4x2 loft joists

Decking Joists - Deckin

  1. Leading timber suppliers for UK and Ireland. Free Next Day delivery for Timber Cladding, Decking and Fencing Internal and External Us
  2. A 4x2 should be fine for storing light things up there. There wouldn't be any point in putting 8x2 joists in to pre-empt a loft conversion as when the structural drawings they wont always specify an 8x2 joist
  3. Sistering joists for loft conversion. Discussion in 'Builders' Talk' started by paul-k, Feb 5, 2010. paul-k New Member. Hi all Im considering convering my loft and doing as much as possible DIY. Its use will be a snug/office. But if you bolt a 4x2 to a 4x2 - you get a 4x4, not an 8x
  4. You can see the 4x2 ceiling joists and another 4x2 going left to right (ceiling binder?) that appears to be nailed to the bottom of the roof rafter and then sits up against the chimney breast on the opposed side of the loft (semi detached house) but not actually tied into anything
  5. The loft is purling design (i think this is the right spelling), anyway on top of the joists I have tie beams running at right angles to the ceiling joists Would it be wise to board on top of these? I would have to add extra 4x2 so the loft boards would span the area. This would then create a gap between joists and new floor. Is this ok to do.
  6. The longer the distance between the supports, the greater the deflection for a given joist size. 4x2 may work for you - when I had my loft converted, it had to be 8x2!! There will probably be a difference though in what will flex when you walk on it, and meeting the loading requirements for habitable rooms as defined in building regs
  7. ing the answer to such questions. Maximum Span Calculator for Wood Joists and Rafters One plugs in w..

Joists-Timber Focu

The binders aren't supported on a wall at either end, just nailed to each joist and hanger. I'd also like to move the loft hatch as it is currently awkwardly placed in the bathroom. Am I ok to put a 4x2 across the length of the joist which has been cut, and then cut it a bit further along (as marked) and brace it on the joists either side Common sense tells you that large floor joists can carry more load, and spacing joists closer together also increases the load-bearing capacity of a floor. But larger is not always better when builders are constructing a home or adding a room addition. The extra two inches of vertical distance when a floor is framed with 2 x 10 joists rather than 2 x 12s can be quite important, for example If the attic joists are not adequate, one way to strengthen the floor for live loads is to sister the old joists. Sistering is the process of adding a new joist next to each existing joist. In the case of 2x6 joists, you can pair them up with additional 2x6 joists by nailing them together, side by side no need to double up any of your new 8x3 floor joists sit a sole plate across them directly beneath your purlin and support your purlins with notched 4x2 as you have suggested or remove purlin and replace with 4x2 and esentially build a stud wall down to your new floor joists. before you complete your stud wall you can also take out that big. Below is a table giving the minimum size floor joists needed for 16″ and 24″ on center (o.c.) spacing when using #2 lumber with 10 pounds per square foot of dead load and 40 pounds of live load. You can always use bigger lumber, closer spacing, or a smaller span; just don't go smaller, further apart, or a longer span

Safe joist size for storage in the loft - MyBuilde

  1. 1. Strip the insulation down to just one layer for the area I want to board and then board it as it was before, traight on to the joists. - The previously boarded area was large though (approx half the entire loft area) and I'd rather keep the same area. 2. Increase the height of the joists to just above the insulation
  2. I think the joists are 4x2 I need to check this and on both of the sides there a plank running diagnal across several joists. Whats the best way to compensate for this raise the others by placing 4x2 or some variant at 90 degrees and floor onto this which I guess would let me put thicker insulation in
  3. Hi, i wouldn't lay your boards directly on to the rockwool, take the 2nd rock wool out and relay all insulation between joists, then fit a 35mm x 45mm batten to the top of the existing ceiling joists to take the loft boarding, make sure you pre drill the timber and use 2 1/2 inch, 8 screws. you don't need many screws into the battens, then lay the boards , t&g loft boards would be best for.
  4. This is part 2 in my series of loft conversion videos. In this video I show how I cut my my joists, make packer plates, get the joists from the garden up int..
  5. 4x2 Carcassing Timber. We hold huge stock of 4x2 (100 x 47mm) treated and untreated timber in strong C24 grade available in various lengths to suit your construction requirements. 15 Items Found. Show

