GardenRoomBeginner Posted March 6 Share Posted March 6 Hi, I have had a look and can’t see any definite answers to this. Apologies if I have missed it. I am looking into what size of joists I need to use for my flat roof on the garden room. It has to span 5.5m with a 300 over hang at the front and 100 at the back. Going with 400 centres. Most of the span tables I have found do not suggest any size of timber for this however, I have seen quite a few videos where they have sistered 6x2 with bolts to help strengthen the joists but again, none of the spans were the same as mine. The question I would like help with is, would sistered 6x2 roof joists be strong enough to carry cold roof and prevent future sagging? Thanks in advance, GRB Link to comment Share on other sites More sharing options...
markc Posted March 6 Share Posted March 6 At 5.5m and flat roof so possible high snow loadings etc. I wouldn’t use solid timber, the self weight (unless looking at 200-250mm timbers) will tend to sag sistered or not, I would be looking at pozi’ joist or cold rolled purlins. Link to comment Share on other sites More sharing options...
BotusBuild Posted March 6 Share Posted March 6 Sounds like a need for Web- or I-joists. As a guide only (!) We have some 5.4m spans and had web joists at 400 c/c. Each joist was 300mm deep and 100mm wide. This is for a warm roof with green roof on top. If you can afford it get a structural engineer to do the calculations for you, with the caveat they will likely overspec for insurance purposes Link to comment Share on other sites More sharing options...
torre Posted March 6 Share Posted March 6 Sounds like a span better suited to engineered posi/i-joists. Plenty of span tables online. Have you tried contacting local joist suppliers? Given a set of plans they'll probably spec out the depth & spacing you need for free when quoting. Deeper but wider spaced may be cheaper if you're not height constrained. 1 Link to comment Share on other sites More sharing options...
Conor Posted March 6 Share Posted March 6 How are the walls built? That's quite a roof. If it's block built you'd want a steel beam running the length to carry the joists so the span is halved. Link to comment Share on other sites More sharing options...
kandgmitchell Posted March 6 Share Posted March 6 The good old 1992 version of the Approved Document to Part A had a wide range of timber span tables in the appendix. For a 5.5m clear span using C16 joists you'll need 63mm x 220mm @ 400mm cntrs assuming a dead load of no more than 0.5kN/m2 (excluding self weight) or 75mm x 220mm @ 400mm cntrs if the dead load is between 0.5 and 0.75kN/m2. Although C16 is not as common these days. If you have C24 joists you could "get down" to 75mm x 195mm @400 cntrs for the lighter dead load. But as been pointed out these are chunky bits of timber and open span joists may be cheaper in the long run. Link to comment Share on other sites More sharing options...
gaz_moose Posted March 6 Share Posted March 6 there is a good facebook page for these garden rooms called, oakwood garden rooms, he also has loads of youtube stuff. its where i learned about epdm roofs. 1 Link to comment Share on other sites More sharing options...
GardenRoomBeginner Posted March 6 Author Share Posted March 6 8 hours ago, markc said: At 5.5m and flat roof so possible high snow loadings etc. I wouldn’t use solid timber, the self weight (unless looking at 200-250mm timbers) will tend to sag sistered or not, I would be looking at pozi’ joist or cold rolled purlins. Thanks. Based on all the replies, I reckon I’ll go for I-joists. Link to comment Share on other sites More sharing options...
GardenRoomBeginner Posted March 6 Author Share Posted March 6 8 hours ago, BotusBuild said: Sounds like a need for Web- or I-joists. As a guide only (!) We have some 5.4m spans and had web joists at 400 c/c. Each joist was 300mm deep and 100mm wide. This is for a warm roof with green roof on top. If you can afford it get a structural engineer to do the calculations for you, with the caveat they will likely overspec for insurance purposes I had seen other projects using I-joists but could never see prices online. I will check with our local timber merchant and see what they say. Thanks for sharing your spec. Link to comment Share on other sites More sharing options...
GardenRoomBeginner Posted March 6 Author Share Posted March 6 39 minutes ago, kandgmitchell said: The good old 1992 version of the Approved Document to Part A had a wide range of timber span tables in the appendix. For a 5.5m clear span using C16 joists you'll need 63mm x 220mm @ 400mm cntrs assuming a dead load of no more than 0.5kN/m2 (excluding self weight) or 75mm x 220mm @ 400mm cntrs if the dead load is between 0.5 and 0.75kN/m2. Although C16 is not as common these days. If you have C24 joists you could "get down" to 75mm x 195mm @400 cntrs for the lighter dead load. But as been pointed out these are chunky bits of timber and open span joists may be cheaper in the long run. Thanks. Not sure I want to risk it with the c24 timbers. Going to investigate the I-joists a bit more now. Link to comment Share on other sites More sharing options...
GardenRoomBeginner Posted March 6 Author Share Posted March 6 5 hours ago, Conor said: How are the walls built? That's quite a roof. If it's block built you'd want a steel beam running the length to carry the joists so the span is halved. Walls are 5x2 timber. Link to comment Share on other sites More sharing options...
oliwoodings Posted March 6 Share Posted March 6 (edited) What's the other dimension of your garden room? If it's less than 5.5m, then why are you choosing to span the longer dimension? EDIT: nevermind just seen your intro post - 6x5.5m, that's a huge space!! Edited March 6 by oliwoodings Link to comment Share on other sites More sharing options...
GardenRoomBeginner Posted March 7 Author Share Posted March 7 1 hour ago, oliwoodings said: What's the other dimension of your garden room? If it's less than 5.5m, then why are you choosing to span the longer dimension? EDIT: nevermind just seen your intro post - 6x5.5m, that's a huge space!! I seen your other comment. Your room was 6x5? What solution did you go for on the roof joists? Link to comment Share on other sites More sharing options...
oliwoodings Posted March 7 Share Posted March 7 18 hours ago, GardenRoomBeginner said: I seen your other comment. Your room was 6x5? What solution did you go for on the roof joists? Mine is divided into two rooms, so the max span is actually 3.5m! I used 6m 2x7s for the joists with them resting on the interior partition wall. 1 Link to comment Share on other sites More sharing options...
oliwoodings Posted March 8 Share Posted March 8 Another thought on your suggestion of sistered joists - I considered this as well at one point, but realised it would tangibly reduce the thermal performance of the roof (because of the amount of timber Vs insulation), unless I went warm roof but then I couldn't get it under 2.5m. Also the cost of sistering became very unattractive compared to other options like I joists etc. Link to comment Share on other sites More sharing options...
GardenRoomBeginner Posted March 9 Author Share Posted March 9 On 08/03/2024 at 09:33, oliwoodings said: Another thought on your suggestion of sistered joists - I considered this as well at one point, but realised it would tangibly reduce the thermal performance of the roof (because of the amount of timber Vs insulation), unless I went warm roof but then I couldn't get it under 2.5m. Also the cost of sistering became very unattractive compared to other options like I joists etc. Good point. I’ve decided on I joists now. Going to speak to local timber merchant and see what they say. Link to comment Share on other sites More sharing options...
GardenRoomBeginner Posted March 9 Author Share Posted March 9 Thanks for all the advice everyone! Link to comment Share on other sites More sharing options...
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