CADD TECHNOLOGIES CADA 1210 – Single Family Residential Roof and Floor Design The following guidelines will assist in the design of roof and floor systems. They are generalized so they must be adapted to the floorplan and foundation design specifics as required. Floor and Roof design is best to be thought of as structural design puzzles. We start from the top down to ensure that load (weight ) and structure work with gravity. Remember, safety is the first order of importance in any structure so we design based on generally accepted practice for simple puzzles but may require the input from an Engineer for not-typical (complex) solutions. We start with the roof design. Roof Design: 1. Roof trusses, beam and girder components are designed to be end bearing. Roof truss assemblies can be comprised of many pieces and are pre-engineered specifically for each project. The basic design principle uses the outside walls of the house as bearing walls and the design layout is a rectangle. All parts of the main assembly (the rectangle must be supported. Any section unsupported will require either beams or a girder truss to support that section. We begin by inserting the largest rectangle that fits inside the floorplan on the bearing walls, this becomes the main assembly and will assist in supporting all other components of the completed roof system. 2. Determine the roof style and draw the roof components as required. a. Ridge b. Hip Ridge c. Valleys d. Overhang (2’ is typical, offset the original rec. by the overhang) 3. Roof trusses will run between the shortest span of the rectangle, this is called the layout, and are typically spaced 2’ apart on centre (O.C.) 4. If the roof style is a hip, you must install a Girder Truss 8’ in from the hip along the layout (both sides) 5. If the rectangle has portions not supported by a bearing wall you must install a Girder or beam along the unsupported length. This girder will run perpendicular to the trusses it is supporting and must be supported at its ends by a bearing wall or beam. 6. Repeat this process for the next largest rectangle until the roof area has been completely described. CADD TECHNOLOGIES CADA 1210 – Single Family Residential Roof and Floor Design 7. Multiple storey roof systems require that we keep in mind a few other factors such as: a. Do these lower roofs extend up to meet upper level roof areas? b. Do these lower level roofs extend only up to upper level walls? c. Expect to make some adjustments as we develop the sections and elevations which may alter the relationship between the house and garage etc. 8. Erase those ridge, valley etc. lines that are redundant to create the finished roof design. Floor Design: 1. Floor systems are very similar to roof systems with one significant difference; roof truss layouts commonly run perpendicular to each other to accommodate the roof design. Floor joist layouts always run in the same direction to accommodate efficient beam design and mechanical, electrical, etc. installations. The exception is in framing out a cantilevered floor section. 2. Beams, or a system of beams, are used together with bearing walls to support the floor system. Beams are supported by a system of intermittent supports of posts and concrete footings as required by the BCBC design tables. 3. We must design the beam layout before we design the floor joist layout. As a rule – Beams run long. Our most common dimensional lumber joist layout uses 2x10 No.2 SPF and can run a maximum clear span of 13’-8” between supports. As a “rule of thumb” we use this as our preferred maximum spacing of beams. However, the use of Engineered Wood Products (EWP) in floor joists has produced systems that work far better then dimensional lumber with much improved maximum clear span potentials. We shall use the TJI brand and span tables as to look at design options. 4. We start by using the simplest possible method, divide the floor space into equal sections with main beams. Main beams are still typically a Built Up Beam (BUB) made from laminated 2x12 Douglas Fir dimensional lumber. The Beam Span Tables in the BCBC will help us to determine the right beam size for our design. Unless the building is very small, all main beams will require intermittent supports as indicated in these tables. Joists and beams are end bearing, therefore, any unsupported length will require the installation of a beam for support (such as a stair opening in the floor) CADD TECHNOLOGIES CADA 1210 – Single Family Residential Roof and Floor Design 5. After we have completed our initial design we can review the information to determine if some modifications may create a more efficient design. 6. Using our joist span tables, determine the correct joist size to accommodate the layout. 7. Using the beam span table, determine the most appropriate beam size to accommodate the layout. a. You must be able to calculate the “supported joist length.” b. Beam sizing will determine how many support locations will be required under the beam. We increase the size of the beam on occasion if doing so will reduce the number of supports required. As before, we use the simplest method first by dividing the overall beam length into equal segments. We can modify these support locations to accommodate other features as long as they remain within the maximum spacing the tables provide for. A multi-level home will require that we repeat the floor design process for each floor. Things to keep in mind: 1. Beams, girders and multi-ply girders all create point loads. All point loads need to be supported by a post or foundation. Point loads must be carried through each floor of the dwelling to the foundation. 2. The type of joist and its installation technique will change the span that it can carry. This may affect where and how many beams are required to support it. 3. The cavities created by the joists are used to run plumbing, heating and electrical so we need to see them as part of a system. 4. Become familiar with span tables and how to read and interpret them.