Sistering joists for loft conversion Screwfix Community

Link to post. Posted May 6, 2008. I know 600kg is a lot. I didnt think it would hold it, but ive just been thinking and last week i put down 875kg worth of sand and cement in the back room, which has a cellar under it, and i suppose the beams under there will probably be the same size as the ones in the attic anyways FSC Standard Kiln Dried Regularised C16 Treated Carcassing 145 x 45mm. FVRT4514. SKU: FVRT4514. Trade Sign in or apply for trade prices. £ 6 .28 inc VAT per metre. Loading. More details. FSC Standard Kiln Dried Regularised C16 Treated Carcassing 120 x 45mm. FVRT4512

A. Loft legs or stilts is no an actual sub-frame they are just stilts located at various intervals under your loft boarding which can raise the floor level by around 175mm which is quite a height and ideal if you need more clearance that 4x2 wood joists It currently has 100x50mm (4x2!) joists at 450mm centres - span tables tell me that's OK for spanning a 1.6metre gap, however they're currently spanning a 3.4metre gap!! They were almost certainly the original joists from when the room above the kitchen was a simple storage loft - it has since had the lean-to roof raised and been converted into. At B&Q, we have a variety of constructional timbers available. Ready for use both indoors and outdoors and for a range of structural and non-structural work including studwork, boxing in, partitions and more. Our treated timber is pressure treated using tanalith e, producing tanalised timber. Tanalised timber is treated to increase the. The loft joists are 4x2 with a max span between load bearing walls of 10ft. The joists have 18 centres. Can I fix my boards directly on to the existing joists (this will compress the loft insulation very slightly) or would it be preferable to put additional batons at right angles and fix onto these

How should I add new joists and flooring to a loft space

  1. Loft boards are fixed together relatively easily, but working withing the confines of the loft's available space can make boarding a loft a tricky endeavour. Do heed the safety advice detailed below and only attempt the task if you are capable. If not, hire a professional. If you want to board your loft, follow this step-by-step guide
  2. Finding deep enough rafters. To achieve the 16-inch depth we wanted for insulation and an air space under the sheathing, we used open-web, parallel-chord trusses as the rafters. These trusses, typically used as joists, have diagonal bracing or struts and are made in Quebec by Open Joist Triforce
  3. Joist hangers attached to the LVL carry the floor joists. Rim joist stock is used on the walls parallel to the joists. In addition, 8-in. x 5-in. gussets connect the inner studs to the outer studs in the exterior wall assembly to transfer load to the outside studs, should any movement of the inside wall occur

bolting two timber joist together for additional strength and cutting treaded rod Our range of sawn treated timber (tantalised timber) is available in a wide choice of sizes from Roof Batten, 3x2, 4x2 and up to 9x3 kiln dried C24 strength graded timber joists. Our high quality eased edge is pressure treated timber making it a great choice for constructions that require strength, durability and a quality finish Wickes Treated Kiln Dried C16 Timber - 45 x 170 x 3600mm. (71) £22.20. £6.17 per M. Wickes Treated Kiln Dried C16 Timber - 45 x 220 x 3600 mm. (62) £29.18. £8.11 per M. Wickes Treated Kiln Dried C16 Timber - 45 x 120 x 4800mm

Floor Joists And Support Beams For Loft Conversion

Screw a base 4x2 to the floor joists, the same to the underside of the rafters and cut the uprights to suit the angle. I've done loads of lofts like this and don't know of any that have blown. Note: be careful when cutting, as sloppy joist layout or warped joists may make the spacing non-uniform. Measure several joists to determine the average spacing. You may have to modify your cuts as you go if the layout is non-uniform. I usually lay out several panels across the joists before cutting or attaching to be sure the spacing will work. In older houses the ceiling joists are often small section, maybe 3x2 or 4x2. This is just about sufficient to support the ceiling itself. Adding significantly to the load that these joists are carrying is not a great idea and can lead to problems. There may also be a few larger section timbers present along with the smaller joists 8 August 2011 at 6:46PM. I beefed up my loft with 8x2 laid at right angles to 4x2 ceiling joists. 8x2s were nailed to the joists but not tied into the walls or roof. This was so I could add 270mm insulation. I then floored the loft with T+G and plasterboarded the ceilings below

