Home & DIY questions, answered
73 short answers from 15 guides. Each set links to the full guide for the worked examples and the reasoning behind the answer.
ADA Ramp Rules: Slope, Rise, Landings and Handrails
How long is a ramp for 3 steps?
Three ordinary steps rise about 21 inches, so at the ADA maximum slope of 1:12 the ramp needs a 21 foot horizontal run. With a 60 inch landing at the top and bottom, the straight footprint is about 31 feet. The rise is under the 30 inch limit, so a single run is enough.
How do I convert a 1:12 ramp slope to percent and degrees?
A 1:12 ramp slope rises 1 unit for every 12 units of horizontal run. As a percentage that is 100 / 12 = 8.33%, and as an angle it is the arctangent of 1/12, or 4.76 degrees. The sloped surface is slightly longer than the run, 24.08 feet for a 24 inch rise.
What does the IRC require for residential ramps?
The International Residential Code, Section R311.8, sets a maximum ramp slope of 1:12, with up to 1:8 only where site constraints make compliance technically infeasible. It requires landings at least 36 by 36 inches at the top and bottom, where doors open onto the ramp and where it changes direction, and guards where the side drops more than 30 inches.
How many landings does a 48 inch ramp need?
A 48 inch rise exceeds the ADA limit of 30 inches per run, so it needs two runs of 24 inches, each 24 feet long at 1:12, and three landings at the bottom, middle and top. With 60 inch landings that is 63 feet in a straight line, which is why many ramps use a switchback.
Does the ADA 1:12 rule apply to loading ramps?
No. Ramps for vehicles, mowers, hand trucks or shed doors are not accessibility ramps, so the ADA 1:12 rule does not apply. The limit comes from the equipment, usually a maximum angle in degrees from its manual. For example, a 36 inch tailgate at 1:6, about 9.5 degrees, needs an 18 foot horizontal run.
Attic Insulation R-Values by Climate Zone, and How to Get There
Is cellulose or fiberglass better R-value per inch?
Loose-fill cellulose gives more R-value per inch than loose-fill fiberglass: about R-3.5 per inch (settled 3.2 to 3.8) against about R-2.5 (settled 2.2 to 2.7). Reaching R-60 therefore takes about 17.1 inches of cellulose but about 24 inches of loose fiberglass. Cellulose settles noticeably, so use the settled thickness.
How do I calculate how much insulation to add to my attic?
Measure the existing insulation's depth in several places and multiply by its R per inch to get the existing R-value, then subtract it from the target. A Zone 5 attic with 3 inches of fiberglass batts has about R-9.3, so reaching R-60 needs R-50.7. Divide that by the new material's settled R per inch for the depth.
How do I reach R-60 with fiberglass batts?
Add batt layers, since R-values in series add: two layers of R-30 make R-60. Lay the first layer between the joists and the second at right angles across them, which covers the wood thermal bridge and the gaps between batts. Use unfaced batts for any layer over existing insulation to avoid a second vapour retarder.
What is the R-value of spray foam insulation per inch?
Open-cell spray polyurethane foam is about R-3.5 to 3.6 per inch and closed-cell foam about R-6 to 7, depending on the product. Spray foam is sold by the board foot, one square foot one inch thick, so 200 square feet at 2 inches is 400 board feet. Real-world kit yield is often lower than the label.
Should I air-seal the attic before adding insulation?
Yes. Gaps around top plates, plumbing and wiring penetrations, attic hatches and recessed lights let warm, moist air into the attic, and sealing them with foam or caulk often saves as much as the insulation itself. It also prevents condensation in the new layer. Keep eave baffles clear and leave clearance around chimneys and non-IC-rated lights.
Baluster Spacing and the 4 Inch Sphere Rule, With the Math
Why is the baluster spacing rule 4 inches?
The IRC's 4 inch sphere rule, in Section R312.1.3, exists because a small child's head is about 4 inches across, so openings in a required guard must not let a 4 in sphere pass. Builders aim a little under 4 in, because balusters are rarely perfectly straight and a slightly bowed one can open the gap.
How do I mark baluster spacing without errors adding up?
