the top course to go completely through the sill (about 3”). Do not install anchor bolts where the plan calls for doors, sliding doors, posts, etc. Try to place them at the sides of openings. Any openings for access into the crawl space, vents, pipes, sewer, girders, etc., should be planned out before construction begins by means of a scaled drawing. This advance planning will save you block cutting (which isn't easy). Both the heights and the widths of all openings should occur in places that are multiples of 8” (assuming you are using 8” x 8” X 16” concrete block). The vents themselves should be multiples of 8” to fit these openings. . Girder pockets can easily be provided by using a face block (which is only 4” wide) and a half or special-cut block to fit around the girder and make the wall solid. Since the girder transfers heavy loads to the wall at that particular point, it's important that the cores of all the blocks under the girder be filled with concrete. The easiest way to bridge the gap left above vent openings is to use precast reinforced- concrete lintels. These lintels are available where you buy the blocks. Steel angles can also be used. They require periodic painting; otherwise, they will rust and fail. CONCRETE BLOCK PATTERNS: The two most common bonding patterns in block construction are the stack bond and the half lap. The stack bond is simply piling blocks directly on each other with all the vertical joints in line. Because of vertical joint alignment, this bond isn't very strong and requires special reinforcement. The half lap, on the other hand, is more popular because of its added strength. Blocks are alternately placed at each other’s midspan, staggering the joints and creating a stronger wall. Pier Foundations You may need to use intermediary piers if the extension width is more than 16’, or you may have chosen to build the extension entirely on piers. The two simplest methods of construction are with concrete blocks and with cardboard tubes. CONCRETE BLOCK: If you choose this approach, footing pads are located and poured as described earlier. While the concrete is still plastic, four reinforcing bars are set in. Once the concrete sets, a double row of blocks is constructed over the footing (the reinforcing bars go through the cores in the block). The blocks are set in a full bed of mortar (the sides with the small openings facing up). After laying a row, check for levelness before proceeding to the next course. Make sure all piers are the required height. Place the anchor bolts and fill all the cores with concrete. CARDBOARD TUBES: In this method, the footings are poured and the vertical steel reinforcing bars placed in the footing while the concrete is still plastic. In order to hold these reinforcing bars in place, metal ties are wrapped around them. The pier reinforcement is then fabricated and tied to that projecting from the footing. Cardboard tubes (which act as the formwork) are then placed over the footing and the reinforcement. Stakes are driven into the ground and diagonal wood bracing is installed around the tube to hold it in place during the pour. A wood collar is placed on top of the tube to make it rigid. Concrete is then poured, vibrated, and the anchor bolts are set in place while the concrete is still plastic. When the concrete is fully set, the cardboard tubing can be peeled away. Steel brackets which act as the connection between the pier and the girders are attached to the anchor bolts. BACKFILLING The next step to complete the foundation work is backfilling. Before backfilling the excavation, however, the drainage tile should be installed. It lies directly adjacent to the bottom of the footing and follows the perimeter of the extension until it eventually connects to either the existing drain age tile or a separate dry well, storm sewer, or other means of water disposal. The tile should slope approximately 1/4” or 1/2” per 10’ of distance. Tiles are placed end to end, leaving only a small gap between them to allow the water to seep in. The gap is covered with tarpaper to avoid clogging by small stones and silt. Gravel or crushed stone is placed all around the tile. Perforated black plastic pipe may be used in place of drainage tile. You are now ready to backfill the excavation and level the inside of the crawl space. Here are a couple of pointers. Be careful of increased earth pressure against a tall un-braced wall. The pressure could knock down the wall. Since you are dealing with a crawl space, the lower wall shouldn't create as much of a problem. You should still be careful. Another piece of advice: don’t make the mistake of backfilling the excavation with an expansive soil such as clay or with topsoil. It hurts the drainage. If you are building the extension in termite country, call in an exterminating company once the foundation work and backfilling have been completed. Next: Constructing and Modifying the
Floors |