Maximum floor joist span for No. 1 and No. 2 Grade of Douglas fir are indicated below. Max. dead load (weight of structure and fixed loads) 10 lbs/ft 2. Live load is weight of furniture, wind, snow and more.. 1 psf (lb f /ft 2) = 47.88 N/m 2; 1 ft = 0.3048 2x4 2x6 2x8 2x10 2x12. Grade. Select Structural No. 1 No. 2 No. 3 Stud Construction Standard Utility. Member Type. Floor Joists Ceiling Joists Rafters (Snow Load) Rafters (Roof Live-Load) Deflection Limit. L/720 L/600 L/480 L/360 L/240 L/180. Spacing (in) 12 16 19.2 24

Where similar lightweight partitions run at right-angles to the joists, the maximum spans in Tables 1 and 2 should be reduced by 10%. For all other additional loads, joist sizes should be designed by an engineer in accordance with Technical Requirement R5. Table 1: Permissible clear spans for domestic floor joists The longer the distance between the supports, the greater the deflection for a given joist size. 4x2 may work for you - when I had my loft converted, it had to be 8x2!! There will probably be a difference though in what will flex when you walk on it, and meeting the loading requirements for habitable rooms as defined in building regs I am planning to run 2 additional 4x2 binders and just want your opinion on the best way to attach them to the joists. Stainless Steel Banding 20mm - Run under the joist and then twist and wrap around and over the top of the binder and screw. Skew screwing from the joists to the binder. Loft is used for light storage and is boarded Floor joists are generally of a minimum size of 4x2 and support a floor by spanning at least a metre or more between supports. The supports to the floor would then be part of a shed foundation which could be a pad foundation or a foundation comprising timber bearers. View of shed floor with joists

timber joists 6x2 grade c24 | in Oakwood, West Yorkshire

Measure the width and length of the existing joist. Purchase the type of lumber that matches the width of the existing joist. Most new construction uses 2-by-8 lumber for joists (although 2-by-6. Extra 4X2 lengths running perpendicular to the existing joists will lift the new floor high enough to have loads of insulation, will spread load better and cut out risk of your ceiling cracking if. - the existing 4x2 ceiling joists. Initially I thought of taking - down the ceilings and replacing with 6x2 joists, but my brother - suggested strengthening the existing joists by screwing -(and/or glueing) 2x2 lengths; in effect, laminating. This would - be considerably quicker, cheaper, and less messy Span tables for timber floor joists. Surveyors and structural engineers use data from tables below to help calculate the size of timbers necessary to give adequate support to timber floors. These tables can help calculate whether floor joists are strong enough to support themselves and the load they carry, without the help of any walls underneath

Carcassing, Joist and Studs. We work closely with leading manufacturers to provide an extensive range of high-quality and cost-effective CLS studwork timber, treated timber carcassing, and untreated timber carcassing.. Carcassing timbers are those that are usually used in structural applications, such as roof joist, floor joists, framing, studwork, and wall partitions Joist span tables - use these tables to determine lengths, sizes and spacing of ceiling joists. Note: Ceiling joists of the first floor of a two-story structure often serve as floor joists for the second floor. If this is the case with your structure, use the Floor Joist Table to determine maximum joist length/load So, let's explore some of the types of loft boarding and their costs. If you want a genuine room up there, you'd need a full loft conversion, which is a big job, but for most purposes you just need the loft boarded over. This also has the added benefit of making your insulation more efficient. And it doesn't have to cost the earth, either Easy to fit supports and steel joist sections allow a solid loft floor to be added easily. The StoreFloor system allows the loft boards to be butted up to the roof rafters so the air can rise through the eaves, follow the line of the underside of the roof and ventilate the main loft, while flowing under the new loft floor through the gap left.