Hook the tape on one post and mark each baluster centre at its cumulative distance, rounding each to the nearest sixteenth, rather than stepping off the same spacing repeatedly. Stepping off 5-7/16 in seventeen times on an 8 ft rail leaves the last baluster about a third of an inch out. A story pole is even faster for identical sections.
How many balusters per stair tread?
Two balusters per tread is common: on a 10 in tread run that gives a 5 in level spacing, and deep treads may take three. Check the resulting gap against the limit for your baluster width; two 1-1/4 in balusters per 10 in tread leave 3.75 in gaps, which passes the 4 inch rule.
How tall does a deck railing have to be?
Guard height is a separate requirement from baluster spacing: 36 in for most residential guards under the IRC and 42 in under the IBC and many local codes. Handrails have their own height, grip size and continuity rules, and posts and their connections are designed to the code's load requirements.
How should I space turned or decorative balusters?
Space turned or decorative balusters on their widest part, because the sphere passes through the narrowest gap between two neighbours. For strict inspections, use a limit below 4 in: a 72 in section with 2-1/2 in turned balusters and a 3-7/8 in limit needs 11 balusters with 3.71 in gaps.
Crown Molding Angles and Miter Math: From 180/n to Compound Cuts
What is the difference between cutting crown molding nested and flat?
Cutting crown molding nested means placing it upside down on the saw with its flats against the table and fence, as installed, so a plain miter of 45 degrees suits a square corner. Cutting it flat means laying it face up on the table, which needs a compound cut: for 52/38 crown at a 90 degree corner, a 31.62 degree miter and a 33.86 degree bevel.
What is the compound miter formula for crown molding?
For crown molding cut flat with spring angle S and wall angle A, the miter is atan(sin S / tan(A/2)) and the bevel is asin(cos S x cos(A/2)). For 45/45 crown in a 90 degree corner that gives a 35.26 degree miter and a 30.00 degree bevel. These match the charts printed in miter saw manuals.
What miter angle do I need for a hexagon or octagon?
Each end of a regular polygon frame is cut at 180 divided by the number of sides on the miter saw scale. A hexagon needs 30 degrees, an octagon 22.5 degrees, a pentagon 36 degrees and a dodecagon 15 degrees. On a protractor or table saw gauge that reads 90 at square, the 30 degree hexagon setting reads 60.
What is coping crown molding and why do carpenters use it?
Coping is an alternative to mitering inside corners. One piece of trim is cut square into the corner, and the other is cut with a 45 degree miter and then back-cut along its profile with a coping saw so it fits over the first. A coped joint hides out-of-square walls and seasonal movement better than a miter does.
How do I mark a layout line for crown molding?
Work out the drop down the wall as w cos S, where w is the width of the molding's back between its two flats and S is the spring angle. A 5-1/4 in back on 38 degree crown drops 4.14 in. Snap a line at that height around the room so the run stays straight even where the ceiling wanders.
Estimating a Deck: Boards, Joists, Screws and Span Tables
How much waste should I add for deck boards?
Add about 10% waste for straight decking, 15% for diagonal decking and more for picture-frame borders. The allowance covers split ends, crooked boards and the last row, which usually gets ripped to width. On a 16 x 12 ft deck with 5-1/2 in boards and 3/16 in gaps, 26 boards plus 10% comes to 29 boards.
What is IRC Section R507?
IRC Section R507 is the part of the International Residential Code that governs deck framing in the US. It contains a deck joist span table and a deck beam span table, by species group, size and spacing, with limits on joist cantilevers past the beam. The American Wood Council's DCA 6 guide presents the same rules with details for ledgers, lateral load connectors and guards.
What if my deck is longer than the deck boards?
When a deck is longer than the stock boards, each row needs two or more pieces, and every butt joint must land on a joist, ideally a doubled one. A 20 ft deck with 16 ft boards takes 52 boards and leaves a lot of offcut, so 20 ft boards or a 10 + 10 split may be cheaper.
How many hidden deck clips do I need?
Hidden clip systems use one clip per joist crossing, roughly half the count of face screws, which take two per crossing. Count crossings as rows of boards times joists, then add starter clips for the first row and screws at the ends of the boards. Face screwing a 16 x 12 ft deck with 26 rows and 13 joists takes 676 screws.
How long should deck joists be?