Best method to strengthen a loft floor ? AVForum

I was playing around with ideas in the begining of my loft thread. Then MrJim, Nu Jerzy Devil and a few others pointed me in the right direction regarding laying joists etc. I was gonna try some counter battening of the old ceiling joists but they had that much of a sagg on them, i decided the way to go was a new floor with joists. Nice and strong Sub-Frame Loft Boarding The most effective way of boarding a loft up until now would be to build a loft sub frame for the new loft boards to be secured. 4x2 timbers are laid in the opposite direction to the original joists, helping to distribute the load across the ceiling however, this method will mean that a considerable amount of additional weight will now be bearing down on the existing beams - If supporting lightweight partitions running parallel to joists, provide two additional joists under each partition. SIZE OF JOISTS (mm) CLEAR SPAN C16 JOIST (M) CLEAR SPAN C24 JOIST (M) 47 X 95 47 X 120 47 X 145 47 X 170 47 X 195 47 X 220 1.77 2.40 2.89 3.38 3.87 4.36 2.04 2.75 3.47 4.20 4.93 5.66 FLOOR JOISTS FLAT ROOF JOISTS Assumptions at. Loft Conversion. Don't Flitch. The first major job is strengthening the floor. Although the existing loft floor is comprised of 4x2 timber (which is quite good for a loft by modern standards), this is nowhere near strong enough for a new floor. Hence the need to play a game of inserting lots of 8 deep beams into a space while dodging all the. Jewson's versatile sawn & carcassing timber is suitable for a variety of construction projects, including decking, structural support, fencing and more. Explore sawn treated timber, great at preventing wood rot even in damp conditions and keeping pesky burrowers away. We also stock untreated and premium varieties to fulfil your needs

Can I use 2x4 for floor joists? - Quor

  1. g out at £299 for loft legs alone. I could keep the insulation as it is, run extra 4x2 joists 90 degree to the current joists at 600 centres, this would cost £190 in timber
  2. Treated Sawn Kiln Dried C24 Timber. Wickes Treated Kiln Dried C24 Timber has been graded to C24 strength for structural use, a stronger grade than C16 for heavier loads. It is also pressure treated to protect against fungal attack, so is ideal for joist work and roof rafters
  3. Joist struts & hangers. Our Joist hangers and joist struts are made from durable, strong steel, ideal for using in timber to timber construction projects like fitting rafters or decking. The joist hanger comes with multiple positioning holes to allow you a range of options of where you screw or bolt to fasten the item in place
  4. We've moved into an old house and want to utilise the loft space, currently there's 3'x 2' joists which run for 6 m across the house, supported in the centre by a dividing wall. Each joist is therefore unsupported for around 2.8 m between each bedroom wall

Our joists are 100mm and that's how much insulation I have between them. I'd have at least double that but the loft is fully boarded and I want to keep it boarded. I might use Yamar1's idea and put 4x2 battens over the joists, double the insulation up, then put the boards back. 200 is still inadequate for today but better than what I have LoftZone StoreFloor is an award-winning loft boarding system designed for installing flooring above the full depth of insulation in an attic. Our DIY kits are lightweight, strong and simple to use with clear, easy-to-follow instructions. StoreFloor can hold up to an impressive 500 kg's per square meter due to its Tri- and Uni-Supports made.

Raise loft floor & strengthen joists DIYnot Forum

Understanding Floor Joist Spans - The Spruc

How to Strengthen Floor Joists Before Finishing an Atti

See the deck joist calculations below and study your local code. Deck Beam Spans: Distance a joist spans between a beam and a ledger or between beams, using No. 2 and better Southern pine or Douglas fir. Joist: 2x6 If joists are spaced: 16 Span: 9 1/2'. Joist: 2x6 If joists are spaced: 24 Span: 8' Selco is a well-established building and timber merchant stocking timber products from well-managed forests. Our timber yards hold a wide range of timber supplies suitable for all projects. Save time and use our click and collect service, or if you're ordering long lengths of timber, speak to your local branch about our reliable delivery service Decking Joists. Our timber decking joists are carefully selected to ensure high quality at competitive prices. Our range comes in a variety of dimensions and lengths, such as 47x100xmm decking joists, 47x125mm decking joists, 3m decking joists, and 4m decking joists Start at one end of the room and lay a board across the joists, leaving a minimum 10mm (3/8in) gap between the board and the walls to allow for expansion. See the advised expansion gaps for different woods. Insert thin strips of wood along the length of the wall to maintain the gap. Fix the board to the joist with nails or screws, using two.

Jiffy Joist Hangers. Timber & Sheet Materials. Timber Decking. Our Timber is graded as C16 or C24 kiln-dried with rounded edge, which produces a neater, faster finish. You will receive a mixture of both grades The floor joists are definitely 4x2 and I think the rafters are 3x2 - the rafters have had that interior felting thing, where strips are cut between the rafters and battened in place. Sadly, loft boards have already been laid to the 4x2 floor joists Joint SpliceA splice of the 4x2 chord member at a chord-and-web joint. Kneewall A short partition stud wall to increase a wall height, typically from the concrete wall plate to the floor decking. Lateral BraceA member placed and connected at right angles to a chord or web member of a truss. Level ReturnA lumber filler place In my bungalow, the previous owner had built a bedroom & sitting room in the roof space. Used the existing 4x2 joists plasterboarded it out & the daughter lived up there. When we said we wanted a full survey done, the seller got jittery, apparently talking to the neighbours a sale had got to that point several times before & then fallen through The only storage system designed specifically for the unfinished areas over a garage or under a roof. These lightweight easy-to-manage panels can be installed by even the most inexperienced homeowner in about 15-minutes. Attic Flooring Panels are easy to install with their interlocking edges. A single 4-pack covers approximately 10 sq. ft

loft conversion advise

C24 Sawn Timber Floor Joist 45x95mm (4x2 Inch) — Armstrong

If using a standard size floor joist creates a problem with your finished floor elevation, look into notching the joist at the supports (read your local building code for notching requirements). Finally, if you can't get a standard size joist (2x6 or larger) to work, you'll have to find a way to prove to your building department will accept 2x4's Treated Battens, Posts & Joists. 4x2 (100x47mm) tanalised sawn joists (4.8m) We're currently experiencing a problem with our payment gateway and can't take payments online. If you're local to Merseyside, you can still order and select cash on delivery at the checkout to pay on delivery. Alternatively, please call us on 0151 207 7488 to arrange. I own a Victorian property which has 4x2 ceiling joists. I want to strengthen these joists as well as increase the height of them. I don't know whether its best to run new joists in between the existing OR use joist hangers to strengthen the existing joists and then nail battons to the tops of the existing joists the give the height im after If they were even 16″ o.c., you are talking 0.79 per square foot for the cost of joists. Floor trusses are going to run around $4.40 per lineal foot, spaced 2′ on center, this makes the cost per square foot for the joists at $2.20. For a floor span of over 24′ trusses are certainly the way to go. To which Rick responded

C16 graded structural timber is the most cost-effective option and meets performance requirements. Kiln dried C16 is generally recommended for most applications if the correct spans are used and is mainly used internally for floor and roof joists. Lengths commonly available are 3.0, 3.6, 4.2 and 4.8 meter Whether you are planning new outdoor decking or installing new roof rafters, C24 timber joists are exactly what you need to ensure strength and stability. At Armstrong Supplies we sell high quality, regularised timber that are suitable for both DIY and professional applications. From 4x2 timber in mm (47mm x 100mm) to 9x3 timber (75x225mm) you.

New stress graded 4x2 timbers are fixed transversely to the top of your existing joists, allowing space for a further 100mm of top layer insulation to be installed on top of your base layer as required. This provides a strong, safe storage area while enabling low profile plumbing and electrical cables to be left in situ C16 Treated Timber Floor Joist 3x2 4x2 5x2 6x2 7x2 8x2 9x2 8x3 4x3 6x3 9x3 4x4. £8.90 to £98.00. (£17.95/Unit) £10.00 postage. Timber Treated, Length 1.2m ! Sizes: 2x4 2x6, Reg. £10.64 to £43.08. Free postage From doing some reading, it seems that I've got two options: put some more timber down to create higher joists (would require 6 height increase), or use loft stilt type things. Although probably more expensive, the loft legs seems better because of reduced disruption to the insulation as well as being easier because no large timber and sawing. It currently has 100x50mm (4x2!) joists at 450mm centres - span tables tell me that's OK for spanning a 1.6metre gap, . Section 25 - Wood Frame Construction (PDF) - Village of Flossmoor Ceilings which are not supported on the underside of floor joists, roof rafters, or roof . angel thirty inches (30) long, unless a double wall is built to. Nicks timber, a timber merchant based in Gloucester for over 150 years of trading. Offering premium timber for a great price to both trade and D.I.Y sectors

Joists Up | off grid homeSawn Timber C24 KD Floor Joist 47x100mm (4x2) — Armstrong