Deck joists run in the projection direction, from the ledger at the house to the rim joist, usually resting on a beam near the outer edge. If the joists butt into the rim, their length is the projection less the rim joist thickness, so a 12 ft projection takes joists 12 ft less 1-1/2 in.
Estimating Concrete, Flooring, Tile and Gravel Without Running Short
How much tile waste should I allow?
Tile waste depends on the layout. Allow about 10% for a straight grid, 10 to 12% for a brick or offset pattern, 15% for a diagonal layout at 45 degrees and 15 to 20% for herringbone, because angled cuts leave triangular offcuts that rarely fit elsewhere. Round up to whole boxes and keep the leftovers, since a later batch will not match exactly.
How do I calculate the square footage of an L-shaped room?
Split the room into rectangles that do not overlap, measure each one, add them, and subtract anything you will not cover. A room made of a 12 by 15 foot rectangle and an 8 by 10 foot rectangle, with a 3 by 4 foot kitchen island, is 180 + 80 - 12 = 248 square feet. Avoid counting the inside corner twice.
How many tons of gravel are in a cubic yard?
Loose, dry gravel weighs about 2,800 lb per cubic yard, or 1.40 short tons. Crushed stone and sand are about 2,700 lb (1.35 tons), topsoil about 2,200 lb (1.10 tons) and wood mulch about 600 lb. These are typical figures rather than a standard, and wet sand and soil can weigh 10 to 20% more, so ask the supplier for their conversion.
How much mulch do I need for my garden beds?
Multiply the bed area by the depth in feet, then convert to cubic yards or bags. 200 square feet of beds at 3 inches deep needs about 2 cubic yards in bulk, or 27 bags of 2 cubic feet, since a cubic yard is 13.5 such bags. Mulch deeper than about 3 inches tends to hold moisture against stems and bark.
How do I convert square feet to square metres?
Divide square feet by about 10.76, which is 3.28 squared, or multiply by exactly 0.09290304. So 248 square feet is 23.04 square metres. Dividing an area by the length factor of 3.28 is a common mistake that gives an answer more than three times too large. A cubic yard is exactly 0.764554857984 cubic metres.
Grass Seed Rates and Sod Ordering for a New Lawn
When is the best time to plant grass seed?
It depends on the grass. Cool-season grasses such as fescues, bluegrass and ryegrass germinate best when soil is around 50 to 65 °F, which in most of the northern US means late summer to early autumn, with spring as second choice. Warm-season grasses such as bermuda, zoysia, centipede, bahia and buffalo need soil of roughly 65 to 70 °F and above, so sow them in late spring and early summer.
Can you put down too much grass seed?
Yes. Sowing grass seed at double the rate produces a crowded stand of thin seedlings that compete for light and water and are more prone to disease, and the extra seed costs money. Uneven spreading is the more common problem: split the seed in two and spread half walking north and south, the other half east and west.
Why is Kentucky bluegrass sown at a lower rate than tall fescue?
Seeding rates aim at a target number of seeds per square foot, and a tall fescue seed is many times heavier than a bluegrass seed. Kentucky bluegrass also spreads by rhizomes and fills in over time, so it can start thinner at 1 to 3 lb per 1,000 ft², while tall fescue is a bunch grass that needs a dense stand at 7 to 9 lb.
What size is a piece of sod?
Common US sod sizes are 16 by 24 inch slabs covering 2.67 square feet, 18 by 24 inch slabs covering 3 square feet, and 2 by 5 foot rolls covering 10 square feet. In the UK and Europe turf is usually sold in rolls of about 1 square metre. Pallets typically cover 400 to 500 square feet, but ask your farm.
How do I lay sod?
Prepare the soil before the sod arrives: grade it, rake it smooth and firm, and moisten it. Lay the pieces with staggered joints like brickwork, pressed tight but not overlapping, then roll and water deeply so the soil below is wet. Sod is perishable, and pallets heat up within a day on a warm day, so schedule delivery for when you can lay it.
How Many Rolls of Wallpaper? The Strip Method Explained
Why does dividing wall area by roll area underestimate wallpaper?
Area division assumes the leftover piece at the end of each roll gets used, but it is usually too short for a full drop and the pattern is in the wrong place. A room with 49 feet of 8 foot wall is 392 square feet, which area division says is 7 US double rolls, yet hanging it in 29 strips at three strips per roll needs 10.