Floor Joist Spans for Home Building Projects Today's

Decking Joist/Bearers 50x50x2400mm. Worthing, West Sussex. Project Surplus. 8 joists / bearers 50 x 50 x 2400mm Made from extruded recycled plastic. Will not rot or warp. Designed as decking subframe but also idea for shed bearers etc. Very strong Cost approx. £120 (£15 each) £90. Ad posted. 9 days ago. Save this ad Joist Hangers, floor joists and Struts are essential in preventing the sideways movements and buckling of beams. Here at Builder depot we offer a wide range of Joist Hangers and Struts that have been manufactured using high strength steel compounds helping you to get the perfect fit for your building project whatever the load or design requirements may be Visit Howarth Barnsley today, offering a wide selection of timber, flooring and so much more. For building supplies Sheffield, visit our store today Joist Hanger Selector. Joist hangers represent a critical design component that helps ensure a building's structural integrity. When factoring in load requirements, member size and costs, deciding on the best solution can be a daunting process

47x150mm Treated Joists C16 3loft conversion-a how to guide

Building up joists to lay down loft boarding

For standard joist hangers, secure the hanger to the joist using only 1-1/2 in. long galvanized joist hanger nails.Avoid longer nails that would poke through the joist and splay out the other side of the hanger. (To hammer inside tight joist spaces, use either short tapping strokes or hold the hammer sideways and strike the nail with the side of the hammer. From roofing caps and roof joist hangers for roofing jobs to screen door corner braces for patio doors, we have it all. Our structural hardware is divided into categories, including angles and straps, brackets and braces, base and cap hardware, angled joist hangers and more. This makes it easy to find exactly what you're looking for quickly This Loughborough Loft conversion took place at the end of 2018. at the start of the job, the loft was merely a dark cold storage space for old boxes and Christmas trees. We began by Stripping of all the old roof tiles and cutting an opening to feed in supporting RSJ steels for the floor. We the started building the dormer using 4x2 timbers for. Builders/Structural Engineers help - posted in General Chat: Just a quick question if anyone can give me some advice. I rent a 2 bed bungalow that has quite a large loft which covers the whole size of the house (9.43m x 7.19m measuring the outside walls). My landlord advertised the bungalow as a 2 bed but said to me I could use the loft as a 3rd bedroom

Flooring a loft for storage — Digital Sp

* Boarding to be fastened on top of flat roof joists with a depth to width ratio greater than 4 The allowable clear spans have been calculated in accordance with BS 5268-2:2002 Structural use of timber - Part 2: Code of practice for permissible stress design, materials and workmanship and BS 5268-7.2:1989 Structural use of timber - Section 7.2. The joists can be built into the wall or hung from joist hangers. The joist hangers must be the correct size for the joists and conform to BS EN 845. The joists should be placed on and fixed to a timber wall plate which is bedded in mortar on top of the inner skin and strapped down using galvanised holding-down straps

Loft Boarding - Raising Joists - MyBuilde

Non-Load Bearing. With timber or metal frame walls it is normally acceptable to support them on the existing floor joists (not the floor boards), either by providing a double floor joist underneath, if they run parallel, or across the existing joists if the floor joists run at 90° to the wall. (In both cases, there may be circumstances where. If your track runs parallel to the ceiling joists, fasten the track to the joists with drywall anchors or secure it with grabber screws and glue. Advertisement. Part 2 of 2: Fastening the Metal Studs and Finishing Up 1. To trim to correct length, cut both side flanges of a steel stud.. Timber 4x2 @ 3m. Oldham, Manchester. Also available in 3.6m- £9.75 4.2m- £10.95 4.8m- £12.50 Call today for a Quote 0161 425 3030 Collection only For any enquiries please call or message us on Gumtree You can also visit us Direct Building Supplies Unit 1 Westwood. £7.25 Browse the Travis Perkin's range of trade standard rough sawn timber such as scaffold boards, untreated & treated timber, roofing battens, studwork timber and site pegs Simpson Strong-Tie Timber to Timber Joist Hanger 90 x 234mm 10 Pack (41273) Product rating. 4.7. out of 5 stars. (65) Compare. Compare. Galvanised Steel. For Supporting Timber Floor Joists

Safeloft. 559 likes · 18 talking about this. Here at Safeloft we specialise in transforming your unused loft space into the best useable storage area we can provide. From loft ladders to loft.. The picture shows a previous loft conversion. It required removing the braces to the purlins hence additional load was placed on the side walls which was supported mid span by the floor. Hence it required 8x3 joists to support the foor and the roof load. Cost £300 for a set of calculations from a structural engineer Decking joists are an essential part of building any quality garden deck. Lawsons' decking joists are all made from quality timber and pressure treated to ensure a longer life. While joists are concealed, they are one of the main structural elements of your deck which is why it's important to install them properly and be confident that you.