What is the strip method for estimating wallpaper?
The strip method counts full-height drops. Divide the wall width, less doors, by the roll width and round up to get strips. Add about 4 inches or 10 cm of trim to the height and round up to whole pattern repeats for the cut length. Divide roll length by cut length, rounding down, then divide strips by strips per roll, round up and add a spare.
Why is American wallpaper priced per single roll?
American wallpaper is usually priced per single roll but sold only in double rolls, also called bolts. A "single roll" price of $40 therefore means $80 for each bolt you actually take home. A US double roll is typically 20.5 inches by 33 feet, about 56 square feet, so check the label and budget by the roll you buy.
Why do wallpaper batch numbers matter?
Wallpaper is printed in runs, and each run gets a batch or lot number. Colours can shift slightly between batches, visibly so on a wall, so buy every roll from the same batch at the same time, plus one spare. The spare covers a miscut, a damaged strip, or a repair years later when the pattern is discontinued.
How much wallpaper do I need for one accent wall?
Enter the accent wall's width directly and count strips. A 3.6 m wall takes 7 strips of 0.53 m paper. With a free match, that is two standard European rolls of 0.53 by 10.05 m.
How Many Studs Do I Need? Wall Framing Takeoffs Explained
Why are wall studs spaced 16 inches apart?
Sixteen inches divides a 48 in sheet of sheathing or drywall three times, and 24 in divides it twice, so sheet edges always land on a stud. Metric framing uses 400 or 600 mm with 1200 mm sheets for the same reason, and some regions use 450 mm. Moving from 16 to 24 in saves about a third of the layout studs.
How do I count layout studs for a wall?
Layout studs = ceil(wall length / spacing) + 1, because there is always a stud at the far end even when the last bay is short. A 12 ft wall at 16 in has 9 bays and 10 studs, and a 10 ft wall has 7.5 bays, rounded up to 8, and 9 studs. Forgetting the "+ 1" is the classic fence-post takeoff mistake.
Is one stud per foot of wall a good estimate?
Only on long, plain partitions. On a short exterior wall with windows it is badly wrong, because openings and corners add kings, jacks, cripples and corner studs. A 16 ft wall with a door, a window, two corners and an intersection needs 25 studs, or 28 with 10% waste, where the rule of thumb says 16.
Do door headers need cripple studs above them?
Not always. Cripples fill the gap between a header and the top plates, so they depend on the header's depth. In a 97-1/8 in wall with an 82-1/2 in rough opening, a 2x10 header (9-1/4 in) fills the space up to the top plates within 7/8 in, so many framers use 2x10 or 2x12 headers above doors and skip cripples. Shallow window headers still need them.
How many plates does a framed wall need?
A standard wall has one bottom plate and a double top plate, so plates total three times the wall length: a 16 ft wall needs 48 linear feet. Lay out the joints in the two top plates at least 24 in apart (IRC R602.3.2), and buy the full bottom plate length even though it is cut out at door openings after the wall is up.
How Much Drywall, Mud and Tape? Estimating a Room
How much waste should I allow for drywall?
Allow 10 to 15% waste on drywall sheets to cover the strips above doors, the pieces around windows and the odd damaged corner. Complicated rooms with soffits, angled ceilings or many small walls need the top of that range. A 540 square foot room at 10% waste needs 540 / 32 x 1.1 = 18.6, so 19 sheets of 4x8.
Should I hang drywall on the ceiling or the walls first?
Hang the ceiling first so the wall sheets support the ceiling edges, then hang the top row of wall board tight to the ceiling and the bottom row below it. Run sheets horizontally on walls up to 9 ft so the tapered edges form one long, easy joint at a comfortable height, and stagger butt joints between rows.
What is a drywall butt joint?
A butt joint is where the un-tapered ends of two drywall sheets meet, and it is the hardest joint to finish flat. Longer sheets mean fewer of them: a 14 x 12 ft bedroom takes 13 sheets of 4x12 instead of 19 of 4x8, and its 14 ft walls have one butt joint per row instead of two, which saves finishing time.
What type of drywall do bathrooms and garages need?
Bathrooms take moisture-resistant board, and showers need cement board behind tile. Fire-rated Type X drywall goes where the code calls for it, such as between an attached garage and the house. These special boards are ordered separately from standard sheets, along with corner bead, adhesive and primer.
What drywall should I use on ceilings with 24 inch framing?
Ceilings framed at 24 in centres often need 5/8 in drywall or a sag-resistant 1/2 in product, especially under a water-based texture. Screw spacing also matters: typically 12 in on centre along each framing member on ceilings, against 16 in on walls, for single-ply screw application under GA-216.
How Much Paint Do I Need? Coverage, Coats and Cans
How do I calculate the wall area of a room for painting?
Wall area is 2 x (length + width) x height, then subtract doors and windows. A 12 by 12 foot bedroom with 8 foot walls has 384 square feet of wall, and one 21 square foot door plus two 15 square foot windows bring it to 333. Floor area misleads: that room's 144 square foot floor has 384 square feet of wall around it.
What is boxing paint?
Boxing paint means pouring several cans of the same colour together into a bucket before you start, so small colour differences between cans of a custom tint do not show as a line on the wall. Buying the whole job at once and keeping the batch label helps too, since two gallons bought a week apart may not match.
Should I buy three quarts of paint or a gallon?
Buy the gallon. A quart is a quarter gallon, and three quarts almost always cost more than one gallon, so once a job would need a third quart a gallon is the better buy. The Paint Calculator applies that rule: 2.73 gallons becomes 3 gallons, while 2.40 gallons becomes 2 gallons and 2 quarts.
Is paint and primer in one a substitute for primer?
Not on bare drywall. Many "paint and primer in one" products are thicker paints that hide better, not a substitute for a real primer. New drywall and plaster absorb unevenly and can flash, showing joints through the paint, so they need a separate primer, as do stains, glossy surfaces you need to key and big colour changes.
Why do metric paint tins quote higher coverage than US cans?
A US can's 400 ft² per gallon works out to about 9.8 m² per litre, since a gallon is 3.785 litres and a square foot 0.0929 m². Metric labels often say 10 to 14 m² per litre because they tend to quote a thinner film on an ideal surface, so a tin's "12 m²/L" sounds more generous than it is.
How to Calculate Board Feet: Hardwood, Dimension Lumber and Metric
What is a board foot?
A board foot is a unit of lumber volume: a slab one inch thick, twelve inches wide and one foot long, which is 144 cubic inches. Hardwood dealers, sawmills and many lumber yards use it to sell wood by volume. A 2x6 eight feet long, for example, is 2 x 6 x 8 / 12 = 8 board feet.
Do you use nominal or actual size to calculate board feet?
Softwood board feet are calculated on the nominal size, so a 2x4 counts as 2 by 4 inches even though its dry, dressed size is 1-1/2 x 3-1/2 in. Using actual sizes understates the board feet by about a third. Hardwood is tallied on its rough thickness, so a 4/4 board planed to 13/16 in still counts as a full inch.
What is a Hoppus foot?
The Hoppus foot is a log measure based on quarter girth that appears in older British or Asian timber trade, and it is roughly 1.27 true cubic feet. It is not the same thing as a board foot, which is 144 cubic inches of sawn lumber, so the two units should never be mixed when comparing prices.
What are the Doyle and Scribner log rules?
Log rules such as Doyle, Scribner and International 1/4-inch estimate how many board feet of lumber a sawmill can cut from a log, after kerf and slabs. They give very different answers for the same log, and Doyle in particular undercounts small logs. Sawn boards are different: their volume is measured directly with the board foot formula.
How to Estimate Fence Posts, Pickets, Rails and Concrete
Does a gate add a fence post?
A gate adds exactly one post compared with an unbroken run of the same fence length, because the posts on either side of the opening are separate posts. A 100 foot run with one 4 foot gate has 96 feet of fence in 12 sections of 8 feet, so it needs 12 + 1 + 1 = 14 posts.
How wide should a fence post hole be?
A fence post hole is usually about three times the post's width in diameter, so roughly 10 to 12 inches for a 4x4, which is 3.5 inches square. A 10 inch hole 24 inches deep around a 4x4 takes about 0.92 cubic feet of concrete, which is two 80 lb bags of concrete mix.
How long should fence posts be?
A fence post's length is the fence height plus the hole depth, and holes are commonly a third to a half of the height above ground. A 6 foot fence with 2 foot holes needs 8 foot posts, a stock length. With 3 foot holes it needs 9 foot posts, which usually means buying 10 footers and cutting them down.
How many rails does a fence need?
Two rails per section are typical for fences up to about 4 feet high and three for a 6 foot privacy fence, while very tall or heavy fences sometimes get four. Total rails are sections x rails per section, so a 100 foot privacy fence with one gate, 12 sections, three rails and 5% extra needs 38 rails.
What should I do before digging fence post holes?
In the US, call 811 or the local utility locating service a few days before digging. Check property lines with a survey or plat map, and check height limits and permit rules with the building department or homeowners association, since front-yard fences are often limited to 3 or 4 feet. Set end, corner and gate posts first, then string a line.
Pool Volume and Chlorine Dosing: Gallons, ppm and Salt
What is the formula for pool volume?
Pool volume is surface area times average water depth, measured to the waterline rather than the top of the wall. For an evenly sloping floor, average depth is (shallow + deep) / 2. Multiply cubic feet by 7.4805 to get US gallons, or use 1,000 litres per cubic metre. A 16 x 32 ft pool averaging 5.75 ft holds 22,023 gallons.
How do I calculate the gallons in an oval pool?
For a true ellipse, multiply length x width x average depth x 5.9 to get US gallons, since pi / 4 x 7.48 is about 5.875. Many above-ground pools sold as ovals are really a rectangle with half circles on each end and hold more, using (length - width) x width + pi x width² / 4 for the area.
What does 1 ppm of chlorine mean in a pool?
In pool water, one part per million means one milligram per litre. Raising free chlorine by 1 ppm in 10,000 US gallons, which is 37,854 litres, needs 37.9 grams of available chlorine. Every chlorine dose follows from that number and the strength of the product, whether liquid chlorine, bleach, cal-hypo, dichlor or trichlor.
Does trichlor raise cyanuric acid in a pool?
Yes. Each trichlor molecule carries three chlorine atoms on one cyanuric acid ring, so every 1 ppm of chlorine from trichlor adds about 0.6 ppm of cyanuric acid, and dichlor adds about 0.9 ppm. A season of trichlor tablets can push cyanuric acid past 100 ppm, so test it every few weeks and switch to liquid chlorine or cal-hypo when it climbs.
Roof Pitch, Stair Layout, AC Sizing and Running Costs Explained
How many steps does a 108 inch floor-to-floor rise need?
A 108 inch rise with a 7.5 inch maximum riser needs 15 risers, because 108 / 7.5 = 14.4 rounds up to 15, giving risers of exactly 7.2 inches. There are 14 treads, one fewer than the risers, and with 10.5 inch treads the stair runs 147 inches at an angle of 34.4 degrees.
Are commercial stair rules different from residential ones?
Yes. Commercial stairs under the IBC are stricter, with a 7 inch maximum riser and an 11 inch minimum tread. Residential stairs under IRC section R311.7 allow a 7 3/4 inch maximum riser and a 10 inch minimum tread, with no more than 3/8 inch of riser variation in a flight and at least 6 ft 8 in of headroom.
How long is each hip on a hip roof?
On a hip roof where every plane shares one pitch, each hip's true length is the square root of 2 x run² + rise². Over a 42 x 32 ft footprint at 6/12, each hip crosses a 16 ft run in plan and rises 8 ft, so it is exactly 24 ft long. With the 10 ft ridge, that puts 106 ft of ridge cap on the hip roof against 42 ft on a gable.
Should I use an appliance's nameplate wattage to estimate running cost?
Not for thermostat-controlled appliances. Fridges, freezers, heaters and air conditioners cycle on and off, so their average draw is a fraction of the label figure, and a plug-in meter or the yearly kWh on an EnergyGuide label is a far better input. Amps times volts also overstates real watts for motors and compressors.
How should a whole-house air conditioner be sized?
A whole-house system should be sized with an ACCA Manual J load calculation, which accounts for windows, orientation, infiltration and duct losses room by room. Shortcuts such as the ENERGY STAR room chart, which gives 12,000 BTU/h for 450 to 550 ft², are fine for choosing a window unit or a space heater but not for a central system.